Field of the invention

The present invention relates to spectroscopy. In particular, the present invention relates to Atomic Absorption Spectroscopy (AAS), and apparatus for performing AAS.

Background

The technique of AAS is dependent on the principle that electrons of elements in an atomic form have discrete energy levels and will absorb energy of specific wavelengths

corresponding to those discrete energy levels. Each atomic element has a unique arrangement of discrete electron energy levels. Accordingly, the wavelength(s) of light absorbed by an atom will be indicative of the discrete energy levels and may be used to identify a particular element. Furthermore, the amount of light absorbed is proportional to the number of atoms present, and so the quantity of atoms present may also be

determined.

A typical AAS system comprises a light source for providing light of a known wavelength, an atomiser for atomising a sample to be measured, a monochromator and a light detector.

Typically, the light source is a hollow cathode lamp (HCL). An HCL usually consists of a glass tube containing a cathode, an anode, and a buffer gas (usually a noble gas).

Applying a large voltage across the anode and cathode will cause the buffer gas to ionize, creating a plasma. The cathode may be constructed from an element to be analysed in the AAS measurement. The buffer gas ions will then be accelerated into the cathode, sputtering off atoms from the cathode. Electrons of both the buffer gas ions/atoms and the sputtered cathode ions/atoms will in turn be excited by collisions with other particles in the plasma. As these excited electrons subsequently fall to lower states, they will emit photons with wavelengths specific to the element to be analysed in the AAS measurement.

Light emitted from an HCL source may be passed through an atomiser containing an atomised sample to be analysed wherein in a proportion of the light is absorbed. The light not absorbed by the atomised sample is then filtered by a monochromator to separate the wavelength(s) of interest for the element to be detected from other wavelengths. The final light signal is focused on a photodetector in order to measure the signal.

In order to increase the accuracy of the AAS measurement, a secondary light source is often used in tandem with the element specific HCL light source. For example, the secondary light source may be a deuterium lamp which produces light with a continuous broadband spectrum typically ranging from 180 nm to 370 nm. By measuring the atomised sample first with the hollow cathode lamp to determine an analyte plus background signal, and subsequently with a broadband light source to determine only a background signal, it is possible to correct for the effects of background absorbance in the analysis measurements.

Another source of error in the AAS measurement may result from variations in the light intensity produced by the HCL and deuterium light sources. Typically, a portion of the light from each light source is not directed through the sample and measured at the detector as a reference beam in order to improve the accuracy of the AAS measurement. Such AAS systems are typically known as double beam (sample beam and reference beam) spectrometers.

Known AAS systems typically use a series of mirrors and lenses to transport light from the light sources through the atomiser and monochromator to the detector. A schematic diagram of a typical double beam spectrometer known in the prior art is shown in Fig. 1 .

The double beam spectrometer uses a chopper to divert the light source between the reference beam light path and the sample beam light path. A chopper is essentially a circular mirror with a number of segments (usually 4) cut from around the circumference. When the light beam is aligned with a cut out segment it passes along the sample beam light path through to the flame. When the light beam is aligned with a remaining reflective segment on the mirror it is reflected around the flame and becomes the reference beam.

A beam combiner combines the two paths for focusing on the monochromator. The beam combiner is may be a circular mirror with a series of holes in it. The sample beam is able to pass through the holes in the mirror, whilst the reference beam is reflected via the mirror surface of the beam combiner.

Known double beam spectrometers, such as the exemplary system shown in Fig.1 include a plurality of mirrors and focusing lenses in order to transport light from the light source(s) through the sample and monochromator to the detector. Such systems using a plurality of optical components are highly sensitive to small variations in the relative alignments of these components. Small variations in the translational or rotational position of one or more optical components (e.g. a mirror) can introduce errors into the alignment of downstream optical components, thus degrading signal intensity and/or increasing noise in the measurement system. Accordingly, such optical systems normally require calibration of the relative alignment of the optical components to ensure that the system operates as intended.

WO 201 1 062630 discloses a photonic measurement instrument using non-solarizing fibre optics. A set of fibre optic cables is used to guide light from one or more light source to each of at least two analysis chambers of a sample holding module. A further set of optical fibres is used to guide light from the analysis chambers to a detector.

Summary of the invention

According to a first aspect of the invention, an adjustable slit mechanism for a

monochromator is provided. The adjustable slit mechanism comprises a first slit member including a first slit edge and a second slit member including a second slit edge, wherein the first slit edge is arranged opposite the second slit edge to define an adjustable slit of the adjustable slit mechanism. The adjustable slit mechanism also includes a slit adjusting cam having a variable radius. Further, a first arm is connected to the first slit member and a second arm is connected to the second slit member. The first arm and the second arm are biased against the slit-adjusting cam such that rotation of the slit adjusting cam adjustably separates the first arm and the second arm for adjusting a width of the adjustable slit.

As such an adjustable slit mechanism is provided which is configured to adjust the slit width (i.e. the opening between the first slit edge and the second slit edge) of a monochromator. By controlling the slit width through rotation of a cam having a variable radius, the slit width may be repeatedly and accurately adjusted to a desired width through control of the rotational position of the slit adjusting cam. Advantageously, the adjustable slit mechanism according to this invention may adjust the position of the first slit edge and the second slit edge, as both the first arm and the second arm are biased against the slit adjusting cam. Accordingly, the position of the centre of the slit may be accurately controlled according to the variable radius of the slit adjusting cam. In a particularly preferred embodiment, the variable radius of the slit adjusting cam is configured to adjust the separation of the first arm and the second arm by moving the each of the first arm and the second arm substantially equally. As such, the centre of the slit may be maintained in substantially the same location for a plurality of different slit sizes. Accordingly, the centre of the slit may be maintained in a central position of a sample light path and/or a reference light path for a range of different slit sizes without any further calibration of the sample light path or reference light path between adjustments of the slit. As such, the adjustable slit mechanism may be incorporated into a robust fibre optic AAS system with minimal calibration.

Preferably, the first arm is biased against the slit adjusting cam at a point opposite to the point at which the second arm is biased against the slit adjusting cam. As such, a variation in the diameter of the slit adjusting cam may be used to adjust the separation of the first arm from the second arm. Such an adjustment in the separation of the first arm and the second arm may provide a corresponding adjustment in the separation of the first slit edge from the second slit edge (i.e. an adjustment of the slit width). Accordingly, a mechanically robust system for adjusting the slit width of a monochromator may be provided.

A resilient biasing element may be provided connected between the first arm and the second arm for biasing the first arm and the second arm against the slit adjusting cam. By resiliently biasing the first and second arms together, the relative positions of the first and second arms may be accurately located against the slit adjusting cam such that the separation of the first and second arms is accurately controlled by the rotation of the slit adjusting cam. Preferably, a pair of resilient biasing elements is connected between the first arm and the second arm. The pair of resilient biasing elements is connected to the first arm and the second arm on either side of the points at which the respective arms contact the slit adjusting cam. Such an arrangement improves the robustness of the adjustable slit assembly as the pair of resilient biasing elements provides resistance to relative rotation of the first or second arms about the slit adjusting cam.

Preferably, each of the first and second arms is supported by a plurality of guide members, the guide members configured to align the first slit edge parallel to the second slit edge.

For example, the guide members may be provided by locating washers which are configured to support the first and second arms and to locate the first and second arms. Preferably, each of the first arm and the second arm comprise an aligning portion, each aligning portion configured to engage with a plurality of guide members. Accordingly, the aligning portions of the first and second arms, when in contact with the guide members, are positioned to prevent relative rotation of the first and second slit edges, when the slit width of the adjustable slit is modified. As such, the aligning portions of the first and second arms restrict the movement of the respective arms to a single axis of movement. Accordingly, the width of the adjustable slit may be accurately controlled for a range of different slit widths.

Preferably, the first and second arms each comprise a cam engaging portion, each cam engaging portion configured to provide a surface which engages with the slit adjusting cam. The cam engaging portions may further preferably consist of plain bearing material or may comprise a component consisting of plain bearing material integrated into the cam engaging portion providing the surface which engages with the slit adjusting cam. In particular, cast iron and babbitt can be used as plain bearing material. Utilising a plain bearing material to provide the surface which engages with the slit adjusting cam may reduce friction between said surface and the slit adjusting cam, thus reducing the wear on the slit adjusting cam over time. This is particularly advantageous, as increased wear on the slit adjusting cam will reduce the accuracy of the adjustable slit mechanism over time.

Also in a preferred embodiment the slit-adjusting cam may consist of plain bearing material or may comprise at least one component, in particular two components, consisting of plain bearing material integrated into the slit-adjusting cam providing the surfaces which engages with the cam engaging portions of the first and second arms. In particular, cast iron and babbitt can be used as plain bearing material. Utilising a plain bearing material to provide the surfaces which engages with the cam engaging portions of the first and second arms may reduce friction between said surfaces and the cam engaging portions of the first and second arms, thus reducing the wear on the cam engaging portions of the first and second arms over time. This is particularly advantageous, as increased wear on the cam engaging portions of the first and second arms will reduce the accuracy of the adjustable slit mechanism over time.

Preferably, each cam engaging portion is configured to engage with the slit-adjusting cam in a plane substantially parallel with the first and second slit edges. Accordingly, any adjustment in the separation of the cam engaging portions through rotation of the slit adjusting cam will be directly reflected in an equivalent adjustment in the separation of the first and second slit edges. As such, a robust, yet highly accurate adjustable slit mechanism may be provided.

In a particularly preferred implementation, the cam engaging portions and the aligning portions of each of the first arm and the second arm are connected to form a substantially L-shaped portion.

Preferably, the first slit edge and the second slit edge of the adjustable slit is configured to provide an entrance slit and an exit slit for the monochromator. Accordingly, both the width of the entrance slit and the width of the exit slit may be adjusted using a single mechanism, thereby improving the robustness of the monochromator whilst also reducing the manufacturing costs.

Preferably the first slit member and the second slit member are connected to the first arm and the second arm respectively by fastenings. As such, the first slit member and the second slit member may be formed from different materials and/or by different

manufacturing methods to the first and second arms and subsequently joined together. For example, a relatively high accuracy forming method may be used to manufacture the first and second slit edges of the first and second slit members respectively, whilst an alternative manufacturing technique may be used to form the first and second arms. Of course, in some embodiments of the invention, the first slit member and the first arm may be formed from the same body using the same manufacturing technique.

Preferably, the first slit edge and the second slit edge are each aligned to be vertical.

Accordingly, gravitational forces acting on the first slit member and the second slit member may not cause the relative alignment of the first and second slit edges to vary over time. Thus, a robust adjustable slit mechanism is provided.

Preferably, the adjustable slit of the adjustable slit mechanism according to the first aspect is adjustable in the range of 0.025 mm to 2 mm, or more preferably in the range 0.025 mm to 0.5 mm. According to a second aspect of the invention a slit assembly for a monochromator (a monochromator slit assembly) for a fibre optic atomic absorption spectrometer is provided. The slit assembly comprises a sample optical fibre for transporting sample light, a reference optical fibre for transporting reference light and a slit for a monochromator. One end of each of the sample and reference optical fibres are arranged to direct light along respective sample and reference light paths through the slit. The slit assembly also comprises a rotatable chopper arranged downstream of the entrance slit configured to alternatively occlude the sample light path and the reference light path by rotation of the rotatable chopper.

The slit assembly of the presently invention includes a slit for both the sample light and the reference light. As such, the slit for the monochromator may provide an entrance slit. As the ends of the sample optical fibre and the reference optical fibre are configured to direct light directly to the slit, it may be that no further alignment optics (e.g. mirrors) are required to direct light into the monochromator. By providing the rotatable chopper downstream of the slit, all the light from the sample optical fibre and reference optical fibre may be directed to the slit. Thus, the slit is arranged to filter all the light transported by both the sample optical fibre and the reference optical fibre. By providing the chopper downstream of the slit only light filtered by the slit is incident on the rotatable chopper.

The rotatable chopper according to the first aspect is advantageously configured to alternatively occlude the sample light path and the reference light path. Thus, the rotatable chopper provides a way of temporally separating the sample light and the reference light which is integrated with the slit for a monochromator. Accordingly, a more compact and robust monochromator slit assembly for a fibre optic double beam AAS system is provided.

Preferably, the slit of the monochromator according to the second aspect of the invention is provided by the adjustable slit assembly according to the first aspect of the invention. As such, it will be appreciated the rotatable chopper may also be accurately aligned with the centre of the (adjustable) slit such that alignment of the rotatable chopper with the sample light and the reference light may be robustly maintained over time.

Preferably, an axis of rotation of the rotatable chopper is aligned with the slit (i.e. the central axis of the slit). By aligning the axis of rotation of the rotatable chopper with the slit, the rotatable chopper can be accurately positioned in the sample light path and the reference light path downstream of the slit. Thus, the chopper assembly according to the present invention may be easily calibrated and controlled and is robust to external environmental disturbances.

