There are numerous applications where there is a requirement to use a liquid sample where the volume of the sample should be known to a high level of accuracy.

An example of an application of this type is in the environmental analysis of water such as ground water. A typical method of conducting this analysis is to concentrate a contaminant of interest (e. g. trace metals, pesticide residues. etc.) by passing a known volume of the water through a solid-phase extraction (SPE) column. Here. the sample volume taken is required to be known to within-1% to allow quantitative analysis down to the parts per billion (ppb) level.

Fixed volume sampling has previously been effected using a number of techniques. In many cases pistons or syringes having a fixed volume, are used to draw up a known volume of liquid (see for example United States Patent US-A- 3607092). Working on a broadly similar principle, tube devices such as those described in United States Patent US-A-4987785, have been proposed in which liquid is caused to flow into a section of tubing having a predetermined volume.

However. these devices can take up gas bubbles, which reduce the volume available for liquid and so reduce the accuracy.

Other devices use ballcocks or ball valves (see for example United States Patent US-A-40832520, United States Patent US-A-4077263). In the former devices. liquid flows into a vessel until a fixed level is reached, whereat a ballcock floats up and causes the flow to stop. Ballvalve devices are similar except that the ball is arranged directly below the liquid inlet such that at a fixed level the ball fioats up

and stoppers the inlet. However, the accuracy and repeatability of these devices is not sufficiently good for some applications.

Also known are timed pumps (e. g. US-A-4121907) where a pump acts for a fixed time, so moving a fixed volume of liquid. However such devices may also suffer from the inaccuracies caused by gas bubbles in the sample, and have the added complication of requiring a measure of the sample flow rate or velocity.

A typical method of measuring a known volume of pure water to high accuracy is to weigh the sample. However, for water samples with various dissolved entities, the density will be unknown, and hence this method cannot be used.

According to the present invention there is provided apparatus for sampling a volume of a liquid, said apparatus comprising a vessel of known volume, said vessel comprising a chamber, an inlet to allow liquid to enter said chamber, characterised in that the chamber is in fluid communication with a gas permeable membrane, and said membrane is arranged such that gas present in said chamber exhausts from the chamber through the membrane, thereby permitting the chamber to fill only with liquid.

The total volume of the vessel therefore comprises the volume of the chamber combined with that of the relevant fluid pathways or conduits leading to or from the chamber, and this can be accurately determined either prior to or after sampling.

The membrane is preferably arranged such that at least some of its area is in contact with any head space formed by gas bubbles in the liquid, so as to allow any gas to exit from the chamber. Thus any gas bubbles that are present in the liquid prior to or during the sampling operation are removed and hence do not compromise the volume measurement accuracy.

Preferably, the gas permeable membrane is arranged in an upper surface of the chamber as any gas bubbles present in the liquid in the chamber will rise up through the liquid and so will contact across the entire surface of the membrane thus maximising efficiency. However, the membrane may be present elsewhere, for example in the side wall of the chamber provided gas bubbles may be drawn through it.

In a preferred embodiment. walls of the chamber include a region which tapers inwardly towards the membrane since gas bubbles will tend then to be channelled towards the membrane as they rise through the liquid. Such a chamber is preferably held substantiallv vertically but a chamber having walls that taper at an angle of x° to its axis will be able to tilt to an angle of (90-x) ° degrees and without trapping gas in any"pockets"formed by the taper (see Figure 2b below).

The inlet is suitably arranged at an upper portion of the chamber. Suitably the chamber contains a separate outlet for the liquid. This is preferably arranged at a lower portion of the chamber to allow for gravity drainage if required.

Alternatively a pump may be provided to remove liquid from the chamber after use.

Suitable gas permeable liquid impermeable membranes are known in the art.

They include polytetrafluoroethylene (PTFE) membranes such as those available from Mupor Ltd., UK. They may be held in place in the chamber by fixing means such as fanges, screws and the like, or where the materials of the membrane and the remainder of the chamber allow, they may be attached to the surrounding chamber walls for example by heat or ultrasonic sealing.

Where necessary, support means can be provided for the membrane. These may take the form of a mesh or grid, for example of a rigid material such as metal or plastics, which holds the membrane to prevent distortion but allows gas to pass through. Alternatively, the membrane may be provided with an annular support

that allows gas to pass through the central region. Preferably also in this arrangement, the membrane extends outwardly around the support, so increasing the available membrane surface.

Suitably, the apparatus further comprises a pump for pumping liquid into the vessel. A suitable pump is a vacuum pump which is arranged to draw liquid into the vessel, for example by inducing a reduced pressure or vacuum in the chamber.

In such an arrangement, the membrane is suitably arranged to form a barrier between the chamber and the vacuum pump. A convenient location for the membrane in this instance is across the vacuum line leading from the chamber to the vacuum pump. In this case, gas present in the chamber is drawn out through the membrane by the pump.

The membrane may be held in place in a vacuum line for example by means of a flange arrangement provided specifically for the purpose.

Alternatively, the pump comprises a positive pump which is arranged to drive sample liquid into the vessel.

