LIBS apparatuses have been known in the art for many years. Numerous examples can be found in patents such as U. S. Patents No. 5,847,825 (Alexander) issued on December 8,1998 and U. S. Patent No. 6,008,897 (Sabsabi et al.) issued on December 28,1999, which are hereby incorporated by reference. In brief, a LIBS apparatus comprises a laser source capable of generating a high power laser beam pulse which is generally in the order of a few nanoseconds, i. e., a few billionths (10-9) of a second. The laser beam pulse is focused on a target point using an optical assembly. The target point corresponds to a location on the surface of a sample that needs to be analyzed.

The extremely rapid heat increase of the material at the target point causes vaporization of the molecules and breakdown of the atoms. An ionization occurs and a plasma is produced. As the plasma cools, the excited components emit optical energy at characteristic wavelengths. This radiation is collected and analyzed to identify the elemental species that are present in the sample based on the presence of characteristic spectral lines. The concentration of the elemental species is proportional to the intensity of the spectral lines that are produced. Calibration procedures and information stored in databases allow a computer to automatically perform the identification of elemental species and the concentration thereof.

The LIBS technique is considered to be nondestructive since only a very small amount of material is ablated by the very short laser pulse and no significant amount of energy is transmitted to the material surrounding the target point.

One possible application of a LIBS apparatus is to monitorthe quality of products. For example, LIBS apparatuses are used to determine the concentration of given ingredients in chemical preparations such as pharmaceutical tablets or other manufactured chemical products. Of course, many other applications are possible, besides performing quality controls, such as identification of unknown compositions in samples.

The procedures followed during quality controls of chemical products with a LIBS apparatus require that a number of identical or similar samples be analyzed before concluding that there is the required concentration of compositions or not. Typically, each sample is provided on a sample holder that is manually positioned with reference to the target point. A visible laser reference line indicates the path of the laser pulse. Although this procedure may be satisfactory for many general applications, it is not well suited for high quality and automated controls in which a number of important criteria have to be complied with. For instance, the measurements that are made have to be reproducible from one similar sample to another, which means that the analyzed locations should be the same from one sample to another in order to follow a common sampling pattern. Moreover, the air flow over the analyzed locations should be the same during all measurements in other to prevent the results from being altered or otherwise disturbed by air flow variations.

SUMMARY One aspect of the present invention is to provide an apparatus and method for performing a laser-induced breakdown spectroscopy analysis of at least two samples where the sampling pattern can be precisely repeated from one sample to another.

Another aspect of the present invention is to provide an apparatus and method which may be used in an automated manner.

It is also an aspect of the present invention to provide an apparatus and method for performing a laser-induced breakdown spectroscopy analysis of at least two similar samples where there is a minimal movement of and around a sample so that the air flow over the analyzed location on the sample is not significantly disturbed between measurements.

The present invention provides a system for performing a laser-induced breakdown spectroscopy analysis of at least two samples, the system comprising: a support member having a medial axis; at least two spaced-apart sample receptacles provided on the support member, the sample receptacles holding a respective sample and positioning the samples at a same distance from the medial axis of the support member and in a same plane that is perpendicular to the medial axis; first means for generating a high power laser beam pulse ; second means for focusing the laser beam pulse at a target point; third means for selectively pivoting and locking in position the support member around the medial axis of the support member; fourth means for performing a spectral analysis of a radiation emitted by a plasma produced by the laser beam pulse on the samples ; and fifth means for offsetting the support member in at least one direction and locking it in position.

In accordance with the present invention, the third means may comprise a motorized rotative actuator mechanically connected to the support member and mounted on an intermediary base. A system in accordance with the present invention, may further comprise a drive mechanism provided between the motorized rotative actuator and the support member. In accordance with the present invention, the motorized rotative actuator may comprise an electric motor.

In accordance with the present invention, the fifth means may comprise: a linear guide having a first and a second side that are slidable with reference to the other, the first side being connected to the intermediary base and the second side being connected to a fixed location; and a motorized linear actuator having a first portion connected to the first side of the linear guide and a second portion connected to the second side of the linear guide.

In accordance with the present invention the motorized linear actuator may comprise an electric motor.