Preferably, the axis of rotation of the rotatable chopper is generally vertical when in use. Thus, gravitational forces will act in line with the axis of rotation of the rotatable chopper, such that the rotatable chopper will retain its alignment in the sample light path and reference light path over time. As such, the generally vertical alignment reduces and/or eliminates any leverage forces that may act on the rotatable chopper which may cause the alignment of the rotatable chopper with the entrance slit to vary over time. As such, small variations in alignment over time which may affect the intensity of light transmitted by the chopper assembly may be reduced and/eliminated. Accordingly, the rotatable chopper according to the present invention is robust and not subject to variation in alignment over time due to gravitational forces.

Preferably, the chopper has a substantially round (circular) shape perpendicular to its axis. Preferably, the chopper may have a shape of polygon with sides of the same length (i.e. a regular polygon). In particular, the polygon may have a number of equal sides.

Preferably, also, a centre of mass of the rotatable chopper generally aligns with the axis of rotation. Thus, the rotatable chopper will rotate about its centre of mass when in use, thereby reducing and/or eliminating a centripetal force, such that the rotatable chopper will retain its alignment in the sample light path and reference light path over time.

Preferably the rotatable chopper is further configured to define at least one sample channel for transmitting the sample light. Preferably, the rotatable chopper is also further configured to define at least one reference channel for transmitting the reference light. Preferably, the sample and reference channels are free space channels through the rotatable chopper for transmitting sample light or reference light through a thickness of the rotatable chopper. Preferably, the sample channel is spaced apart from the reference channel along the axis of rotation of the chopper. By providing a sample channel and a reference channel for transmitting light through the rotatable chopper, the rotatable chopper provides a mechanically robust means for transmitting and/or occluding the sample and reference light paths independently of each other. Further, by providing the chopper with free space channels for transmitting light, the rotatable chopper does not utilise any further optical elements for supplying to the monochromator the sample and reference light, resulting in a time dependent measurement of the light intensity of both light sources.

In particular, the rotatable chopper may be configured to define at least two sample channels arranged transverse with respect to each other about the axis of rotation of the chopper and at least two reference channels arranged transverse with respect to each other about the axis of rotation of the chopper. Preferably, the rotatable chopper is configured to alternatively occlude the sample light and the reference light as the reference channels are offset from the sample channels about the axis of rotation. Thus, in a single rotation of the rotatable chopper the sample light path may be transmitted four times and the reference light path may be transmitted four times. By offsetting the sample channels from the reference channels about the axis of rotation, the transmission of the sample light paths and the reference light paths may be temporally spaced (time resolved) as the rotatable chopper rotates.

Preferably, the chopper assembly comprises a sensor configured to detect a rotational position of the chopper assembly. Thus, the position of the rotatable chopper can be accurately determined and calibrated over time to reduce or prevent small variations in alignment over time which can affect the intensity of light transmitted by the chopper assembly.

Preferably the chopper assembly comprises a stepper motor configured to drive the rotatable chopper. In particular, it is preferable that the stepper motor has a step angle of no greater than 10°, or more preferably 8°. By using a stepper motor with a relatively small step angle, the rotational position of the rotatable chopper can be accurately controlled. Furthermore, the detent torque of the stepper motor ensures that the positional accuracy of the rotatable chopper is maintained when the chopper is held in a fixed position, for example during an AAS experiment.

In an alternative embodiment of this invention, the slit assembly may be provided as part of an optical system. As such, the slit assembly may comprise a slit, means for transporting sample light to the slit, and means for transporting reference light to the slit. The means for transporting sample light and reference light may be arranged to direct light along respective sample and reference light paths through the slit. The slit assembly also comprises a rotatable chopper arranged downstream of the slit configured to alternatively occlude the sample light path and the reference light path by rotation of the rotatable chopper.

For example, the means for transporting sample light and reference light may be provided by sample and reference optical fibres, or may be provided by an arrangement comprising one or more optical components, said optical components including mirrors and/or lenses.

According to a third aspect of the invention, a monochromator assembly is provided. The monochromator assembly comprises the monochromator slit assembly according to the second aspect of the invention, a collimating element, and a diffracting element. The collimating element is arranged downstream of the rotatable chopper to direct sample light or reference light from a first position of the slit to the diffracting element. The diffracting element is arranged to reflect light incident on it from the collimating element, back to the collimating element. The collimating element is further configured to direct sample light and reference light from the diffracting element to respective second positions along the slit, the respective second positions being spatially separated from the first positions.

Preferably, the collimating element is arranged downstream of the rotatable chopper.

Accordingly, the collimating element and diffracting element only direct light (sample light of reference light) which is not occluded by the rotatable chopper.

Preferably, the monochromator assembly according the third aspect incorporates the adjustable slit mechanism of the first aspect. Accordingly, the diffracting element may be rotatably adjustable, such that, in combination with the adjustable slit mechanism the monochromator may selectively output sample light or reference light with a bandwidth in the range of 0.1 nm to 2 nm.

Description of the Figures

Embodiments of the present invention will now be described with reference to the accompanying figures in which:

Figure 1 shows schematic diagram of an exemplary double beam spectrometer known in the prior art.

Figure 2 shows a schematic diagram of a fibre optic AAS according to an embodiment of the invention; Figure 3 shows an isometric representation of a lamp holder assembly according to an embodiment of the invention;

Figure 4 shows a further isometric representation of a lamp holder assembly according to an embodiment of the invention;

Figure 5A shows an isometric representation of an optical fibre assembly according to an embodiment of the invention;

Figure 5B shows a top down representation of an optical fibre assembly according to the invention;

Figure 6 shows an isometric view of an adjustable slit mechanism for a

monochromator according to an embodiment of the invention;

Figure 7 shows a plan view of an adjustable slit mechanism for a monochromator according to an embodiment of the invention;

Figure 8 shows a further isometric view of the adjustable slit mechanism shown in Figure 6 with the optical sensor assembly hidden from view;

Figure 9 shows a plan view of the adjustable slit mechanism for a monochromator as shown in Figure 7 with the optical sensor assembly hidden from view;

Figure 10 shows a further isometric view of the mechanism for driving and controlling the rotational position of the slit adjusting cam in which part of the optical sensor assembly and the mounting plate are shown in transparent outline;

Figure 1 1 shows a diagram of an exemplary slit adjusting cam having a variable radius according to an embodiment of this invention;

Figure 12 shows a three-dimensional view of the exemplary slit adjusting cam shown in Figure 1 1 ;

Figure 13 shows a rear plan view of the adjustable slit mechanism according to an embodiment of the invention;

Figures 14a, 14b and 14c show exemplary views (front, side, and sectional view along line H-H respectively) of a sample optical fibre, a reference optical fibre and a combined optical fibre connector;

Figure 15 shows a sectional view along line E-E of Figure 14b;

Figures 16a, 16b and 16c shows detail views C, D, and A of the sample optical fibre, the reference optical fibre and the combined optical fibre connector as indicated in Figures 14a and 14b;

Figure 17 shows a detail view F of the end of the combined optical fibre connector. Detailed Description

This invention relates to atomic absorption spectrometry (AAS), in particular fibre optic AAS. It is not, however, limited to AAS. It is understood that a fibre optic AAS is an AAS in which light is guided between at least some of the optical components of the AAS using optical fibres, for example optical fibres may be used to transport light from one or more light sources to a sample compartment, or transport light from a sample compartment to an entrance aperture of a monochromator.

Nevertheless, the inventive adjustable slit mechanism can be used in any kind of optical apparatus and/or optical instrument, in particular an optical spectrometer, when light of different sources shall be detected. The use is not limited to visible light and can be also used for the electromagnetic radiation of various wavelengths of the electromagnetic spectrum.

This invention also refers to AAS using light sources which emit light. It is understood that use of the word light in this disclosure refers to electromagnetic radiation of any suitable wavelength for use in AAS. As such, it is understood that light, as mentioned in this disclosure is not limited to wavelengths of only visible light (i.e. approximately 400-800 nm) but also includes electromagnetic radiation of other wavelengths, both shorter and longer than visible light. For example, light, as used in this disclosure is intended to include at least ultraviolet light and infrared light. As such, light, as used in this disclosure may include UV-Visible light, which may comprise wavelengths in the range of 200 nm to 1000 nm.

According to an embodiment of the present invention an atomic absorption spectrometer (AAS) 1 is disclosed.

Figure 2 shows a schematic diagram of an AAS 1 according to an embodiment of the present invention. As shown in Figure 2, the AAS 1 comprises at least one hollow cathode lamp 10 housed in lamp holder assembly 20, a broadband light source 12, a fibre optic cable assembly 30, a sample compartment 40, a sample atomiser 50, a sample optical fibre 60, a reference optical fibre 70 and a monochromator assembly 80, and a detector 90. As shown in Figure 2, the fibre optic assembly 30 is connected to the lamp holder assembly, 20, the broadband light source 12 and to focussing elements 32, 34 for the sample compartment 40. The fibre optic assembly 30 is configured to combine and transport light from the lamp holder assembly 20 and the broadband light source 12 along a sample light path and a reference light path. Light from each of the lamp holder assembly and broadband light source is coupled into each of the sample light path and the reference light path. Further details of the fibre optic assembly 30 are provided below.

The fibre optic assembly 30 outputs light on a sample light path and a reference light path to a plurality of first focussing elements 32, 34. As shown in Figure 2, two focusing elements 32, 34 are provided in order to guide sample light on the sample light path and a reference light on a reference light path. The focussing elements 32, 34 are configured to guide light output from the fibre optic assembly 30 into the sample compartment (40).

The sample compartment 40 houses at least one sample atomiser 50. The sample atomiser 50 as shown in Figure 2 is a burner. The burner is configured to atomise a sample contained within it and to expose said atomised sample to the sample light of the sample light path. Accordingly, sample light of the sample light path may be absorbed by the atoms of an atomised sample within the burner for performing an AAS experiment.

The sample compartment 40 is also configured to provide free space for the reference light of the reference light path to travel through the sample compartment without further interference. A suitable sample compartment 40 for housing a sample atomiser 50 and a sample atomiser 50 as part of a double beam spectrometer may be provided by the 100 mm Universal Burner provided as part of the iCE 3300 Double Beam AA Spectrometer offered by Thermo Fisher Scientific Inc. In other embodiments, the sample atomiser 50 may be another type of sample atomiser known in the art, for example a graphite furnace or a vapour generation cell.

As shown in Figure 2, the sample compartment 40 provides an output for the sample light path and the reference light path. A plurality of second focusing elements 36, 38 are positioned at the output to the sample compartment 40 for focusing the light of the sample light path and reference light path into a first end of a sample optical fibre 60 and a first end of a reference optical fibre 70 respectively. The sample optical fibre 60 and the reference optical fibre 70 are provided for transporting light from the sample chamber 40 to the monochromator assembly 80. As shown in Figure 2, the sample optical fibre 60 and the reference optical fibre 70 may be bent in order to provide a more compact arrangement for the AAS 1 . For example, as shown in Figure 2, the direction of travel of the sample light and the reference light runs through 180° through the bend in the optical fibres 60, 70. Providing a change of direction of travel of the sample light and reference light using optical fibres is advantageous as this reduces or avoids using a number of mirrors and lenses which may be sensitive to vibrations and

environmental disturbances.

The sample optical fibre 60 and the reference optical fibre 70 may each be multiple core optical fibres or they may be single core optical fibres. Preferably, the sample optical fibre 60 and the reference optical fibre 70 are optimised to be highly transparent for light with wavelengths typically used for AAS. For example, a preferred wavelength range of around 190-800 nm may be a typical range of wavelengths used in AAS. Preferably, the sample optical fibre 60 and the reference optical fibre 70 are optimised to reduce and/or prevent solarisation.

Preferably, the optical fibres (i.e. optical fibre assembly 30, sample optical fibre 60, reference optical fibre 70) used in this invention utilise fibre optic cores comprising silica. Accordingly, the optical fibre cables of this invention may have a working range of from 190 nm to 1 100 nm. The optical fibres may also be resistant to solarisation, thereby improving the long-term optical performance of the optical fibres. An optical fibre having a core size of at least 550 pm, for example optical fibre CF01493-43 produced by OFS, Georgia, USA, may be a suitable optical fibre for use in this invention.

Second ends of the sample optical fibre 60 and the reference optical fibre 70 are arranged in a fixed positional relationship with an entrance slit to a monochromator assembly 80.

The entrance slit may be provided by adjustable slit assembly 81 (300 in Figs. 6, 8, 9 & 13) as further described below.

The monochromator assembly 80 is configured to filter the sample light and/or the reference light incident on the entrance slit and output a relatively narrow wavelength band of light at an exit slit 89. As such, the function of the monochromator assembly 80 is to select light of a narrow wavelength band. Preferably the selected light comprises light of one specific wavelength which is related to photons absorbed by a specific element contained in the investigated sample. This wavelength is isolated by the monochromator assembly 80. The exit slit 89 is coupled with a detector 90 for detection of the intensity of the light of narrow wavelength band passing the exit slit 89. The monochromator assembly 80 may also include a rotatable chopper 500 within the monochromator assembly 80 for temporally separating the reference light from the sample light as it travels to the detector 90. The rotatable chopper 500 is described in more detail below.