In a particularly preferred embodiment, the apparatus is provided with means for halting the pump when the vessel is full. This may comprise a sensor device, which is operatively interconnected with a controller for the pump. Suitable sensors include a pressure sensor or a level sensor, or a combination of one or more of these. For example, where liquid is drawn into the vessel by means of a vacuum pump, the pressure in the vacuum line drops significantly when the vessel is full and all gas bubbles have been drawn out. Thus a pressure sensor arranged in the vacuum line detects this pressure change which results in a signal being passed to the controller which halts the vacuum pump. Conversely, where the apparatus comprises a pump arranged to drive liquid into the vessel, the pressure in the vessel would increase significantly when the chamber was full, and a pressure sensor in the feed line would detect this change.

Level sensors, for example optical devices, may be used to detect when liquid reaches exactly the level of the top of the vessel. It is difficult to ensure the accuracy of the position of such a level sensor. Therefore pressure sensors may be preferred in this context. However, in a particularly preferred embodiment, a level sensor is provided in addition to the pressure sensor, and is arranged to detect a'nearly full'condition of the chamber. A controller can then be arranged to slow the pump in response to a signal from the level sensor. This reduces the pressure shock to the upper surface of the chamber, which may contain the membrane, when the liquid reaches it.

In yet a further preferred embodiment, the apparatus comprises multiple vessels each including a chamber as described above, which are arranged such that they may be filled concurrently for example by connecting pumping lines to a single pump. The chambers or vessels may be of the same or different volumes. Each vessel fills at a similar rate, assuming any differential pressure caused by the pipework is similar for each chamber. Where the vessels are of different volumes, the smallest vessel will become full first. At this point, the liquid in the smaller chamber will be in contact with the gas permeable membrane, and no more liquid will be drawn into this chamber. However, the larger chambers. will continue to fill until the liquid level in each chamber also contacts the respective gas permeable membrane.

Such an embodiment may be useful where samples are to be subjected to multiple analyses and/or where different volumes of liquid may be required for each test.

If required, the volume of a chamber (and thus the vessel) may be adjustable. The adjustment is suitably measurable so that the volume of the vessel is always known or can be calculated. For example, a wall of the chamber may be of a flexible material which is deformable under the influence of an externally applied pressure, so as to alter the volume of the chamber. In this case, suitable means for

applying the deforming pressure, such as a piston or a screw, is provided externally of the chamber. These devices are electronicallv controlled so as to ensure that accurate deformation occurs.

Alternatively, the volume of the vessel may be adjusted by providing a piston or plunger with a seal in the chamber. In yet another embodiment, the gas permeable membrane is moveable, for example by attaching it to the end of a probe which can move up and down within the chamber. In this way the height of the liquid in the chamber, and hence the volume, can be selected prior to taking a sample.

The adjustment of the volume of the vessel is effected in a measurable way. This provides for greater flexibility of use of the apparatus, in particular where there are situations where the supply or source of liquid is low and there may not be sufficient liquid available to fill the vessel. In this case, a volume such as the available volume of liquid may be pumped into the vessel and afterwards, the volume of the vessel may be adjusted until the volume of liquid completely fills the vessel. Further pumping will ensure that any air bubbles are removed as described above. At this point, the actual volume of liquid will be the volume of the vessel.

A further aspect of the invention comprises a method for sampling a volume of a liquid, such as water, said method comprising filling a vessel of an apparatus as described above with liquid, and ensuring that gas bubbles have escaped through the gas permeable membrane of the chamber.

In this method, the volume of the vessel may be predetermined and liquid is caused to enter the vessel until it is completely filled. Alternatively, where the apparatus has a chamber of adjustable volume, a volume of liquid insufficient to fill the vessel is pumped in and the volume of the vessel is then adjusted so that that the liquid completely fills the vessel in the absense of gas bubbles. The

volume of the vessel in this configuration is then noted and so provides an accurate measure of the actual volume of liquid in the sample.

Suitably, liquid enters the vessel under the influence of a pump which either drives or draws the liquid. Any gas bubbles in the sample then come into contact with the suitably arranged gas permeable membrane and so leave the vessel, preferably under the influence of the pump. Sensors, where present, detect the full condition of the vessel and so control the action of the pump or the volume adjustment means depending upon the way the apparatus is operated.

The vessel then contains a precisely known volume of liquid. with no gas bubbles.

This liquid can then be evacuated or pumped through the outlet and collected for use, for example in analysis. At this point, it may be passed though a solid-phase extraction (SPE) unit such as an SPE cartridge or disc to extract analytes from a precisely known volume of liquid. Preferably, the liquid sample is drawn through an analytical device such as an SPE unit prior to entry into the vessel, so that again, the SPE unit will extracts analytes from a precisely known volume of liquid. After filling, the SPE unit is removed for analysis, and the contents of the vessel may then be discarded.

The invention will now be particularly described by way of examples only and with reference to the drawings in which; Figure 1 shows diagrammatically an embodiment of the sampling apparatus of the invention, in contact with a sample liquid: Figures 2 (a)- (c) are diagrams of alternative arrangements for the upper portion of the chamber of the apparatus of the invention: and Figure 3 illustrates diagrammatically an adjustable form of the apparatus of the invention.

