GB2263769A

GB2263769A - Vapour sampling device

Info

Publication number: GB2263769A
Application number: GB9301505A
Authority: GB
Inventors: Bernard Fields
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-01-31
Filing date: 1993-01-26
Publication date: 1993-08-04
Also published as: GB9301505D0

Abstract

A desorbable vapour sampling apparatus for the sampling and analysis of a component of a gas consists of a porous diffusion layer (7), an absorbing layer (8) and a solvent extraction chamber characterised by the solvent extraction chamber (11) being the cavity of a glass vial (6), the absorbing layer being between the diffusion layer and the solvent extraction chamber and outside of the solvent extraction chamber during sampling, the sampling rate of the apparatus as determined by the distance from the sampled gas to the absorbing layer being independent of the dimensions of the extraction chamber and the position of the absorbing layer during sampling and during solvent extraction being different. After exposure the absorbing layer (8) is transferred into the chamber (11), e.g. by detaching with a needle, and then solvent extracted. Sampler is attachable to clothing. HCHO in air may be detected. <IMAGE>

Description

Title: Desorbable vapour sampling device and method.

This application describes a diffusive air sampling device used in the measurement of average concentrations of air pollutants to which people may be exposed for example whilst working in a chemical factory. The device contains components for controlling the sampling rate, for absorbing the pollutant and for extracting the pollutant into a solvent after sampling so that the amount of pollutant can be measured for example by chromatography.

Measurements of time averaged concentrations of chemicals in air may be performed using an apparatus described in " Personal sampler for nitrogen dioxide ", by E D Palmes and A F Gunnerson J DiMattio and C Tomczyk published in Am. Ind.

Hyg. Assoc. J., vol 37, p570-577, 1976. This apparatus comprises of a hollow tube which is closed off by a surface at one end.

The surface is provided by a removable closure. The hollow tube acts as conduit in which substantially only diffusional transport of the gas occurs. Three steel gauzes placed one on top of the other on the surface provide a means by which a triethanolamine liquid reagent is held by surface tension between the wires of the gauzes. This liquid reagent is used to retain nitrogen dioxide during the exposure. In the above paper the liquid reagent was placed on the gauzes prior to exposure and the analysis was completed after exposure by manually adding a solvent containing a reagent which formed a colour with the absorbed analyte. Devices of this design however have poor accuracy because external air motion causes some convection in the conduit and most diffusive air samplers used in practice have some mechanism for reducing the effect of external air motion.

Another diffusive sampling apparatus described in U.S. Patent number 3,950,980 comprises of multiple gas flow attenuating layers separated from an absorbing layer by at least one air space. The attenuating layers serve the purpose of reducing convective air motion so that the air in the air space is placid. The diffusion of gaseous component in-the placid layer controls the sampling process. In this device the absorbing layer was physically removed from the sampling device in order to perform extraction of the absorbed component of interest.In a later U.S. patent number 4,102,201 the inventors of the apparatus in U.S. Patent number 3,950,980 described means for solvent extracting the reagent layer while it remained in the apparatus and means either for removing part of the sample for analysis or a significant proportion (for further preparation).

This patent claimed improved analytical precision by the avoidance of sample contamination associated with transferring the absorbing layer from a sampling device into a glass vial in a conventional manner. However there are a number of limitations associated with the procedures described in U.S. patent 4,102,201. These partly stem from the fact that the sampling chambers described in U.S. patent 4,102,201 and the earlier patents are not ideal as solvent extraction chambers being manufactured from polymers and of a shape and construction determined by the sampling requirements rather than the extraction requirements.Compared with a glass extraction vial these chambers lack good transparency so that visual indications of the efficiency of the solvent extraction are difficult to obtain, the chambers described are not completely impervious so that long term storage could lead to ingress of contaminants.

The use of polymers in the manufacture of the chamber walls limits the choice of solvents or conditions under which solvents can be used. An example in U.S. patent 4,102,201 requires the time for extraction using carbon disulphide to be strictly controlled to 30 minutes because of its interaction with the polypropylene walls of the sampling device. The location of the absorbent layer against the sampler base wall both hinders access of the solvent to the sample and creates an interdependency between the sampling rate on the one hand and the volume of solvent that can be used in the extraction on the other.Furthermore the device does not lend itself easily to automation because of its shape and because parts need to be exchanged or removed in order to perform solvent extraction.It is to avoid problems such as these that the standard practice for the solvent extraction of vapours sampled onto absorbents has been to perform such extractions in a glass analytical vial.