As shown in Figure 2, the monochromator assembly 80 is arranged in an Ebert-type configuration. As such, the skilled person will understand that the monochromator uses a single collimating mirror 82 to direct light incident from the entrance slit to a grating 84. The grating 84 is provided with a blaze angle such that reflected light path is directed to an exit slit 89. In the exemplary embodiment, the grating 84 is configured to diffract the light incident on it, and to reflect the light back on to the single collimating mirror 82, as per the Ebert-type configuration. Light reflected back from the single collimating mirror is focused on an exit slit 89, which is spatially separated from the entrance slit. In the exemplary embodiment shown in Figure 2, the entrance and exit slits to the monochromator are both provided by the separate (spatially separated) sections of the adjustable slit assembly 81 .

In the embodiment shown in Figure 2, the grating is provided with a blaze angle such that the angle of the incident light path and angle of the reflected light path are the same (i.e. a Littrow-type grating). One suitable grating 84 is a diffraction grating supplied by

Richardson Gratings having a line density of 1800 lines/mm. Such gratings are well known in the art, as is the arrangement of such gratings in an Ebert-type monochromator, and so these are not further described herein.

Diffracted light from the grating 84 is reflected back to the collimating mirror 82, which in turn reflects light back towards the exit slit of the adjustable slit assembly 81. The diffraction grating 84 causes a wavelength dependent spread in the light reflected back towards the exit slit 89. The angle of the grating 84 relative to the collimating mirror 82 may be adjusted in order to change the narrow band of wavelength(s) of the diffracted light aligned with the exit slit. As such, the relative angle of the grating 84 may be adjusted in order to provide a narrowband light output at the exit slit wherein the wavelength range may be adjusted. Accordingly, the monochromator assembly 80 may be configured to separate light of a specific wavelength to be measured of the reference light and/or the sample light from light of other wavelengths which may be emitted by other emissions sources such as the flame.

The light exiting from the exit slit of the adjustable slit assembly may be coupled to a detector 90 where it may be converted into a photocurrent and integrated. As shown in Figure 2 a further mirror 86 is provided to couple light into the detector 90. According to the embodiment shown in Figure 2, the detector is a photomultiplier tube. It will be appreciated that other photodetectors, such as an avalanche photodiode, may also be suitable for use as a detector 90.

Although an Ebert-type monochromator assembly 80 is shown in the embodiment of Figure 2, it will be appreciated that the AAS 1 of the present invention is not limited to such a monochromator. For example, in other embodiments, a Czerny-Turner type

monochromator including multiple collimating mirrors may be used. Furthermore, distinct (i.e. separate) adjustable slit assemblies may be used for one or more of the entrance slits and/or exit slits.

Accordingly, the AAS 1 may be used to guide light from the lamp holder assembly 20 or light from the broadband light source 12 to sample atomiser 50 whereby it may interact with an atomised sample. Light travelling though sample atomiser 50 is sample light according to this invention. The intensity of the sample light guided through the sample atomiser 50 is reduced due to the absorption of photons by atoms of the atomised sample. Sample light may be transported by sample optical fibre 60 to the entrance slit provided by adjustable slit assembly 81. Sample light passing through the entrance slit is reflected by the collimating mirror 82 and the grating 84 such it is diffracted and spatially dispersed.

Accordingly, a small (narrow) wavelength band of the sample light may be aligned with the exit slit 89 of the adjustable slit assembly 81 . Accordingly, the monochromator assembly may be used to select a small bandwidth of the sample light to be detected by the detector 90. Similar functionality is provided for the light directed on the reference light path, wherein the reference light is not exposed to the atomised sample. Therefore, the light intensity of the reference light on the reference light path is not reduced due to absorption by elements of the sample. In particular, light of only one specific wavelength may be related to a photon absorption by a specific element of the sample. Next, the lamp holder assembly 20 will be described in detail with reference to Figures 3 and 4.

Figure 3 shows a schematic diagram of a lamp holder assembly 20 according to an embodiment of the present invention. As shown in Figure 3, the lamp holder assembly 20 comprises two hollow cathode lamps 102, 104, a fibre optic cable 106, a focusing element 108, an arm 1 10, a drive shaft 1 12, a first stepper motor 1 14, a counterweight 1 16, a support member 1 18 and a housing assembly 120.

The lamp holder assembly 20 as shown in Figure 3 comprises two hollow cathode lamps 102, 104. Each hollow cathode lamp is mounted on a hollow cathode lamp mount 130a, 130b. Each hollow cathode lamp mount 130a, 130b is configured to provide a power connection for the hollow cathode lamp 102, 104 and to fixedly support the hollow cathode lamp 102, 104 in a generally vertical/generally upright position. As shown in Figure 3, the output window of a hollow cathode lamp 102, 104 is arranged to be orientated generally upwardly such that light is output from the hollow cathode lamp in a generally upward direction with respect to the housing as shown in Figure 3.

The lamp holder assembly 20 is provided with a plurality of lamp holder mounts 130a,

130b, 130c, 130d, 130e, 130f, 130g, 130h, 130i, 130j for supporting a plurality of hollow cathode lamps 102, 104. More specifically, the lamp holder assembly 20 shown in Figure 2 comprises ten lamp holder mounts for ten hollow cathode lamps 102, 104. It will be appreciated that the number of hollow cathode lamps and hollow cathode lamp mounts is not limited to this number and that in other embodiments there may be fewer than 10 or more than 10 hollow cathode lamps and corresponding hollow cathode lamp mounts. As shown in Figure 2, some of the hollow cathode lamp mounts may not be fitted with a hollow cathode lamp. By providing a lamp holder assembly 20 with the option to mount a plurality of hollow cathode lamps 102, 104 it is possible to provide a selection of different hollow cathode lamps for use with an atomic absorption spectrometer 1 . In particular, the hollow cathode lamps mounted in the lamp holder assembly 20 may have different emission profiles such that different wavelengths of light may be provided by the range of hollow cathode lamps for use in one or more AAS experiments. It will be appreciated that hollow cathode lamps and means for mounting hollow cathode lamps are well known in the art and so the hollow cathode lamps 102, 104 and the hollow cathode lamp mounts 130a,

130b are not discussed further herein. It will further be appreciated that any light source configured to produce light with a suitably narrow bandwidth for AAS may be used as an alternative to an HCL lamp, for example electrodeless discharge lamps, or boosted hollow cathode lamps.

As shown in Figure 3, the one end of fibre optic cable 106 is connected to one end of arm 1 10 at a fibre optic mount 122. The end of the fibre optic cable 106 is connected to the fibre optic mount 122 by a fastener 124. The optical fibre 106 is also supported at a point along its length by a support member 1 18. The support member 1 18 provides a support point for the optical fibre 106 at one or more points along the length of the optical fibre, locating the optical fibre 106 relative to the support arm 1 10.

The arm 1 10 is provided to rotatably position the optical fibre 106 and focusing element 108 above one of the hollow cathode lamps 102, 104 which are mounted in the lamp holder assembly 20. As such, the arm 1 10 is rotatable (a rotatable arm) and may be rotated to selectively couple light from one of the hollow cathode lamps 102, 104 into the fibre optic cable 106.

In order to maximise the amount of light focussed into the optical fibre 106 from a hollow cathode lamp 102, 104, a focussing element 108 is provided within the fibre optic mount 122 in a fixed position relationship with the end of the fibre optic cable 106 at the end of the arm 1 10. The optical fibre 106 may be a single core optical fibre or may be a multiple core optical fibre. Optical fibres with multiple cores can be produced to cover a larger area, thereby increasing the total power of light that may be transmitted down the fibre. For example, the optical fibre 106 may have at least five optical fibre cores; more preferably, the optical fibre has at least seven optical fibre cores or at least nine optical fibre cores. As such it will be understood that an optical fibre 106 according to this invention, may be a single optical fibre or a multi-core optical fibre.

The focussing element 108 is configured to couple light which is incident on it over a broad area into a smaller area and at an acceptable angle for transmission down an optical fibre 106 by total internal reflection. The focussing element 108 may be mounted within a hollow portion of the fibre optic mount (not shown). The focussing element 108 may be a lens, or a mirror, or a combination thereof, suitable for focussing light incident on a portion of the focussing element into the optical fibre 106. As shown in the embodiment of Figure 3, the focusing element 108 is preferably a lens only. It is advantageous to use only a lens as the focusing element 108 of the fibre optic mount 122 to focus the light into the optical fibre, in order to provide a robust means for focusing the light consistently into the optical fibre 106. By contrast, focusing elements incorporating a mirror may be more susceptible to variations in alignment over time, which may require repeated calibration in order to compensate for said variation in alignment.

Preferably, the focussing element 108 is a lens which is spaced apart from the end of the optical fibre 106 and held in a fixed position arrangement relative to the end of the optical fibre 106 within the fibre optic mount 122. The focussing element 108 and the end of the optical fibre 106 are arranged on the end of the support arm 1 10 such that the fibre optic mount 122 can be rotatably positioned above the output window of each of the one or more hollow cathode lamps 102, 104. Light output through the window of a hollow cathode lamp 102 may be directed onto the focussing element 108 when the fibre optic mount 122 of the support arm 1 10 is positioned above the hollow cathode lamp 102. The focussing element 108 then focuses said a substantial portion of the light onto the end of the optical fibre 106 such that a substantial portion of the light can be transmitted down the optical fibre by a total internal reflection. As such, the rotatable support arm 1 10 of the lamp holder assembly 20 is configured to selectively couple light from each of the plurality of the hollow cathode lamps 102, 104 into the fibre optic cable 106.

The support arm 1 10 is also attached to a drive shaft 1 12 for varying the rotational position of the support arm 1 10. As such the support arm 1 10 is rotatable about an axis of rotation which is determined by the axis of the drive shaft 1 12. As shown in Figure 3, the drive shaft 1 12, and the axis of rotation, is at least substantially vertical (generally upright), preferably vertical. By substantially vertical, it may be meant a deviation from vertical of less than 5 ⁵ , preferably less than 2-, more preferably less than 1 ⁵ , still more preferably less than 0.5 ⁵ , yet more preferably less than 0.2 ⁵ , even more preferably less than 0.1 ⁵ . By providing the drive shaft 1 12 in this orientation with respect to the housing 120 (and the intended orientation of the lamp holder assembly in use) the drive shaft 1 12 and the axis of rotation therein is not subjected to leverage forces as a result of gravity that may cause the alignment of the axis of rotation by the alignment of the drive shaft 1 12 to vary over time. The stability of the alignment of the axis of rotation of the support arm allows for the support arm to be accurately and consistently positioned above a plurality of different hollow cathode lamps 102, 104 arranged around the axis of rotation. It will be appreciated that variations in the alignment of the focusing element 108 of the fibre optic mount 122 relative to the direction of light output of an output window of a hollow cathode lamp may cause a variation in the intensity of light coupled into the optical fibre from hollow cathode lamps 102, 104 in different positions about the axis of rotation. Preferably, the light output of the hollow cathode lamp is focussed by the focussing element 108 in the direction of the axis of rotation. Therefore, the variation between the relative alignment of the light output of the hollow cathode lamps and the axis of rotation should preferably be less than 2-, more preferably less than 1 ⁵ , still more preferably less than 0.5 ⁵ , yet more preferably less than 0.2 ⁵ , even more preferably less than 0.1 ⁵ .

The support arm is also connected to a counterweight 1 16 at an opposing end of the support arm 1 10 to the fibre optic mount 122. The counterbalance 1 16 is provided at an opposing end of the support arm to the fibre optic mount 122. The counterbalance 1 16 is configured to counteract the leverage force resulting from the weight of the support arm 1 10, the optical fibre 106, the focussing element 108 the fibre optic mount 122 and the support member 1 18 acting on the support arm 1 10 about the drive shaft 1 12 on which the support arm 1 10 is supported. Thus, the counterweight 1 16 ensures that no further leverage forces act on the drive shaft as a result of the weight of the support arm and components attached to it. Thus, the effects of gravity (leverage forces acting on the support arm 1 10 as a result of gravity) over time may be reduced and/or eliminated such that the variation in the alignment of the axis of rotation the support arm over time may be reduced or eliminated.

In order to further increase the accuracy of the rotational position of the support arm 1 10, the support arm 1 10 may also include a positional sensor 126. The positional sensor 126 is configured to detect at least one rotational position of the support arm 1 10 with respect to the lamp holder assembly 20. For example, as shown in Figure 3, the positional sensor 126 comprises an optical sensor mounted on the housing assembly 120, although any sensor suitable for measuring rotational position may be used. The positional sensor 126 detects the presence of a marker 128 on the support arm 1 10 when the support arm is in a specific rotational position (a“home” position). Accordingly, the positional sensor 126 may be used to repeatedly calibrate the position of the support arm 1 10 such that the accuracy of the rotational position of the support arm 1 10 can be maintained.

The lamp holder assembly 20 also comprises a housing assembly 120, for example as shown in Figure 3. The housing 120 comprises an upper support plate 142, a base plate 144 and a connecting portion 146. The connecting portion 146 connects the upper plate 142 to the base plate 144. The drive shaft 1 12 may be supported by the housing assembly 120 at two or more points. As shown in Figure 3, the drive shaft 1 12 is supported at a first upper end by the upper support plate 142 and at a second, lower end by the base plate 144. Accordingly the housing assembly 120 provides at least two fixed support points for the drive shaft in order to ensuring that the axis of rotation of the drive shaft is accurately maintained over time with respect to the housing assembly 120.