In the illustrated apparatus (Figure 1). a sample tube 1 is connected to a chamber 2 via an inlet port 3 by way of an inlet valve 4.4. drain channel channel is connected connected the chamber 2 by way of a drain valve 6. The drain channel 5 is provided in a lower region of the chamber 2 so as to allow for gravity drainage, although a pump may be provided to allow for forced drainage.

The chamber 2 has a tapering top portion 7 and an opening 8 at the top which is connected to a pump 9 by way of a pumping line 10. A gas permeable, liquid impermeable membrane 11, such as a PTFE membrane, is held across the pumping line 10 by means of a clamped flange arrangement 12.

A pressure sensor 13 is provided in the pumping line 10 and a level sensor 14 is arranged in the line intermediate the opening 8 and the membrane 11.

In use, the pump 9 operates to evacuate the chamber 2 and so draw in liquid from a reservoir 15 via tube 1 and valve 4. Although a reservoir 15 is illustrated in this embodiment, the source of liquid is immaterial and the sample may be drawn directly from a source such as a natural source, like a river, stream. ground water supply, well or the sea.

Air from above the liquid level is evacuated until the chamber 2 is full. When the liquid reaches the membrane 11, it is unable to pass through it, and the pump 9 then runs to remove any residual bubbles in the system. The pressure in the pumping line will then fall suddenly and this is detected by the pressure sensor 13, which is monitored by a controller (not shown). The level sensor 14 is arranged to detect a'nearly full'condition and slow the pump, in order to reduce pressure shock to the membrane when the liquid reaches it.

The pattern of response of the pressure and optionally the level sensor can be checked by the controller to determine that the correct sample volume has been

drawn. Correct operation is expected to give a gradually falling pressure while air is being pumped and liquid drawn up the tube 1-the precise profile will depend on the head of liquid and how this changes with time-but with a sudden drop when the liquid reaches the membrane. Algorithms are provided in the controller to distinguish between correct and incorrect filling patterns.

In the illustrated embodiment, the volume of sample liquid is drawn from a reservoir 15 through an SPE cartridge 16. It is clear then that the total volume of the liquid drawn through the absorbent bed of the cartridge 16 is the total of the volume of the cartridge 16 above the absorbent bed. tube 1 and vessel 2 and the pumping line 10 up to the membrane 11. This volume has in this case been predetermined.

The rate of drawing of sample through the cartridge can be set by the rate of the pump, or by an air bleed inserted into the pump line (not shown). Constant rate is not normally required but can be stabilise by e. g. using a large bleed and fast pump, or a significant pressure drop in the pump line such that the additional effect of changing head of water is negligible, or by active feedback control of the bleed or pump speed.

Once the sample volume has been drawn, it is necessary to empty the vessel. This is accomplished by closing inlet valve 4 and opening drain valve 6. Drainage might be forced by reversing the pump, or providing a separate drain pump (not shown).

The inlet port 3 is arranged near (or preferably at) the top of the chamber 2. so that liquid is between the port 3 and the membrane 11 for as little time in the filling cycle as possible. This will ease identification of correct filling using pressure sensing.

Alternative designs of pumping unit might be used. For instance, a liquid pump might be located in tube 1, using positive pressure to fill the vessel and expel air through the membrane 11. Two pumps might be used if necessary or desired.

In the illustrated embodiment, the chamber 2 as well as the entire sampling vessel system is kept vertical by mounting it on a gimbal arrangement (not shown).

The apparatus may be modified to allow for operation in a wider range of orientations, provided only that the vessel contains no areas in which gas pockets could become trapped and fail to reach the membrane 11. For example, a chamber ^ with walls that taper at an angle of x degrees to its axis will not trap gas, even if tilted up to an angle of 90-x degrees from the vertical (Figure 2a). In this embodiment, x will typically be from 5 to 80, in particular from 30 to 50.

If a greater inclination is required, a larger membrane 11, extending over an entire end of the chamber 2 may be provided (Figure 2b). In this case, any gas bubbles in the chamber 2 will rise to the top of the chamber to form a head space 18. The pump (not shown) acting to force gas in the direction of the arrow will continue to act provided the head space contacts the membrane 11. In this case a case a support mesh 17 is provided to prevent distortion of the membrane.

In an alternative arrangement (Figure 2c), the membrane is supported by means of an annular support 19, held within a correspondingly shaped end region 20 of the vessel 2. In this case, the available area of membrane for gas escape comprises a central region 21 and a circumferentially arranged area 22.

In the embodiment illustrated in Figure 3, a side wall 23 of the chamber is flexible, allowing adjustment or calibration of the sample volume by means of an actuator 24 in contact with the side wall 23.

The internal volume of the chamber may be adjusted by the screw 25 of the actuator 24 which are driven by a motor 26. Using suitable encoding means (not shown) on the screw, the position of the screw 25 and hence volume of the chamber 2 can be accurately determined. The volume of the chamber 2 is adjusted prior to or after taking a sample. A sample of the required volume can then be drawn into the vessel as described above.

Variations may be made to the aforementioned embodiments without departing from the scope of invention.