As will be apparent from the discussion in later sections the sensitivity of a diffusive sampling apparatus is inversely proportional to the distance between the absorbent layer and the sampled gas. In many analyses particularly where the device is being used to measure short exposures against low prescribed concentration limits, the analysis is only successful where this distance is small. It will be shown that the present invention may take the form of a very thin diffusion layer adjacent to the absorbent layer to provide a high sensitivity sampler which may be used in conjunction with a chromatographic'analysis.

The device of the present invention recognises the long established advantages of using a glass analytical vial for performing solvent extraction of analytes absorbed into an absorbent layer in an air sampling apparatus. Furthermore it recognises that for good sampling sensitivity the absorbent layer is best placed close to the external air and is ideally juxtaposed with any attenuating layer. Ideally also the dimensions of the solvent extracting chamber, which in this case is the cavity of the vial, are optimised dependent on the analytical finish. Therefore it is advantageous if the distance between the external air and the absorbent layer are independent of the dimensions of the extraction chamber. Hence in this invention the absorbent layer is located outside the extraction chamber during sampling and is transferred into the extraction chamber after exposure.Finally the device provides a system which results in free access of the extracting solvent to the chemicals within the absorbent layer during extraction because during extraction the absorbent layer is not attached to any part of the sampler. This provides efficient and rapid extraction.

The device of the present invention therefore utilises a glass analytical vial as an integral receptacle in a diffusive sampling apparatus where the absorbent layer is an involatile reagent immobilised on an inert porous medium. The absorbent layer is disposed across the vial opening or within the vial neck close to the external air but external to the extraction chamber (the vial cavity) so that the sampling rate of the device is not affected by the size of the vial which may be any size. The device provides a mechanism for physically transferring the whole absorbent layer into the vial cavity in a manner which prevents sample contamination and overcomes the limitations associated with the prior art outlined above. The device may be sealed after exposure using a metal foil lined cap to provide a truly impervious container for the absorbent layer to prevent contamination prior to analysis. During extraction the solvent has free access to the absorbent layer within the vial. The device allows new analytical procedures to be conveniently applied for example the use of very small amounts of extraction solvent to improve sensitivity. Since the extraction may take place in a standard analytical vial it allows the use of standard laboratory equipment (sample vial shakers) two assist the extraction and allows the automation of analysis when using the device in conjunction with an autosampler.

Objectives of the invention One objective of the invention is provide a desorbable diffusive vapour sampling device which is simple, inexpensive, and disposable.

Another objective is to provide a diffusive vapour sampling device incorporating a glass analytical vial for the purpose of performing solvent extraction so that the device can be used in accordance with standard procedures and equipment and so that samples can be readily processed using an autosampler.

Another objective of the invention is to provide an analytical procedure for vapour sampling and analysis with advantages of completely impervious containment of sample during storage, of efficient extraction and of visibility of the extraction process.

Another objective is to provide a flexible diffusive vapour sampling system incorporating a glass extraction vial where the sampling rate of the device is selectable in a manner which is independent of the selectable solvent extraction chamber dimensions.

Another objective of the invention is to produce a diffusive vapour sampling device with improved analytical sensitivity whereby the device is designed to allow maximum uptake of sampled components and achieve recovery from the absorbing layer with small amounts of solvent.

Another objective is to provide a mechanism for transferring the absorbent layer of a diffusive sampling device into a standard glass extraction vial in a manner which is rapid, safe and convenient and avoids contamination of the sample during transfer.

Embodiments of the preferred forms of the invention The sampling apparatus described here is designed such that it allows the solvent extraction of absorbed components in an integral impervious, inert, transparent glass analytical vial.

The dimensions of the vial are selectable depending on the requirements of the analytical finish used and the requirement of automated sample processing when the device is used in conjunction with prior art means of sample processing.

In one form of the invention illustrated in figure 1 the apparatus consists of a member 1 with a flat end wall 2 containing an opening 3 which defines the extent of the diffusive access into the device. The member also contains walls 4 perpendicular to the end wall with an internal screw thread 5 for the attachment of a glass vial 6. A diffusion element 7 consists of a right cylinder of porous polymer which lies with one end across opening 3 in member 1 such that access from the outside must be through the diffusion element. The diffusion element consist of a porous polymer such as polyethylene or polypropylene where the average pore size may be in the range 0.2 to 250.micrometers but is normally about 20 micrometers so that bulk gas diffusion described by Fick's law normally applies to the motion within the pores.The absorbent layer 8 consists of at least one woven polymer gauze bearing an involatile reagent. The absorbent layer has a diameter larger than the internal diameter of the vial neck and smaller than the internal diameter of walls 4. A separator layer 9 for the purpose of preventing contamination of the diffusion element 7 with reagent from the absorbent layer consists of at least one woven polymer gauze. The absorbent layer is held in a fixed position outside the vial neck 10 when the glass vial is attached to member 1 using the screw thread 5.