Figure 4 shows a further schematic diagram of the lamp holder assembly 20 wherein, compared to the diagram of Figure 3, only one of the plurality of hollow cathode lamps 102 and one of the plurality of lamp holder mounts 130a is shown. Figure 4 shows in more detail the drive mechanism for the support arm 1 10 and the drive shaft 1 12.

The drive mechanism for the rotational movement of the support arm 1 10 is provided by a first stepper motor 1 14. The first stepper motor 1 14 is connected to the support arm and drive shaft via a worm gear 150 and a worm 152. As shown in Figure 4 the worm gear 150 is connected about the drive shaft 1 12 and the worm 152 is fixed to the axle of the first stepper motor 1 14. The worm 152 is rotatably driven through rotation of the axle 154 of the first stepper motor 1 14. Rotation of the worm 152 rotates the worm gear 150 which is attached to the driveshaft 1 12 in order to transfer the rotational movement from the axis of the first stepper motor 1 14 to the axis of the drive shaft 1 12. By using a first stepper motor 1 14 to drive the movement of the support arm 1 10, the detent torque of the first stepper motor 1 14 can be used to ensure that the support arm is maintained in a fixed position when the first stepper motor 1 14 is not being driven. Thus, the rotational position of the support arm 1 10 may be robustly maintained by the first stepper motor 1 14 in a simple manner without requiring further mechanical components.

As shown in the embodiment in Figs. 3 and 4, the first stepper motor 1 14 is mounted on a mounting plate 156 through which the axle 154 of the first stepper motor 1 14 extends. The mounting plate 156 is in turn mounted on the base plate 144 by fasteners to provide a secure attachment point for the first stepper motor 1 14. It will be appreciated that the worm 152 and worm gear 150 arrangement for connecting the first stepper motor 1 14 to the drive shaft 1 12 is only one possible arrangement for driving the drive shaft 1 12 with the first stepper motor 1 14. The skilled person will appreciate that other drives (driving means), in particular gear drives (geared driving means) may also be suitable. The skilled person will also appreciate that the orientation of the axle of the stepper motor and/or mounting plate 156 is not limited to any particular orientation, and as such the arrangement of the stepper motor may be varied within the housing.

As shown in Figure 4, the base plate 144 includes a plurality of attachment points 148 for attaching lamp holder mounts 130a, 130b, 130c etc. to the base plate. Similar attachment points are arranged about the axis of rotation of the drive shaft at a fixed radial distance. Accordingly, it will be understood that the plurality of hollow cathode lamps 102, 104 will be arranged about the axis of rotation at a fixed radial distance when the lamp holder assembly 20 is fully assembled. By providing the plurality of hollow cathode lamps 102,

104 about the axis of rotation at a fixed radial distance, the fibre optic mount 122 of the support arm 1 10 can be accurately positioned above each of the hollow cathode lamps 102 104 in the same relative position.

Preferably the housing for the lamp holder assembly 20 is constructed from aluminium, for example, sheet aluminium. The sheet aluminium provides a lightweight yet rigid structure for fixing the alignment of the drive shaft. It will be appreciated that the housing assembly 120 is shaped to provide two fixing points for supporting the drive shaft at opposing distal ends. Accordingly, various modifications of the base plate 144 connecting member 146 and upper plate 142 are envisaged by this invention which may also provide the necessary fixing points for the drive shaft 1 12 and support arm 1 10.

The lamp holder assembly may also comprise a controller (not shown) configured to control the operation of the first stepper motor 1 14 and the positional sensor 126 in order to position the arm above one of the hollow cathode lamps. The controller may also control the operation of the hollow cathode lamps. The functionality of the controller for the lamp holder assembly may be incorporated into a controller for an AAS.

As described above, the lamp support arm 1 10 is rotatable about the drive shaft 1 12 such that the fibre optic mount 122 can be positioned above (or relative to one of the plurality of hollow cathode lamps 102, 104 in order to couple light from the one hollow cathode lamp 102 into the fibre optic cable 106. The mounts for the hollow cathode lamps 130a, 130b, 130c are arranged in a circular pattern about the drive shaft, corresponding to the path of rotation of the end of the support arm about the drive shaft. This allows the end of the support arm to be positioned above each of the hollow cathode lamps mounts 130a, 130b, 130c. Thus, light from a lamp mounted on said hollow cathode lamp mount positioned below the end of the support arm may be coupled into the optical fibre 106. Accordingly, rotation of the support arm 1 10 can be used to select one of the hollow cathode lamps for use in an AAS experiment. Light from the selected hollow cathode lamp 102, 104 is focussed into the optical fibre whereas the other lamps remain unused. Preferably, the entire lamp holder assembly is encased in a further common housing (not shown) to prevent ambient light from unintentionally being focussed into the optical fibre thereby reducing the accuracy of the experiment.

Next, a method of operating the lamp holder assembly 20 will be described with reference to Figures 2, 3 and 4.

In a particularly preferred embodiment of the method for operating the lamp holder assembly 20, the support arm 1 10 is first sent to a home position to calibrate a rotational position of the support arm 1 10. A home position of the support arm 1 10 may be set with reference to the positional sensor 126 of which is configured to detect a fixed rotational position of the support arm 1 10.

Next, a user may select a desired hollow cathode lamp 102, 104 for use by specifying the lamp holder mount position to be used. The desired lamp holder mount position may be selected by a user via a user interface to the controller. From the home position, the controller (not shown) is configured to drive the first stepper motor 1 14 a predetermined number of steps to rotational position corresponding to a desired hollow cathode lamp mount position. Optionally, once the support arm 1 10 has reached the desired

predetermined position, the rotational position of the support arm 1 10 may be further calibrated to find the optimal rotational position for that hollow cathode lamp using a calibration routine.

For the calibration routine, the fine step adjustments of the first stepper motor 1 14 can be used to provide relatively small angle rotational adjustments via the gearing on the worm 152 and the worm gear 150. Variations in the intensity of the light (for example of light of a specific wavelength emitted by the HCL lamp) coupled into the optical fibre 106 as a result of the adjustments may be detected by a detector 90 of an AAS system 1 . Intensity measurements may be performed by measuring the light intensity as reference light in the AAS 1 . By taking a plurality of such intensity measurements at different angular positions, an optimal rotational position for the support arm relative to the hollow cathode lamp may be determined. Thus, the predetermined number of steps to the desired rotational position may be updated based on the optimal position determined by the calibration routine.

The optimal rotational position of the support arm 1 10 may be stored by the controller for the selected hollow cathode lamp 102. Thus, the support arm 1 10 can return to the optimal position directly when said lamp is reselected by a user of the lamp holder assembly 20. The calibration method described above may be repeated for one or more of the hollow cathode lamps 102, 104 in the lamp holder assembly 20. For example, a calibration program may be performed upon start-up of the AAS 1 in which the optimal positions of the support arm for each of the hollow cathode lamps 102, 104 in the lamp holder assembly 20 is recorded. Alternatively, the optimal position for a hollow cathode lamp may be determined the first time it is used after start-up of the AAS 1.

Thus, during an AAS experiment the support arm may quickly select a hollow cathode lamp 102 and move to the optimal position for said hollow cathode lamp without further calibration. Preferably, the support arm may move to the home position between each adjustment of the position of the support arm 106 such that the number of steps to the desired optical position is always counted from the home position to ensure that the support arm precisely moves to the desired location. Accordingly, the support arm may be robustly and accurately positioned in an optimal position for any hollow cathode lamp of the lamp holder assembly 20.

As discussed above, a fibre optic assembly 30 for a fibre optic AAS 1 is provided according to an embodiment of this invention. The fibre optic assembly 30 is configured to provide a fibre optic light path between the lamp holder assembly 20, the broadband light source 12 and the focussing elements 32, 34 for the sample compartment 40. The fibre optic assembly 30 is configured to couple light from the lamp holder assembly 20 (first light source) to the focusing elements 32, 34 to form a sample light path and a reference light path. The fibre optic assembly is also configured to couple light from the broadband light source 12 (second light source) to the focusing elements 32, 34 along the sample light path and the reference light path. As such, light from each of the lamp holder assembly 20 and broadband light source 12 is coupled into each of the sample light path and the reference light path. Figure 5A shows an isometric diagram of a fibre optic assembly 200 according to an embodiment of this invention. Figure 5B shows a further top-down view of the fibre optic assembly 200. The fibre optic assembly 200 comprises a first light source input 202 and a second light source input 204 and a sample light output 206 and a reference light output 208. The first light source input 202 is connected to the sample light output by 206 a first sample optical fibre 210. The first light source input 202 is connected to the reference light output 208 by a first reference optical fibre 212. The second light source input 204 is connected to the sample light output 206 by a second sample optical fibre 214. The second light source input 204 is connected to the reference light output 208 by a second reference optical fibre 216.

The first light source input 202 comprises a connecting element 220 for connecting the sample light input 202 to a first light source. For example, the connecting element may be a fastener such as the fastener 122 configured to connect the optical fibre 106 to the lamp holder assembly as described above. The first light source input 202 also comprises a first coupling assembly 222 configured to couple light incident on the first light source input 202 into the first sample optical fibre 210 and the first reference optical fibre 212.

The coupling assembly 222 comprises a plurality of optical cores 224 (224a, 224b, 224c, 224d, 224e, 224f, 224g) disposed at the end of the coupling assembly 222 which is to be exposed to light from the first light source. The optical cores are typically provided as UV resistant optical cores as are known in the art for use in optical fibres. Accordingly, the plurality of optical cores are arranged to be able to couple light from the first light source into the coupling assembly 222 by total internal reflection. The coupling assembly 222 also includes a splitting element 226 configured to split the light coupled in the plurality of optical cores 224 into the first sample optical fibre 210 and the first reference optical fibre 212. Preferably, the splitting element 226 couples light equally into the first sample optical fibre 210 and the first reference optical fibre 212, but other ratios of light coupling may be provided. For example, it may be advantageous to couple a greater portion of light into the first sample optical fibre 210 for use in the sample light path.

The coupling assembly 222 may be housed in a protective covering 228 to connect and hold all the elements in place and prevent outside radiation from being coupled into the optical fibres. Similarly, the second light source input 204 also comprises a connecting element 220a for connecting the second light input 204 to a second light source, a coupling assembly 222a (including splitting element 226a), and a protective housing 228a. The coupling assembly 222a in the second light source input 204 is configured to couple light incident on the second light source input 204 into the second sample optical fibre 214 and the second reference optical fibre 216. As such, coupling assembly 222a may be substantially the same as coupling assembly 222 for the first light source input 202.

Each of the first sample optical fibre 210, first reference optical fibre 212, second sample optical fibre 214, and reference optical fibre 216 may be provided as a U.V resistant optical fibre. As such, it is understood that the optical fibre is configured to be resistant to solarisation. Each of the optical fibres 210, 212, 214, 216 may be provided as a single core optical fibre, or more preferably, a multicore optical fibre. Preferably, each of the optical cores has a diameter of 0.55 mm. In one embodiment, the multicore optical fibre comprises 7 optical fibre cores, each having a diameter of 0.55 mm, which is preferable to allow the optical fibres to be suitably flexible (i.e. capable of having a bend radius of less than 15 cm without a risk of micro-fractures). Of course, it will be appreciated that optical fibres with a different number of cores, or different core diameters may be equally suitable for use in the optical fibre assembly 200.

In the embodiment shown in Figures 5A and 5B, the first sample optical fibre 210 and the first reference optical fibre 212 may each have a length of about 450 mm, while the second sample optical fibre 214 and the second reference optical fibre 216 may each have a length of 250 mm.

The sample light output 206 is configured to output light coupled from the first sample optical fibre 210 and the second sample optical fibre 214. The sample light output 206 may comprises a connecting element 220b for connecting the output to a focusing element 32. By using a connecting element 220b, the end of the sample light output 206 may be held in a fixed positional arrangement with the focusing element 32, thereby providing a robust arrangement for the guidance of the sample light path. In order to couple light from the first sample optical fibre 210 and the second sample optical fibre 214 to the output, a coupling assembly 222b may be provided, substantially as described above, wherein light travels from first and second sample optical fibres 210, 214 to the sample light output 206. In contrast to the splitting element 226, the combining element 226b is configured to combine light from the first sample optical fibre 210 and the second sample optical fibre 214 into a plurality of optical cores 224. The plurality of optical cores 224 then couple light to the end of the sample light output 206.

Similarly, the reference light output 208 also comprises a connecting element 220c for connecting the reference light output 208 to a focusing element 34, a coupling assembly 222c and a protective housing 228c. The coupling assembly 222c in reference light output 208 is configured to couple and combine light from the first reference optical fibre 212 and the second reference optical fibre 216 in a similar manner to the sample light output 206 as described above.

It will be appreciated the optical fibres 210, 212, 214, 216 of the optical fibre assembly 200 shown in Figures 5A and 5B are not limited to the lengths or dimensions discussed above. Indeed, the lengths of the respective fibres, and the relative alignments of the first light source input 202, second light source input 204, sample light output 206 and reference light output 208 may be varied, not least due to the inherent flexibility of the optical fibres.