In accordance with the present invention the fifth means may comprise: a first linear guide having a first and a second side that are slidable with reference to the other, the first side of the first linear guide being connected to the intermediary base, the first linear guide extending in a first direction; and a first motorized linear actuator having a first portion connected to the first side of the first linear guide and a second portion connected to the second side of the first linear guide; a second linear guide having a first and a second side that are slidable with reference to the other, the first side of the second linear guide being connected to the second side of the first linear guide and the second side of the second linear guide being connected to a fixed location, the second linear guide extending in a second direction; and a second motorized linear actuator having a first portion connected to the first side of the second linear guide and a second portion connected to the second side of the second linear guide.

In accordance with the present invention the first and second directions may be perpendicular. In accordance with the present invention, the first and the second motorized linear actuator may comprise an electric motor.

In accordance with the present invention the receptacles may be removable from the support member.

In accordance with the present invention each receptacle may further comprise means for gripping a corresponding sample.

In accordance with the present invention, the support member may comprise a plate.

A system in accordance with the present invention may further comprise means for providing reproducible air flow at the target point. In accordance with the present invention, the means for providing reproducible air flow may comprise a removable hood to at least cover the support member.

A system in accordance with the present invention, may further comprise sixth means for controlling the third and fifth means. In accordance with the present invention, the sixth means may comprise a computer. In accordance with the present invention, the computer may also controls the first and fourth means.

The present invention also provides a method for performing a laser-induced breakdown spectroscopy analysis of at least two samples held by a respective sample receptacle on a support member, the samples being positioned at a same distance from a medial axis of the support member and in a same plane that is perpendicular to the medial axis, the method comprising the steps of: a) positioning a relative location on one of the samples at a target point; b) generating a high power laser beam pulse ; c) focusing the laser beam pulse at the target point; d) performing a spectral analysis of a radiation emitted by a plasma produced by the laser beam pulse on the sample ; e) offsetting the support member in at least one direction so as to allow another location on each sample to be positioned at the target point and repeating steps b) to d) after each offset until a desired set of locations have been analyzed; and pivoting the support member around the medial axis of the support member so as to position a location on another sample at the target point and repeating steps b) to e) after each pivot until all samples are analyzed, the locations on the samples forming a sampling pattern which is repeated from one sample to another.

In accordance with the present invention, in step e), the support member may be offset in two perpendicular directions.

A method in accordance with the present invention may further comprise the step of covering at least the support member before performing the spectral analysis for providing reproducible air flow around the target point.

The present invention further provides a method for performing a laser-induced breakdown spectroscopy analysis of at least two samples held by a respective sample receptacle on a support member, the samples being positioned at a same distance from a medial axis of the support member and in a same plane that is perpendicular to the medial axis, the method comprising the steps of: a) positioning a location on one of the samples at a target point; b) generating a high power laser beam pulse ; c) focusing the laser beam pulse at the target point; d) performing a spectral analysis of a radiation emitted by a plasma produced by the laser beam pulse on the sample ; e) pivoting the support member around, the medial axis of the support member so as to position a location on another sample at the target point and repeating steps b) to d) after each pivot until the location is analyzed on all samples; and offsetting the support member in at least one direction so as to allow another location on each sample to be positioned at the target point and repeating steps b) to e) after each offset until a desired set of locations have been analyzed, the locations on the samples forming a sampling pattern which is repeated from one sample to another.

In accordance with this aspect of the present invention, in step D, the support member may be offset in two perpendicular directions. A method in accordance with this aspect of the present invention may further comprise the step of covering at least the support member before performing the spectral analysis for providing reproducible air flow around the target point.

These and other aspects and advantages of the present invention are described in or apparent from the following detailed description of a preferred embodiment made in conjunction which the apparent figures.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is a schematic view of a typical LIBS system.

FIG. 2 is an upper perspective view of a sample holder and a positioning mechanism according to a preferred embodiment of the present invention.

FIG. 3 is view similar to FIG. 2, showing only the positioning mechanism.

FIG. 4 is a bottom perspective view showing the positioning mechanism of FIGS. 2 and 3 from an opposite side.