When the apparatus is required to measure exposure to persons it is placed in a vial holder which has a facility for attachment to clothing. After exposure the absorbent layer is transferred into the vial cavity 11 for storage or analysis by unscrewing slightly member 1 to loosen the gauzes of the absorbent and separator layers then inserting the transfer tool illustrated in figure 2 through the diffusion element 7. The transfer tool consists of a pointed needle 12 protruding a short distance from a concentric flat ended tube 13. Needle 12 is attached to a holding rod 14. Needle 12 and tube 13 penetrate the soft diffusion element 7 and needle 12 penetrates the woven gauzes of the separator and absorbent layers. Tube 13 is unable to penetrate these layers because it is flat and of a substantially greater diameter than the separation of the gauze filaments.

Tube 13 pushes the gauzes through the vial neck into the top of the vial cavity. When the transfer tool is withdrawn the gauzes are pulled from needle 12 bathe top of the vial cavity because the diameter of the gauzes is greater than the diameter of the vial neck. After transfer of the absorbent layer into the vial cavity the vial may be sealed with a solid cap lined with a thin aluminium disc to enclose the sample in an wholly impervious contaminant free environment for storage prior to analysis.

When the contents of the absorbent layer are to be analysed solvent is added and the vial sealed with an impervious cap and agitated to effect dissolution of the absorbed components. Since the vial cavity is of greater diameter than the absorbent layer gauzes the gauzes move freely in the solvent liquid and this helps to effect an efficient and rapid extraction. Solvent may also be added and withdrawn through a septum held in place with a vial cap which contains an opening. Such a procedure allows the analysis procedure to be automated once the absorbent layer has been pushed into the vial cavity.

Where the diffusion element consists of a hard or thick material a small hole is made through the diffusion element during sampler preparation for ease of insertion of an absorbent layer transfer tool. The area of the hole is less than ca. 10% and preferably about 2% of the exposed area of the diffusion element so that its effect on the nature of the sample uptake is small.

In a second form of the invention shown in figure 3 the absorbent layer is a compressible porous polymer disc 15 bearing an involatile reagent. The disc is held in a fixed position in the vial neck by virtue of internal forces in the disc reacting to its compressed state. A porous polymer diffusion element disposed across the vial opening and held in place by a screw cap limits convective motion within the sampling device.

Optionally a diffusive air gap 16 serves to attenuate diffusive uptake and to prevent reagent migration into the pores of the diffusion element. After exposure the diffusion element and screw cap may be discarded and replaced with a solid cap lined with an aluminium thin disc to enclose the sample in an wholly impervious contaminant free environment for storage prior to analysis. A solid cap may also be used prior to exposure to prevent contamination of the absorbent layer during storage of the sampler. When the analysis is to be performed the solid cap is removed and the absorbent layer is pushed into the vial cavity. Solvent is added and the vial sealed with an impervious cap and agitated to effect dissolution of the absorbed components.

When the sample has been extracted into the solvent in the analytical vial it may be sealed with a septum in a conventional manner and placed in an autosampler, for example the Gilson 231/401 Auto-sampling Injector manufactured by Gilson Medical Electronics Inc. Box 27, 3000 W Beltline Hwy., Middleton, WI 53562, USA which will automatically inject a series of samples into an HPLC analyser.

DESCRIPTION OF MASS UPTAKE OF THE INVENTION The mass uptake of the sampler M where an efficient absorbing medium is used and where bulk gas diffusion applies may be estimated for a component using Ficks law in its following form: M=DPACat/Lt Equation (1) where M is the mass of component collected expressed in grammes, D is the binary diffusion coefficient of the component in the sampled gas expressed in square centimetres per second, P is the dimensionless fractional void volume of the diffusion element, A is the cross sectional area expressed in square centimetres of the diffusion path perpendicular to the net flux of component, C is the concentration of the component outside of the a apparatus expressed in grammes per cubic centimetre, t is the sampling time expressed in seconds, L is the length of the diffusive path expressed in centimetres, and t is the tortuosity of the pores in the diffusion path. Where the diffusion path takes the form of an open conduit P and t are both equal to unity. Where the diffusion path consists of a porous polymer part and an open conduit part a change in slope of the concentration gradient of gas components occurs at the interface of the two parts however data provided in the examples shows that the uptake of the sampler is proportional to the average concentration of gas component outside of the sampling device.

The small orifice (where present) in the diffusion element described in the first form of the invention and in example 2 represents about 2% of the exposed area of the element and has a negligible effect on mass uptake.