Accordingly, the optical fibre assembly 30, 200 according to this invention may be a robust means for providing a sample light path and reference light path for a fibre optic AAS system incorporating secondary light source.

According to a further embodiment of the present invention, Figure 6 shows an isometric view of an adjustable slit mechanism 300 for a monochromator. For example, the embodiment of the adjustable slit mechanism 300 shown in Figure 6 may provide the adjustable slit assembly 81 of the monochromator assembly 80 as shown in Figure 2. As such, the adjustable slit assembly 81 is arranged to receive sample light and reference light from the sample optical fibre 60 and reference optical fibre 70 respectively.

The adjustable slit mechanism comprises a first slit member 302 and a second slit member 304, a slit adjusting cam 306, a first arm 308, a second arm 310, and an optical sensor assembly 31 1 .

The first slit member 302 provides a first slit edge 312 for the adjustable slit mechanism 300. The second slit member 304 provides a second slit edge 314 for the adjustable slit mechanism 300. The second slit member 304 is arranged opposite the first slit member 302 such that the first and second slit edge 312, 314 oppose each other to define slit for a monochromator. The first and second slit members 302, 304 are configured to provide the first and second slit edges 312, 314 as straight edges suitable for defining a precise slit width of the monochromator.

The first slit member 302 is mounted on the first arm 308. As shown in Figure 7, the first slit member 302 is mounted on the first arm 308 by a plurality (two) of fasteners 316. By mounting the first slit member 302 on the first arm 308 using a plurality of fasteners 316, the first slit member 302 may be mounted in a precise location on the first arm 308.

Accordingly, the location (alignment) of the first slit edge 312 relative to the first arm 308 may be precisely controlled. Similarly, the second slit member is 314 mounted on the second arm 310 by a plurality of fasteners 316.

To provide the first and second slit edges 312, 314 of the first and second slit members 302, 304, the first and second slit members 302, 304 are formed using a (relatively) high accuracy shaping process. In this context, a high accuracy shaping process is considered to be a shaping process configured to form shapes having an accuracy of ±0.05 mm or less. Preferably, the chemical etching process has an accuracy of ± 0.025 mm. In one preferred embodiment, the first and second slit members are formed from spring steel and further shaped using a chemical etching process. In the chemical etching process, a photo resist mask is applied to the part to be etched in a desired pattern, followed by a

subsequent exposure of the un-masked regions of the part to a chemical etchant.

Chemical etching processes for spring steel are known in the art, and so art not described in further detail herein. As the first and second slit members 302, 304 are formed separately from the first and second arms 308, 310, the size of the part to be chemically etched is reduced. Of course, the skilled person is aware of alternative high accuracy shaping processes that may also be used to form the first and second slit members, such as laser machining or micro-polishing.

The first arm 308 is shown in more detail in Figure 8, which shows a further isometric view of the adjustable slit mechanism 300 with the optical sensor assembly 31 1 hidden from view. Figure 9 also shows a plan view of the adjustable slit mechanism 300 with the optical sensor assembly 31 1 hidden from view. As shown in Figures 8 and 9, the first arm 308 comprises a first mounting portion 320, a first aligning portion 322, and a first cam- engaging portion 324. The first mounting portion 320 of the first arm is configured to provide mounting locations for attaching the fasteners 316 in order to mount and precisely locate the first slit member 302 on the first arm 308. The first aligning portion 322 of the first arm 308 is connected to the first mounting portion 320 at one end of the first aligning portion and extends away from the mounting portion. The first aligning portion 322 of the first arm 308 provides a means for adjusting the position of the first slit member 302 and aligning the direction of adjustment of the first slit member 302. The first aligning portion 322 includes one or more straight edges which are provided to align the first arm as discussed in more detail below. The first cam-engaging portion 324 is connected to the first aligning portion 322 at its end opposing the first mounting portion 320. The first cam-engaging portion 324 is configured to provide a surface for engaging with the slit adjusting cam 306. As shown in Figure 9, it is preferable for the cam-engaging portion 324 to include a plain bearing material 326 mounted upon it to provide the surface for engaging with the slit adjusting cam 306. In the shown embodiments, a ring of plain bearing material is arranged on the cam engaging portion 324. The ring of plain bearing material may be fixed by a screw 327 or may be pivoted by the screw 327 on the cam engaging portion 324. The surface for engaging with the slit adjusting cam 306 is discussed in more detail below.

As shown in the embodiment of Figure 8, the combination of the first aligning portion 322 and the first cam engaging portion 324 provides a section of the first arm 308 which is substantially L-shaped. The combination of the first mounting portion 320, the first aligning portion 322 and the first cam engaging portion 324 provides a first arm 308 which is substantially U-shaped. Of course, it will be appreciated that the shape of the first mounting portion 320, the first aligning portion 322 and/or the first cam-engaging portion 324 of the first arm 308 may be varied, and so the present invention is not limited to these shapes as shown in the embodiment in Figure 8. For example, the nature of the mounting and fasteners may be varied as is known in the art, and so the shape of the first mounting portion 320 may be varied accordingly. Further, the shape of the cam-engaging portion 324 may be varied depending on the shape and relative position of the slit-adjusting cam 306.

Similarly, the second arm 310 is shown in Figures 8 and 9. The second arm 310 comprises a second mounting portion 328, a second aligning portion 330 and a second cam-engaging portion 332 which are connected in a similar U-shape as the first arm 308. As such, the second arm 310 has features similar to the feature of the first arm 310 as discussed above. As for the first arm 308, the shape of the second arm 310 is not limited to the shapes as shown in the embodiment in Figure 8.

It will be appreciated from the embodiment shown in Figure 8 that the second cam- engaging portion 332 and the first cam engaging portion 324 each extend in a direction which is aligned with the first and second slit edges 312, 314. As such, the surfaces of the first and second cam engaging portions 324, 332 which engage with the slit adjusting cam 306 may be parallel to the first and second slit edges 312, 314. In the case where the surfaces of the first and second cam engaging portions 324, 332 which engage with the slit adjusting cam 306 are provided by a plain bearing material 326, it will be appreciated that the point on the plain bearing material 326 which contacts the slit adjusting cam 306 can be considered to be arranged tangentially to the slit adjusting cam 306. The plane formed by such a tangent will be substantially parallel to the first and second slit edges 312, 314. As such, it will be appreciated that the separation of the first and second cam engaging portions 324, 332 is adjustable in a direction substantially normal to the first and second slit edges 312, 314.

The first and second arms 308, 310 may each be formed from stainless steel. As such, each of the first and second mounting portions 320, 328, the first and second aligning portions 322, 330, and the first and second cam engaging portions 324, 332 may each be formed from stainless steel. Preferably, the stainless steel used is SAE grade 316 stainless steel.

The adjustable slit mechanism 300 also comprises a mounting plate 340 on which the adjustable slit mechanism 300 is mounted. The first arm 308 and the second arm 310 are mounted on the mounting plate 340 by a plurality of locating washers (guide members)

342. Each of the first arm 308 and the second arm 310 are mounted on the mounting plate 340 using at least two locating washers 342.

The mounting plate 340 on which the first and second arms 308, 310 are mounted is manufactured from a stainless-steel component. The mounting plate 340 comprises a number of threaded holes for attaching the locating washers 342 to the mounting plate 340. The mounting plate 340 also comprises an aperture for mounting a second stepper motor 360. The various threaded holes for locating the locating washers 342 and the hole for mounting the second stepper motor 360 are manufactured using a high precision jig in order to provide accurate location of the parts. Of course, it will be appreciated that other relatively high accuracy machining techniques may also be used such as laser processing.

As shown in Figures 8 and 9 the first arm 308 is mounted on the mounting plate 340 using two locating washers 342. The locating washers 342 are arranged on the mounting plate 340 to engage with the first aligning portion 322 of the first arm 308. The locating washers 342 are each mounted on the mounting plate 340 by a fastener. The fastener is preferably a spring-loaded bolt. The spring-loaded bolt is connected to the mounting plate 340 through a threaded hole in the mounting plate 340 at the desired location for the locating washer 342. The locating washer 342 is then mounted about the bolt thread and a spring applies tension between the bolt head and the locating washer 342 to locate the washer against the mounting plate 340.

The locating washers 342 each comprise an arm retaining portion 344 and an arm aligning portion 346. The arm retaining portion 344 is configured to retain the arm it is in contact with against the mounting plate 340. The arm aligning portion 346 is configured to provide alignment for the arm it is in contact with such that the first slit edge 312 is aligned with the second slit edge 314. Further, the arm aligning portions 346 of the fasteners are aligned with each other such that the relative alignment of the first slit edge 312 and the second slit edge 314 is maintained as the width of the adjustable slit is adjusted. Of course, it will be appreciated that the guide members of the present invention are not limited to locating washers 342, and guide members may be provided in a range of different shapes and sizes for locating and guiding each of the first and second arms 308, 310 on the mounting plate 340.

As shown in Figures 8 and 9, the arm aligning portion 346 of the locating washer 342 is provided as a circular washer of a first diameter which is provided to engage with an edge of the first arm or the second arm to align and locate that edge against the washer. It is preferably that the thickness of the arm aligning portion 346 is the same as the thickness of the first arm 308 and/or the second arm 310. The arm retaining portion 344 is connected to the arm aligning portion 346 and has a second diameter which is larger than the first diameter of the arm aligning portion 346 such that the arm retaining portion 344 overlaps (in the plan view shown in Figure 9) the respective arm so as to retain and locate the respective arm against the mounting plate 340. As such the locating washers 342 are configured to locate the first arm 308 or the second arm 310 with respect to the mounting plate 340. The spring of the spring-loaded bolt tensions the locating washers 342 against the mounting plate 340 such that the locating washers and the respective first and second arms 308, 310 are held securely in place.

As shown in Figure 9 the first arm 308 is mounted on the mounting plate 340 by two locating washers 342. The locating washers 342 are arranged to engage with an edge of the first aligning portion 322. The locating washers 342 are arranged on a lower edge of the first aligning portion 322 such that the locating washers 342 support the first arm 308 when the first aligning assembly is orientated in its intended upright orientation. The two locating washers 342 of the first arm are also positioned such that in combination with the substantially straight edge of the first aligning portion 322 the first arm 308 is free to move in a substantially horizontal direction. Further details of the movement of the first arm 308 will be discussed below.

The second arm 310 is mounted on the mounting plate 340 through the use of four locating washers 342. As shown in Figure 9, three locating washers 342 are arranged to contact a substantially straight edge of the second aligning portion 330 of the second arm 310. A fourth locating washer 342 is provided at a lower end of second cam engaging portion 332 to provide vertical support for the second arm 310 to subsequently prevent rotational movement of the second arm 310 about the locating washer 342 in contact with the second cam engaging portion 332. As such the locating washers 342 are configured to locate and align the second arm 310 on the mounting plate 340.

In the embodiment shown in Figures 6 through 10 the first arm 308 and the second arm 310 are mounted on the mounting plate 340 using locating washers 342 such that there is a small vertical separation between the first aligning portion 322 and the second aligning portion 330 (not shown in the figures). No contact between said aligning portions is required because the locating washers 342 are able to accurately locate each of the first arm 308 and the second arm 310 of the mounting plate 340 without any further support. Of course, in other embodiments it will be appreciated that the two arms may be provided in contact in order to provide further alignment. Lubrication between said components may be provided in order to reduce frictional forces in such an embodiment. A plain bearing material may be provided at a contact area of at least one of the first and second aligning portions 322, 330 to reduce friction. In order to further locate the first arm 308 and the second arm 310, the first arm 308 and the second arm 310 are each resiliently biased against the slit adjusting cam 306. As shown in Figure 9, the first cam engaging portion 324 is resiliently biased against the slit adjusting cam 306 at a first point on the circumference of the slit adjusting cam 306, and the second cam engaging portion 332 is resiliently biased against the slit adjusting cam 306 at a second point on the circumference of the slit adjusting cam 306. As shown in the embodiment in Figure 9, the first point is opposite the second point. As such, the first arm 308 is resiliently biased against the slit adjusting cam 306 at a point opposite the point at which the second arm 310 is resiliently biased against the slit adjusting cam 306 (i.e. on opposing sides of the slit adjusting cam). The first arm 308 and the second arm 310 are each resiliently biased against the slit adjusting cam 306 by a first elastic element 350 and a second elastic element 352. Preferably, the first elastic element 350 and the second elastic element 352 are each connected to the first arm 308 and the second arm 310, and may be arranged between the two arms 308, 310. The first and second elastic elements 350, 352 are spaced apart from each other on the first and second arms 308, 310. The elastic elements 350, 352 are configured to resiliently bias the first arm 308 and the second arm 310 towards the slit adjusting cam 306. In an alternative embodiment, each elastic element may be connected to and arranged between the mounting plate 340 and the first arm 308 or the second arm 310.

As shown in Figure 9, the first elastic element 350 is connected to the first cam engaging portion 324 of the first arm 308 at one end of the elastic element 350. The other opposing end of the first elastic element 350 is connected to the second cam engaging portion 332 of the second arm 310. Similarly, the second elastic element 352 is connected at opposing ends to the first cam engaging portion 324 and the second cam engaging portion 332. The first and second elastic elements 350, 352 are spaced apart along the respective lengths of the first and second cam engaging portions 324, 332. The first and second elastic elements 350, 352 are preferably spaced apart on either side of the first and second points at which the cam engaging portions 324, 332 contact the slit adjusting cam 306.