DETAILED DESCRIPTION FIG. 1 schematically illustrates a system (10) according to a possible embodiment of the present invention. This system (10) is used for performing a laser-induced breakdown spectroscopy analysis of at least two samples (12). A laser (20) is disposed as to deliver high-power laser beam pulses. The output of the laser (20) is aimed at an optical assembly which is used for focusing the laser beam pulses at a target point (T). Each location to be analyzed on the samples (12) is successively positioned at the target point (T). The optical assembly may comprise a lens (22), a mirror (24), an optional optical fiber (26) and/or any other necessary parts, as apparent to a person skilled in the art.

An optical spectrometer (30) is provided for performing a spectral analysis of a radiation emitted by a plasma produced by the laser beam pulse on the sample being analyzed. The spectrometer (30) is equipped with a gated detector (32) to enable measurements of the time limited pulsed emission spectrum of the plasma. The spectrometer (30) also includes an optical assembly to bring the radiation to the detector (32). For instance, the mirror (24) may be a dichroic mirror sending the radiation to a lens (34). The radiation is then transmitted to the entrance of the spectrometer (30) by an optical fiber (36). Other ways of achieving the collection of the radiation are possible. For instance, the radiation that is sent to the spectrometer (30) may follow a path which is independent from that of the laser beam pulses.

A delay generator (38) is preferably included in the optical spectrometer (30) for gating off the unwanted time period in the formation of the plasma or at the end thereof. The data acquired by the spectral analysis are preferably sent to a computer (40) for determining the element concentrations or the nature of the composition through prior calibration. The laser (20) and the spectrometer (30) are preferably controlled by the computer (40). Of course, other kinds of arrangements are possible, as apparent to a person skilled in the art, including manually interpreting the data.

FIG. 2 shows a sample holder (50) and a positioning mechanism (60) according to a preferred embodiment of the present invention. These components are used to bring each location to be analyzed on a sample (12) into a coincident position with the target point (T). The sample holder (50) comprises a support member (52), preferably in the form of a circular plate, and a plurality of sample receptacles (54) which are removably connected to the periphery of the support member (52) by appropriate means, as apparent to a person skilled in art. In the illustrated embodiment, the connecting arrangement comprises dovetail connectors.

Each sample receptacle (54) of the illustrated embodiment comprises a bore or depression (56) in which a corresponding sample (12) is tightly inserted. Alternatively, the samples (12) may be held in place in numerous other ways, including glue or releasable retention mechanisms with prongs, claws or vices (not shown) which are fixed to the support member (52).

The support member (52) is rotatable around a medial axis (M), which is a vertical central axis in the illustrated embodiment. All sample receptacles (54) hold respective samples (12) and the samples (12) are positioned at a same distance from the medial axis (M) of the support member (52) and in a same plane, which plane is perpendicular to the medial axis (M).

The positioning mechanism (60) comprises an arrangement for selectively pivoting and locking in position the support member (52) around the medial axis (M). In the preferred embodiment shown in FIGS. 2 to 4, the arrangement comprises a motorized rotative actuator (70) that is mechanically connected to the support member (52). The rotative actuator (70) preferably comprises an electric motor that is mounted on an intermediary base (72). The arrangement further comprises a drive mechanism (74) provided between the rotative actuator (70) and the support member (52). The precision of the movement depends on the selection of the components. For instance, the drive mechanism (74) can have a very low ratio and the movement of the support member (52) will be very slow compared to that of the actuator (70).

The drive mechanism (74) can further provide the locking capability to prevent the support member (52) from moving when the actuator (70) is not in operation. The drive mechanism (74) may comprise a worm or a Geneva mechanism.

The support member (52) is preferably mounted over the drive mechanism (74) by a bearing-supported hub (75) over which projects a spindle (76). A locking cap (78) or any other similar device is used to lock the support member (52) in place.