Accordingly, the first elastic element 350 and the second elastic element 352 are connected to the first arm 308 and the second arm 310 such that the slit adjusting cam 306 (adjustably) tensions the first elastic element 350 and the second elastic element 352 by separating the first arm 308 from the second arm 310. Accordingly, the first elastic element 350 and the second elastic element 352 resiliently bias the first arm 308 towards the second arm 310 such that both arms remain in contact with the slit adjusting cam 306.

Thus, the slit width of the adjustable slit mechanism may be precisely controlled by the slit adjusting cam 306.

As shown in the embodiment of Figure 9, the first elastic element 350 and the second elastic element 352 are springs (helical springs). The springs are each attached to the first arm 308 and the second arm 310 by fasteners. Of course it will be appreciated that the first and second elastic elements 350 and 352 are not limited to helical springs, and that any other type of elastic element or equivalent thereof suitable for resiliently biasing the first arm 308 and the second arm 310 towards each other known to the skilled person would be suitable.

The first arm 308 and the second arm 310 may be manufactured from for example stainless steel other suitable materials similar to stainless steel are well known to the skilled person. The first arm 308 and the second arm 310 may be manufactured into their desired shapes using milling and grinding processes. Such known processes may provide parts with the desired shape and smoothness to provide suitable alignment surfaces as discussed above.

As discussed above, the slit adjusting cam 306 is located between the first cam engaging portion 324 of the first arm and the second cam engaging portion 332 of the second arm 310. The slit adjusting cam 306 is a mounted on an axle which is connected to a step motor 360 such that the slit adjusting cam is rotatable about an axis of rotation. The slit adjusting cam 306 has a variable radius about its axis of rotation. The variable radius of the slit adjusting cam is provided in order to provide a means for adjusting the separation of the first cam engaging portion 324 of the first arm 308 from the second cam engaging portion 332 of the second arm 308. With reference to Figures 8 and 9, it will be appreciated that changing the relative separation of the separation of the first cam engaging portion 324 from the second cam engaging portion 332 causes a corresponding change in the separation of the first slit edge 312 from the second slit edge 314. As such, rotation of the slit adjusting cam 306 causes and (adjustable) separation of the first slit edge 312 from the second slit edge 314.

Preferably, the variable radius of the slit adjusting cam 306 is provided such that for at least one rotational position of the slit adjusting cam 306, the diameter of the slit adjusting cam 306 is such that the separation between the first arm 308 and the second arm 310 provided by the slit adjusting cam 306 is sufficient to allow the first slit edge 312 to be in contact with the second slit edge 314. As such the variable radius of the slit adjusting cam 306 about its circumference includes sections configured to allow the adjustable slit of the

monochromator to be fully closed. Such sections preferably may be the minimum radius of the variable radius of the slit adjusting cam 306. The slit adjusting cam 306 further includes one or more sections about the circumference in which the variable radius increases such that rotation of the slit adjusting cam 306 increases the separation of the first arm 308 from the second arm 310. Accordingly, rotating the slit adjusting cam 306 adjustably separates the first slit edge 312 from the second slit edge 314. Preferably the variable radius of the slit adjusting cam 306 is provided such that at the first point at which it contacts the first cam engaging portion 324, the radius is the same as the radius for the slit adjusting cam 306 at the second point at which it contacts the second cam engaging portion 332. As such, the slit adjusting cam 306 equally separates the first arm 308 and the second arm 310 in opposing directions (relative to its axis of rotation). Advantageously, by equally separating the first arm 308 and the second arm 310 the central location of the adjustable slit remains constant. By providing a mechanism in which the centre of the adjustable slit remains constant, incident light on said adjustable slit can be aligned with the centre of the adjustable slit for one slit width and no further alignment is necessary as the central location of the slit will remain constant. Accordingly, a robust adjustable slit mechanism for a monochromator may be provided.

As shown in Figure 8, it is preferable that the slit adjusting cam 306 engages with the first arm 308 and the second arm 310 via rings 326, 326’. Preferably, the rings 326, 326’ are made of a plain bearing material, or at least the outer surface of the rings 326, 326’ consist of a plain bearing material. The rings 326, 326’ are mounted on each of the first arm 308 and the second arm 310 at respective cam engaging portions 324, 332. According to the embodiment shown in Figure 8, it is preferable that the rings 326, 326’ are mounted on the first and second arms 308, 310 by fasteners 327, 327’. The rings 326, 326’ provide contact points for the slit adjusting cam 306 which reduce the friction between the rotating slit adjusting cam 306 and the first and second arms 308, 310, in particular if their surface consists of a plain bearing material. In an alternative embodiment, the slit adjusting cam may directly contact the first arm 308 and the second arm 310. It is advantageous to use rings 326, 326’ to reduce the friction caused by the rotation of the slit adjusting cam 306 in order to reduce wear of the slit adjusting cam 306. Figure 10 shows a further isometric view of the mechanism for driving and controlling the rotational position of the slit adjusting cam 306. In the sectional view of Figure 10, part of the optical sensor assembly 31 l and the mounting plate 340 are shown in transparent outline to show more detailed view of the adjustable slit mechanism. Optical sensor assembly 31 1 includes an optical disc 370, and an optical sensor 372. The optical disc 370 includes one or more gradations configured to provide an indication of the rotational position of the optical disc. The optical sensor 372 is configured to sense the one or more gradations on the optical disc 370 in order to output information regarding the rotational position of the slit adjusting cam 306. By sensing the rotational position of the slit adjusting cam 306 using the optical sensor assembly 370 the second stepper motor 360 can control the rotational position of the slit adjusting cam 306 through a precise number of controlled steps such that the slit width of the adjustable slit mechanism 300 may be precisely controlled. A controller is provided (not shown) which is configured to utilise information provided by the optical sensor assembly 31 1 to control the second stepper motor 360 to provide a desired rotational position of the slit adjusting cam 306 in response to a demanded slit width by a user. Using a second stepper motor 360 to rotate the slit adjusting cam 306 means that the detent torque of the stepper motor ensures that slit adjusting cam 306 remains in a fixed rotational position when the second stepper motor 360 is not driven. Accordingly, the adjustable slit mechanism 300 may be robust and resistant to vibrations.

Figure 1 1 shows a diagram of an exemplary slit adjusting cam 306 having a variable radius according to an embodiment of this invention. Figure 12 shows a three-dimensional view of the exemplary slit adjusting cam shown 306 in Figure 1 1. The exemplary slit adjusting cam 306 has an inner diameter 380 for mounting on the axle of the second stepper motor 360.

In the exemplary embodiment the inner diameter has a dimension of 8mm. The slit adjusting cam 306 also has an outer diameter 382 which has a variable radius about its circumference. As shown in Figure 1 1 , the dimension of the variable radius increases from a minimum at a first point at a nominal 0° about the circumference to a maximum at a second point 160° around the circumference from the nominal 0° point. This variable radial profile around the circumference is repeated from a point 180° about the circumference through to a point 360° about the circumference. As such, in the preferred embodiment, the radius at any given point of the circumference of the slit adjusting cam 306 is the same as the radius of an opposing point of the circumference of the slit adjusting cam (i.e. a point on the opposite side of the central axis). Thus, the slit adjusting cam has a variable radius wherein the diameter (variable diameter) of the slit adjusting cam has a midpoint which is aligned with the axis of rotation of the cam about its circumference. In the exemplary embodiment, the variable radius increases from a minimum of 7.5mm at a nominal 0° point at a rate of 1 pm per degree. So, after 10° of rotation the radius of the slit adjusting cam 306 has increased by 10 pm. Due to the variable radial profile being repeated 180° around the circumference, the diameter of the slit adjusting cam 306 will have increased by 20 pm after 10° of rotation. After 160° of rotation, such that the variable radial profile reaches its maximal point, the diameter of the slit adjusting cam will have increased by 320 pm.

As described above, the slit adjusting cam 306 shown in Figure 1 1 is configured to provide an adjustable slit width for the adjustable slit mechanism from 0 pm (i.e. slit completely closed) to 320 pm. It should be noted that in Figures 1 1 and 12, the base diameter of the outer diameter is indicated only to provide a clearer indication of the variable profile, and that in practice this base circumference is not necessarily distinguishable in the finished cam 306. It will also be appreciated that the variable radial profile shown in Figure 1 1 is merely exemplary, and that the profile may be adapted depending on the desired range of slit widths required for the adjustable slit mechanism of the monochromator.

For example, the slit adjusting cam 306 may have a diameter which increases from a minimum diameter to a maximal diameter by at least 2mm. Accordingly, a slit adjusting cam may be provided which is configured to provide an adjustable slit width ranging from at least 0.025mm to no greater than 2mm, or more preferably an adjustable slit width ranging from, for example, at least 0.025 mm to no greater than 0.5 mm.

The slit adjusting cam 306 according to the exemplary embodiment shown in Figures 1 1 and 12 is manufactured from spring steel. More specifically, the slit adjusting cam 306 may be made from Oil Hardened Non-Shrinking (OHNS) steel. The slit adjusting cam 306 is first cut to a broad outer diameter and desired inner diameter 380 using standard machining techniques. The variable radial profile of the slit adjusting cam 306 is then cut from the broader outer diameter using a wire cutting machine. The wire cut slit adjusting cam 306 is then hardened using heat treatment process, e.g. an oil hardening process. Preferably, the slit adjusting cam is hardened to at least 30 HRC (Rockwell Hardness), more preferably at least, 35 HRC, more preferably at least 40 HRC, and most preferably at least 45 HRC. No further machining or polishing of the part is required as the wire cutting process produces a sufficiently accurate and smooth finish to the variable radial profile of the cam. A press-fit bush may be mounted in the internal diameter of the slit adjusting cam 306 in order to adapt the slit adjusting cam 306 for mounting on an axle of the second stepper motor 360. Of course, other high precision machining techniques for producing a cam having a desired radial profile are known to the skilled person.

Figure 13 shows a rear plan view of the exemplary embodiment of the adjustable slit mechanism 300. As shown in Figure 13, the adjustable slit mechanism 300 provides both the entrance slit 88 and exit slit 89 of a monochromator assembly 80 (see also Fig. 2, where the entrance slit 88 is provided by the adjustable slit assembly 81 ). To further define the entrance and exit slits 88, 89, the mounting plate 340 further comprises an entrance aperture 390 for sample light and reference light to pass through the mounting plate 340 and an exit aperture 392 for sample light to pass through the mounting plate 340 in the opposite direction. As shown in Figure 13, the first and second slit edges 312, 314 of the adjustable slit mechanism are visible through each of the entrance aperture 390 and the exit aperture 392.

In the exemplary embodiment, the adjustable slit is utilised as both the entrance slit 88 and the exit slit 89. At least 1 1 mm of the length of the adjustable slit is provided as the exit slit, 89 wherein the sample light path is about 4 mm long, and the reference light path is about 4 mm long, and about 3 mm of separation is provided (by the optics of the system) between the light paths. The portion of the adjustable slit mechanism forming the exit slit 89 is separated for the portion of the adjustable slit mechanism forming the entrance slit by a gap of about 24 mm. The entrance slit 88 may also be provided using about 1 1 mm length of the adjustable slit mechanism.

Next, a method of operating the adjustable slit mechanism 300 shown in Figures 6 to 10 will be described. After assembling the adjustable slit mechanism 300 as described above, an initial setup and calibration of the machine may be performed. The first slit edge and the second slit edge 312, 314 may be aligned in the adjustable slit mechanism 300 such that the slit edges 312, 314 come into contact along their lengths (i.e. the slit can be completely closed along its length). In the embodiment shown in Figures 6 to 10, the first and second slit edges 312, 314 are aligned parallel to each other, such that both slit edges are vertical. The alignment of the first and second slit edges 312, 314 are provided by the first arm 308 and the second arm 310 in contact with locating washers 342. Locating washers 342 are provided to restrict the movement of the first and second slit members 302, 304 to a single axis of movement. As such, as the width of the slit is adjusted by the slit adjusting cam 306, the first and second slit edges 312, 314 remain parallel to each other.

The alignment of the first and second slit edges 312, 314 may be confirmed by checking that the adjustable slit can be completely closed so that no light passes through the slit.

The calibration of the slit adjusting cam 306 can subsequently be checked by utilising the slit adjusting mechanism in a monochromator system, such as the monochromator 80 shown in Figure 2. By rotating the slit adjusting cam 306 to a known angular position that provides a desired slit width, incident light (sample light or reference light) can be imaged on the adjustable slit and a measurement of the light using the detector 90 can be made. Based on the bandwidth of the detected light, the slit width of the monochromator can be inferred, as is known in the art. Accordingly, the system can be simply calibrated confirm that the actual slit width of the monochromator 80 at the known angular position of the slit adjusting cam 306 corresponds to the expected slit width and gives the desired optical performance. Once this initial setup is performed, due to the robust nature of the adjustable slit mechanism 300 no further calibration of the adjustable slit mechanism is required. For example, no further calibration of the width of the adjustable slit mechanism is required on start-up of the system as the controller is configured to adjust the angular position of the slit adjusting cam 306 based on information from the optical sensor assembly 31 1 .

The second stepper motor 360 is used to adjust the angular position of the slit adjusting cam 306 in response to a request from the controller to change the slit width. The controller may include a memory which stores information regarding the variable radial profile of the slit adjusting cam, for example, which angular positions of the slit adjusting cam correspond to specific slit widths of the adjustable slit mechanism. The controller is configured to utilise said information to instruct the second stepper motor 360 to rotate the slit adjusting cam 306 to a desired angular position corresponding to the desired slit width i.e. corresponding to a diameter of the slit adjusting cam 306.

In order to transport sample light and reference light to the adjustable slit mechanism 81 , 300, optical fibres may be used. For example, when the adjustable slit mechanism 300 is used as part of a fibre optic AAS as shown in Figure 2, a sample optical fibre 60 and a reference optical 70 may be used to transport light to the entrance aperture 390. Figures ! 4a, 14b and 14c show exemplary views of sample optical fibre 60 and reference optical fibre 70 for such a purpose. Figure 14a shows a front view of a sample optical fibre 60, a reference optical fibre 70, and a combined fibre optic connector 400 for focussing sample light and reference light onto the adjustable slit. As shown in Figure 14b the sample optical fibre 60 and the reference optical fibre 70 may each be connected at one end to sources of sample light and reference light respectively, for example from second focussing elements 36, 38 as shown in Figure 2. Figure 14c shows a detailed view along Section H-H as shown in Figure 14b.

Figure 15 shows a detailed view along Section E-E of Figure 14b. Figure 15 shows a sectional view of the reference optical fibre 70.

Figures 16a, 16b, and 16c show further detailed views of the fibre optic core arrangements at respective ends of the sample optical fibre 60 and reference optical fibre 70, as indicated in Figures 14a, 14b, and 14c. Figure 16a shows a detailed view of the plurality of optical fibre cores within reference optical fibre 70. As indicated in Figure 16a, there are seven optical fibre cores within the reference optical fibre 70. Similarly, Figure 16b shows a detailed view of sample optical fibre 60, also having seven optical fibre cores. Figure 16c shows a detailed view“A” of one embodiment of the arrangement of the fibre optic cores from the sample optical fibre 60 and the reference optical fibre 70 of Section H-H as indicated in Figures 14b and Figures 14c. As shown in Figure 16c the combined fibre optic connector 400 is arranged to guide the fibre optic cores of each of the reference optical fibre 70 and sample optical fibre 60 into an arrangement suitable for injecting light into the entrance aperture 390 of the adjustable slit mechanism 300.

Preferably, the combined fibre optic connector 400 is shaped to separate the fibre optic cores of the reference optical fibre 70 from the fibre optic cores of the sample optical fibre 60 and also to align each of the sets of optical fibre cores in a single plane. An example of such an alignment is shown in Figure 17, which provides an end view as indicated by detail “F” in Figure 14a. As indicated in Figure 17, each of the labelled fibre optic cores as shown in Figure 16a and Figure 16b are aligned by the combined fibre optic connector 400 along a plane which can subsequently be aligned with the centre of the slit of the adjustable slit mechanism 300. Accordingly, the combined fibre optic connector 400 is configured to output sample light on a sample light path and reference light on a reference light path, the sample light path and the reference light path being spaced apart by a spacing section of the combined fibre optic connector 400.

According to a further embodiment of the invention, a monochromator slit assembly for a fibre optic atomic absorption spectrometer is provided. The monochromator slit assembly comprises a sample optical fibre, a reference optical fibre, a slit for a monochromator, and a rotatable chopper. In one particularly preferred embodiment of this invention, the monochromator slit assembly including the rotatable chopper may be incorporated into the adjustable slit mechanism 300 as described above.

For example, a monochromator slit assembly according to an embodiment of this invention is shown in Figure 6. As shown in Figure 6, the monochromator slit assembly comprises a sample optical fibre 60 for transporting sample light, a reference optical fibre 70 for transporting reference light, a slit for a monochromator which is provided by the adjustable slit mechanism 300, and a rotatable chopper 500 which is also shown in a separate detail view.

As shown in Figure 6, the rotatable chopper 500 is arranged downstream of the adjustable slit mechanism 300 such that light output from the sample optical fibre 60 and the reference optical fibre 70 may travel through the adjustable slit mechanism 300 such that it is incident on the rotatable chopper 500. As such, the rotatable chopper 500 is understood to be arranged downstream of the adjustable slit mechanism 300. The rotatable chopper 500 is configured to alternatively occlude light output from the sample optical fibre 60 and the light output from the reference optical fibre 70. As explained above, the light output from the sample optical fibre 60 on a sample light path is spatially separated from the light output from the reference optical fibre 70 on a reference light path due to the arrangement of the combined optical fibre connector 400 which spatially separates the sample light path from the reference light path.

The rotatable chopper 500 includes a plurality of sample channels 502, 504 which are each configured to transmit the sample light path when one of the sample channels are aligned with the adjustable slit, and a plurality of reference channels 506, 508 which are each configured to transmit the reference light path when one of the reference light channels are each aligned with the reference light path. Each of the sample channels 502, 504 and the reference light channels 506, 508 are provided as through holes of the rotatable chopper 500 which run through the central axis of the rotatable chopper.

The rotatable chopper 500 is provided as an elongate member which may be substantially tubular and may have an axis of rotation which is aligned with a central axis of the adjustable slit of the adjustable slit mechanism 300. As shown in Figure 6, the rotatable chopper is provided such that the axis of rotation of the rotatable chopper 500 is generally vertical when in use. This is advantageous as gravitational forces acting on the rotatable chopper 500 will act in line with the axis of rotation of the rotatable chopper 500 such that the alignment of the axis of rotation of the chopper will remain aligned (with respect to the adjustable slit mechanism) with the central axis of the adjustable slit of the adjustable slit mechanism 300 over time. Accordingly, a robust monochromator slit assembly for a fibre optic AAS is provided.

As shown in the embodiment in Figure 6, the rotatable chopper 500 includes two sample channels 502, 504 and two reference channels 506, 508. The sample channels 502, 504 are spaced apart from the reference channels 506, 508, in accordance with the spacing between the sample light path and the reference light path incident on the adjustable slit. Thus, it will be understood that as the rotatable chopper 500 rotates about its axis of rotation the sample light channel 502 may be aligned with the sample light path in a first angular position such that sample light path passes along the sample light channel 502 whilst reference light on the reference light path blocked by the rotatable chopper as no reference light channel is aligned with the reference light path. The rotatable chopper may be moved to a second position by a clockwise rotation of the rotatable chopper of about 45°. In the second position, reference light channel 508 may be aligned with the reference light path output by reference optical fibre 70 through the adjustable slit. In such a second position, the reference light on the reference light path will be transmitted by the rotatable chopper 500 through reference light channel 508. In the second position, sample light will be occluded by the rotatable chopper 500, as no sample channel is aligned with the sample light path.

Thus, rotation of the rotatable chopper 500 causes the rotatable chopper to alternatively occlude the sample light paths and the reference light paths as each of the sample light channels 502, 504 and reference channels 506, 508 are aligned with the adjustable slit. It will be understood that in an intermediate position, for example at a rotation between the first position and the second position (e.g. 45°), the rotatable chopper may be provided to occlude both the sample light path and the reference light path such that no interference between the sample light and the reference light may occur (i.e. the sample light and the reference light are not transmitted at the same time).

In order to control the rotational position of the rotatable chopper 500 the rotatable chopper is driven by a third stepper motor 510 which is mounted beneath the rotatable chopper 500 as shown in Figure 6. The rotatable chopper 500 and the third stepper motor 510 are mounted on a mounting platform 512 which provides a stable base for the rotatable chopper 500 and ensures that the rotatable chopper 500 is aligned with the adjustable slit of the adjustable slit mechanism 300. In order to control the rotational position of the rotatable chopper 500, the third stepper motor 510 is controlled by a controller (not shown). The controller may be the same controller used to control the first stepper motor 1 14 and/or the second stepper motor 360. The controller is configured to receive information about the rotational position of the rotatable chopper 500 from a sensor. In the embodiment shown in Figure 6 the sensor is an optical sensor 514. The optical sensor 514 is configured to detect optical bar 516, which is attached to the rotatable chopper 500, when it passes through the optical sensor 514. Accordingly, the optical sensor 514 may be configured to detect a home position of the rotatable chopper 500. The controller may then control the third stepper motor 510 to rotate the rotatable chopper 500 by a desired number of steps of the third stepper motor 510 to a desired angular position in order to align the sample channel or a reference channel with the adjustable slit as desired depending on the measurement required to be taken by the monochromator. For example, as shown in Figure 6, the rotatable chopper 500 is orientated such that reference light (R) of the reference optical fibre 70 is transmitted by the rotatable chopper 500 whilst the sample light of the sample optical fibre 60 is occluded by the rotatable chopper 500. In order to precisely control the rotation of the third stepper motor 510, the third stepper motor 510 preferably has a step angle of less than 10°.

It will be appreciated that in Figure 6 the rotatable chopper only extends about half the length of the adjustable slit. This is because the upper portion of the adjustable slit is used as the exit slit for reference light and sample light. It is preferable that the rotatable chopper 500 is located directly downstream of the adjustable slit (i.e. before the collimating mirror 82 of Figure 2) such that at any one time only one of the sample light or the reference light but not both may be incident on the optics of the monochromator (i.e. collimating mirror 82 and grating 84).

As such with reference to the adjustable slit mechanism shown in Figures 6 through 9 and the monochromator 80 shown in Figure 2, a monochromator assembly may be provided. Such a monochromator assembly comprises the adjustable slit mechanism 300, the rotatable chopper 500, a collimating element (for example collimating mirror 82), and a diffracting element (for example diffraction grating 84), as shown in Figure 2. In particular, adjustable slit mechanism 300 is provided to act as both an entrance and an exit slit (i.e. a common entrance and exit slit) for the monochromator assembly.

One advantage of such a monochromator assembly is that the monochromator assembly is robust, as the sample light path and the reference light path output from respective sample optical fibre 60 and reference optical fibre 70 remain aligned with a centre of the adjustable slit mechanism and a central axis of the rotatable chopper 500. Accordingly, the light output through the exit aperture 392 of the monochromator may be accurately controlled for a range of different adjustable slit sizes as the central axis of the adjustable slit remains aligned with the sample optical fibre 60 and reference optical fibre 70 for a range of different slit widths. Accordingly, the need to calibrate the monochromator for a range of different slit widths may be reduced and/or eliminated.

In a particularly preferred embodiment the adjustable slit mechanism 81 , 300 may provide both the entrance and exit slit for the monochromator assembly 80 such that the

monochromator may output light having a bandwidth in the range of 0.1 nm to 2 nm. For example, such a range of bandwidths may be achieved by a slit width having an adjustable range between 25 pm and 2 mm in combination with a suitable diffraction grating, or a slit width being adjustable between 25 pm and 320 pm in combination with a suitable diffraction grating. In both exemplary cases, it will be appreciated that it is also possible to entirely close the adjustable slit (i.e. a slit width of 0 pm). The design of suitable diffraction gratings and collimating mirrors in combination with a common entrance and exit slit is well known in the art and is thus not discussed further herein. As such, any component of this monochromator may be replaced by any component having the same physical effect.

Accordingly, a fibre optic AAS 1 is provided according to this invention. The fibre optic AAS 1 includes a number of components which are described according to the embodiments of this invention. One particularly advantageous embodiment is the lamp holder assembly 20. The lamp holder assembly 20 incorporates a rotatable arm and a fibre optic connection such that light may be focussed from one of a plurality of sample light sources into a fibre optic cable in a robust and highly repeatable manner.

In a further advantageous embodiment of this invention, an adjustable slit mechanism 300 is provided. Said adjustable slit mechanism may provide an entrance and/or exit slit (common adjustable slit) for a monochromator 80. Such an adjustable slit mechanism may be used in various types of monochromators but is particularly advantageous when used in combination with fibre optic cables as part of a fibre optic AAS. Fibre optic AAS systems reduce complexity and improve robustness through the reduction of mirror based focussing elements, which may require repeated calibration before use to ensure sufficient alignment of the various optical components. Fibre optic AAS systems may include optical fibres for guiding sample light and reference light which may be held in a fixed positional relationship with an entrance slit to a monochromator. Accordingly, the adjustable slit mechanism, having a constant central axis for the slit, is particularly advantageous as the central location of the adjustable slit mechanism will retain its position relative to the sample optical fibre 60 and reference optical fibre 70 over an extended period of time such that further calibration of the slit alignment may be reduced and/or avoided. Accordingly, a robust yet highly accurate adjustable slit mechanism for a fibre optic AAS may be provided.

Furthermore, the adjustable slit mechanism may be formed into a monochromator slit assembly comprising a rotatable chopper 500. In this assembly it is advantageous that the rotatable chopper 500 has an axis of rotation which may be aligned with the central axis of the adjustable slit. Thus, the monochromator may filter sample light and/or reference light as part of a double beam AAS system. The alignment (vertical alignment) of the rotatable chopper may ensure that over time the relative alignment of the monochromator slit assembly is robust and does not require repeated calibrations.