The positioning mechanism (60) further comprises an arrangement for offsetting the support member (52) in at least one direction and locking it in position. Although combining the rotation with a linear translation of the support member (52) in one direction would allow to position the target point (T) anywhere on the samples (12), the positioning mechanism (60) of the preferred embodiment is movable in two perpendicular directions. It comprises a first linear guide (80) having a first (82) and a second side (84) that are slidable with reference to the other. The first linear guide (80) can be any kind of linear motion system, such as a linear slide, a linear positioning stage of the linear bearing, crossed-roller, dovetail guide or roller bearing type. Linear air bearing stages can also be used.

The first side (82) of the first linear guide (80) is connected to the intermediary base (72). A first motorized linear actuator (86) is provided for moving the first side (82) with reference to the second side (84). The first actuator (86) has a first portion connected to the first side (82) and a second portion connected to the second side (84).

The second linear guide (90) is essentially similar to the first linear guide (80). The second linear guide (90) has a first (92) and a second side (94) that are slidable with reference to the other. The first side (92) of the second linear guide (90) is connected to the second side (84) of the first linear guide (80) and the second side (94) of the second linear guide (90) is connected to a fixed location (96), such as a table or any other object which supports the positioning mechanism (60). The second linear guide (90) comprises a second motorized linear actuator (98) having a first portion connected to the first side (92) of the second linear guide (90) and a second portion connected to the second side (94) thereof.

Each motorized linear actuator (86,98) preferably comprises an electric motor which is connected to a lead-screw, follower/nut, belt-drive (not shown) and controlled by the computer (40). The electric motors can be stepping motors, dc servo-motors or direct linear motors. Each arrangement locks to the parts in position when it is not in operation to obtain a high precision. The control of the motors can be accomplished by the computer (40) through a programmable motor controller or directly to an indexer or motor driver (not shown). Also, control of the motor movements can be accomplished by a standalone programmable motion control system.

Optionally, the support member (52) and the positioning mechanism (60) may be covered by a removable hood (not shown). The hood comprises access ports for the laser beam pulses and for the collection of the radiation emitted by the plasma. This allows to further reduce the variation of air flow and thus allows to reproduce the same air flow conditions at the target point (T) in all measurements. Another way of achieving this is to install the LIBS apparatus in a confined space or in a room where the flow of air is negligible.

In use, the samples (12) are provided on the support member (52). A first location to be analyze on a first sample (12) is positioned at the target point (T). A first laser beam pulse is generated to produce a plasma on the sample (12) and a first spectral analysis of the radiation emitted by the plasma is performed. More than one measure can be taken at the same location. Thereafter, the support member (52) is offset by the positioning mechanism (60) so as to allow another location on the sample (12) to be aligned with the target point (T), in accordance with the sampling pattern determined by the quality control procedure.

Preferably, all locations are analyzed on a sample (12) before moving to the next sample (12). Alternatively, the same relative location can be analyzed on each sample (12) before offsetting the support member (52) and analyzing another relative location on each sample (12). A location previously analyzed on a sample (12) can be analyzed again before changing the position of the support member (52). The same location can also be analyzed after all other locations of the sample (12) or after that other samples (12) have been analyzed. Therefore, measurements are made until a desired set of relative locations have been analyzed on a sample (12), ranging from one to a plurality of locations.

The computer (40) preferably-controls all operations. The sampling pattern is programed in the computer (40) in the form of rasters with x and y coordinates with reference to a reference point and the computer (40) is adapted to send signals to the various actuators of the positioning mechanism (60). Measurements are made until all samples (12) are analyzed.

The present invention can thus analyze all samples (12) according to a same sampling pattern and with a minimal movement of and around the samples (12) so that the air flow is not significantly disturbed between measurements. Yet, the whole procedure may be performed in an automated way when using the appropriate components and computer (40).

Although a preferred embodiment of the invention has been described in detail herein and illustrated in the accompanying figures, it is to be understood that the invention is not limited to this precise embodiment and that various changes and modifications may be effected therein without departing from the scope or spirit of the present invention. For instance, the support member (52) may have other forms than the illustrated circular plate. It may have almost any shape. It may even be in the form of a cylinder (not shown), whereby the sample receptacles (54) are located on the surface thereof.

It should be noted that more than one sampling pattern may be used with a same set of samples (12).

Although electric motors are disclosed as the actuators in the preferred embodiment, other kinds of actuators may be used, including manually controlled arrangements.