According to another embodiment of this invention a fibre optic cable assembly is provided. Such an assembly provides an effective and robust means for transporting light from a first light source and a second light source to a sample housing 40 as part of a double beam fibre optic AAS.

Embodiments of the invention may be summarized in the following clauses: 1. An adjustable slit mechanism for a monochromator comprising:

a first slit member including a first slit edge;

a second slit member including a second slit edge;

the first slit edge arranged opposite the second slit edge to define an adjustable slit of the adjustable slit mechanism;

a slit adjusting cam having a variable radius;

a first arm connected to the first slit member;

a second arm connected to the second slit member;

wherein the first arm and the second arm are resiliently biased against the slit adjusting cam such that rotation of the slit adjusting cam adjustably separates the first arm and the second arm for adjusting a width of the adjustable slit.

2. An adjustable slit mechanism according to clause 1 , wherein the first arm is resiliently biased against the slit adjusting cam at a point opposite to a point at which the second arm is resiliently biased against the slit adjusting cam.

3. An adjustable slit mechanism according to clause 1 or 2, further comprising a resilient biasing element connected between the first arm and the second arm for biasing the first arm and the second arm against the slit adjusting cam.

4. An adjustable slit mechanism according to clause 3, wherein a pair of resilient biasing elements are connected between the first arm and the second arm, the pair of resilient biasing elements connected to the first arm and the second arm on either side of the points at which the respective arms contact the slit adjusting cam.

5. An adjustable slit mechanism according to any preceding clause, wherein the variable radius of the slit adjusting cam is configured to adjust the separation of the first arm and the second arm by moving the each of the first arm and the second arm substantially equally.

6. An adjustable slit mechanism according to any preceding clause wherein each of the first arm and the second arm are supported by a plurality of guide members, the guide members configured to align the first slit edge parallel to the second slit edge. 7. An adjustable slit mechanism according to clause 6, wherein each of the first arm and the second arm comprise an aligning portion, each aligning portion configured to engage with a plurality of guide members.

8. An adjustable slit mechanism according to any preceding clause, wherein the first and second arms each comprise a cam engaging portion, each cam engaging portion configured to provide a surface which engages with the slit adjusting cam.

9. An adjustable slit mechanism according to clause 8, wherein each cam engaging portion further comprises a plain bearing material configured to provide the surface which engages with the slit adjusting cam.

10. An adjustable slit mechanism according to clause 8 or 9 wherein each cam engaging portion is configured to engage with the slit-adjusting cam in a plane substantially parallel with the first and second slit edges.

1 1. An adjustable slit mechanism according to any of clauses 8 to 10 when dependent on any of clauses 6 or 7, wherein the cam engaging portions and the aligning portions of each of the first arm and the second arm are connected to together to form a substantially L-shaped portion.

12. An adjustable slit mechanism according to any preceding clause, wherein the first slit edge and the second slit edge of the adjustable slit is configured to provide an entrance and an exit slit for the monochromator.

13. An adjustable slit mechanism according to any preceding clause, wherein the first slit member and the second slit member are connected to the first arm and the second arm respectively by fastenings.

14. An adjustable slit mechanism according to any preceding clause, wherein the slit width is adjustable in the range of 0.025 mm to 2 mm, or in the range 0.025 mm to 0.5 mm.

15. A slit assembly for a monochromator for a fibre optic atomic absorption

spectrometer comprising:

a sample optical fibre for transporting sample light; a reference optical fibre for transporting reference light;

an adjustable slit provided by the adjustable slit mechanism according to any one of clauses 1 to 14,

wherein one end of each of the sample and reference optical fibres are arranged to direct light along respective sample and reference light paths through the adjustable slit ; and

a rotatable chopper arranged downstream of the adjustable slit mechanism and configured to alternatively occlude by rotation the sample light path and the reference light path.

16. A slit assembly according to clause 15, wherein an axis of rotation of the rotatable chopper is aligned with the adjustable slit.

17. A slit assembly according to clause 15 or 16, wherein the axis of rotation of the rotatable chopper is generally vertical when in use.

18. A slit assembly according to any one of clauses 15 to 17, wherein the rotatable chopper is further configured to define:

at least one sample channel configured to transmit the sample light; and

at least one reference channel configured to transmit the reference light;

wherein the sample channel is spaced apart from the reference channel along the axis of rotation of the chopper.

19. A slit assembly according to clause 18 wherein the rotatable chopper is configured to further define:

at least two sample channels arranged transverse with respect to each other about the axis of rotation of the chopper; and

at least two reference channels arranged transverse respect to each other about the axis of rotation of the chopper.

20. A slit assembly according to any one of clauses 15 to 19, comprising a sensor configured to detect a rotational position of the rotatable chopper.

21 . A slit assembly according to any one of clauses 15 to 20 comprising:

a stepper motor configured to rotate the rotatable chopper, wherein the stepper motor has a step angle of less than 10°.

22. A monochromator assembly, comprising:

the slit assembly of any of clauses 15 to 21 ;

a collimating element;

a diffracting element;

wherein the collimating element is configured to direct sample light and reference light path incident on it from respective first positions along the slit to the diffracting element;

the diffracting element is arranged to reflect sample light and reference light incident on it from the collimating element back to the collimating element; and

the collimating element is further configured to direct sample light and reference light from the diffracting element to respective second positions along the adjustable slit, the respective second positions being spatially separated from the first positions.

23. A monochromator assembly according to clause 22, wherein the collimating element is arranged downstream of the rotatable chopper.

24. A monochromator assembly according to clause 22 or clause 23 and incorporating the adjustable slit mechanism of any one of clauses 1 to 14, wherein the diffracting element is rotatably adjustable, such that in combination with the adjustable slit mechanism the monochromator may selectively output sample light or reference light with a bandwidth in the range of 0.1 nm to 2 nm.

25. A monochromator slit assembly for a fibre optic atomic absorption spectrometer comprising:

a sample optical fibre for transporting sample light;

a reference optical fibre for transporting reference light;

a slit for a monochromator,

wherein one end of each of the sample and reference optical fibres are arranged to direct light along respective sample and reference light paths through the slit; and

a rotatable chopper arranged downstream of the slit and configured to alternatively occlude the sample light path and the reference light path. 26. A monochromator slit assembly according to clause 25, wherein an axis of rotation of the rotatable chopper is aligned with the slit.

27. A monochromator slit assembly according to clause 25 or 26, wherein the axis of rotation of the rotatable chopper is generally vertical when in use.

28. A monochromator slit assembly according to any one of clauses 25 to 27, wherein the rotatable chopper is further configured to define:

at least one sample channel configured to transmit the sample light; and at least one reference channel configured to transmit the reference light;

wherein the sample channel is spaced apart from the reference channel along the axis of rotation of the chopper.

29. A monochromator slit assembly according to clause 28, wherein the rotatable chopper is configured to further define:

at least two sample channels arranged transverse with respect to each other about the axis of rotation of the chopper; and

at least two reference channels arranged transverse respect to each other about the axis of rotation of the chopper.

30. A monochromator slit assembly according to any one of clauses 25 to 29, comprising a sensor configured to detect a rotational position of the chopper assembly.

31 . A monochromator slit assembly according to any one of clauses 25 to 30, comprising:

a stepper motor configured to rotate the chopper assembly,

wherein the stepper motor has a step angle of less than 10°.

32. A monochromator slit assembly according to any one of clauses 25 to 31 , wherein the slit is an entrance slit for a monochromator.

33. A monochromator assembly, comprising:

the monochromator slit assembly according to any of clauses 25 to 32; a collimating element;

a diffracting element; wherein the collimating element is configured to direct sample light and reference light path incident on it from respective first positions along the slit to the diffracting element;

the diffracting element is arranged to reflect sample light and reference light incident on it from the collimating element back to the collimating element; and

the collimating element is further configured to direct sample light and reference light from the diffracting element to respective second positions along the slit, the respective second positions being spatially separated from the first positions.

34. A monochromator assembly according to clause 33, wherein the collimating element is arranged downstream of the rotatable chopper.

35. A monochromator assembly according to clause 33 or 34, wherein the slit for a monochromator is provided by an adjustable slit mechanism comprising:

a first slit member including a first slit edge;

a second slit member including a second slit edge;

the first slit edge arranged opposite the second slit edge to define an adjustable slit of the adjustable slit mechanism;

a slit adjusting cam having a variable radius;

a first arm connected to the first slit member;

a second arm connected to the second slit member;

wherein the first arm and the second arm are resiliently biased against the slit-adjusting cam such that rotation of the slit adjusting cam adjustably separates the first arm and the second arm for adjusting a width of the adjustable slit, wherein the diffracting element is rotatably adjustable, such that in combination with the adjustable slit mechanism the monochromator may selectively output sample light or reference light with a bandwidth in the range of 0.1 nm to 2 nm.

36. A monochromator assembly according to clause 35, wherein the first arm is resiliently biased against the slit adjusting cam at a point opposite to a point at which the second arm is resiliently biased against the slit adjusting cam.

37. A monochromator assembly according to clause 35 or 36, further comprising a resilient biasing element connected between the first arm and the second arm for biasing the first arm and the second arm against the slit adjusting cam. 38. A monochromator assembly according to any one of clauses 35 to 37, wherein the variable radius of the slit adjusting cam is configured to adjust the separation of the first arm and the second arm by moving the each of the first arm and the second arm substantially equally.

39. A monochromator assembly according to any one of clauses 35 to 38, wherein each of the first arm and the second arm are supported by a plurality of guide members, the guide members configured to align the first slit edge parallel to the second slit edge; and

the first and second arms each comprise:

a cam engaging portion, each cam engaging portion configured to provide a surface which engages with the slit adjusting cam; and

an aligning portion, each aligning portion configured to engage with the plurality of guide members,

wherein the cam engaging portions and the aligning portions of each of the first arm and the second arm are connected to together to form a substantially L-shaped portion.

40. A light source assembly for a fibre optic atomic absorption spectrometer (AAS) comprising:

a plurality of light sources arranged about an axis;

an arm arranged to rotate about the axis;

a fibre optic cable;

a focusing element configured to couple light from a selected light source of the plurality of light sources into one end of the fibre optic cable;

wherein the fibre optic cable and focusing element are arranged on the arm such that the arm is rotatable relative to the plurality of light sources to select a light source for the fibre optic cable.

41 . A light source assembly according to clause 40, wherein

the axis of rotation of the arm is generally vertical when in use.

42. A light source assembly according to clause 40 or 41 , wherein

the arm includes a counterweight at an opposing end of the arm to the focusing element. 43. A light source assembly according to any one of clauses 40 to 42, wherein the focusing element is a lens.

44. A light source assembly according to any one of clauses 40 to 43, wherein

the focusing element is arranged on the arm relative to the fibre optic cable to directly couple light into the fibre optic cable.

45. A light source assembly according to any one of clauses 40 to 44, wherein

the focusing element and the end of the fibre optic cable are aligned with a direction in which light is output from the selected light source.

46. A light source assembly according to clause 45, wherein

the optical fibre, the focusing element and the selected light source are arranged to be aligned substantially vertically when in use.

47. A light source assembly according to any one of clauses 40 to 46, comprising: a drive shaft connected to the arm at one end, the drive shaft aligned with the axis of rotation.

48. A light source assembly according to clause 47, comprising

a stepper motor configured to drive the drive shaft at an opposing end to the arm.

49. A light source assembly according to clause 47 or 48, comprising

a housing configured to support the drive shaft at both ends.

50. A light source assembly according to any one of clauses 40 to 49, comprising

a controller configured to calibrate the rotational position of the arm with respect to a light source in response to a signal indicative of the light power focused into the fibre optic cable from the light source.

51 . A light source assembly according to any one of clauses 40 to 50, wherein

the plurality of light sources comprises a plurality of Hollow Cathode Lamps.

52. A light source assembly according to any one of clauses 40 to 51 , wherein the plurality of light sources comprises at least 5 light sources, preferably at least 10 light sources.

53. A fibre optic atomic absorption spectrometer comprising:

a light source assembly according to any one of clauses 40 to 52;

a sample atomiser;

a sample optical cable;

a monochromator; and

a detector;

the light source assembly configured to output sample light to the monochromator through the sample atomiser and the sample optical cable, and the monochromator configured to filter said sample light and to output filtered light to the detector.

54. A double-beam fibre optic atomic absorption spectrometer comprising:

a light source assembly according to any one of clauses 40 to 52;

a reference light source;

a sample atomiser configured to receive a sample to be analysed;

a sample optical cable;

a reference optical cable;

a monochromator; and

a detector;

the light source assembly and the reference light source configured to output light to the monochromator on:

i) a sample light path through the sample atomiser and sample optical cable; and

ii) a reference light path through the reference optical cable;

the monochromator configured to filter said sample light and reference light and to output filtered light to the detector.

It will be understood by those skilled in the art that the invention is not limited to the embodiments described above and shown in the drawings, and that many additions and modifications can be made without departing from the scope of the invention as defined in the appending claims.