The parameters relating to the diffusion element P,A L and 6 are controlled in its production and can be readily measured and the diffusion coefficient in air for many analytes is known and published. A theoretical uptake for many analytes can therefore be calculated. Preferably however mass uptake rates for individual compounds are be determined by exposure of the device to atmospheres of known concentration and measurement of the amount of component absorbed in a fixed time so that a characteristic mass uptake for any sampler design and any component may be determined which is dependent only on the concentration to which the sampler is exposed and the time of exposure.Therefore if this mass uptake is known a measurement of the time of exposure and the mass of component absorbed by the sampler in use enables a calculation of the average concentration to which the sampler was exposed.

The materials of construction are important in the correct functioning of the apparatus and depend on the need to immobilise the reagent which may be a liquid and the inertness of the parts of the apparatus toward the chemicals used for a particular analysis. Polypropylene is suitable for member 1 and is commercially available as a standard vial cap. Polypropylene, polyethylene or polytetrafluoroethylene woven polymer gauzes of filament diameter approximately 0.lmm, aperture size approximately 0.lmm and open surface area of approximately 25% are suitable for the absorbent layer substrate 8 and the separator gauze 9 but wide variations on these parameters are also useable. Plain or twill weaves are effective but twill weaves are preferred.

Porous polypropylene and polyethylene sheets of various void volumes and pore sizes in the range 0.2 to 250 micrometers but normally about 20 micrometers are suitable for the diffusion element 7 and the absorbent layer substrate 15.

An important aspect of the invention is the ability to choose the the volume of the glass vial without affecting the sampling rate so that the vial cavity volume is suited to the amount of solvent required for a convenient analytical finish. The combination of the use of the invention in the form of figure 1 where the sampling rate is very high (L is small) together with the use of a small solvent volume for the extraction solution provides a device sensitive to low concentrations of pollutants for short exposure periods. When vials of standard size are used this has the advantage of allowing an analytical finish using an autosampling system to automate analysis for instance by gas chromatography, high performance liquid chromatography or flow injection analysis. Vials manufactured from special glasses such as amber glass or borosilicate glass may be preferred for some analyses.

Example 1 An absorbent layer was prepared from a 9mm diameter disc of twill woven polypropylene fabric 100 micrometer aperture size and 0.lmm thread diameter which was saturated by dipping in a reagent comprising of 1%v/v orthophosphoric acid, 3%v/v methanol and 95%v/v acetonitrile saturated with 2,4 dinitrophenylhydrazine. The volatile solvents were evaporated off in a stream of air. A diffusion element was prepared from a 9mm diameter, lmm thick disc of porous polypropylene of fractional void volume of 49% and average pore size of 18 micrometers sold in the United Kingdom by Porvair Ltd under the trade name Vyon T which was placed in an inverted polypropylene screw cap which had a 7mm diameter round aperture in its top.A separator layer was prepared from a clean 9mm disc of twill woven polypropylene fabric 100 micrometer aperture size and 0.lmm thread diameter. The separator layer was placed on the diffusion element in the cap followed by the absorbent layer.

The cap was screwed onto a 12mm diameter 32mm high glass vial with a 6.7mm internal diameter neck and a vial cavity internal diameter of 9.5mm. This vial is suitable for use in most commercially available autosamplers. 34 such samplers were prepared. 29 were exposed to known concentrations of formaldehyde at concentrations and for the time given in the results, six at each concentration except for the exposures at 2.07ppm where five samplers were exposed. One sampler was analysed unexposed at each concentration and used as an experimental blank. A transfer tool was prepared by removing the pin head from a 0.7mm diameter 30mm long stainless steel domestic pin. The cut end was pushed into a 0.4mm diameter 1.5cm deep hole drilled into the centre of a 5mm diameter 80mm long PTFE rod so that the PTFE rod held the pin firmly. The pin was allowed to protrude 13mm from the end of the PTFE rod. An llmm length of PTFE tubing 1.2mm outside diameter and 0.6mm internal diameter was prepared with both ends cut at right angles to the length of the tube.

The cut length was pushed over the protruding pin until the tubing made contact with the PTFE rod. The point of the pin was left protruding 2mm from the end of the tubing.

After exposure the absorbent layer and separator layer were pushed into the cavity of the vial by first loosening the cap slightly then pushing the transfer tool through the diffusion element and into the glass vial and withdrawing. The absorbent layer and separator layer gauzes fell to the bottom of the vial cavity when the transfer tool was withdrawn. 150 microliters of a 50% v/v mixture of acetonitrile and water was added from a micropipette and the vials were capped and the contents swirled to effect dissolution. The mass of formaldehyde absorbed by the samplers was determined by high performance liquid chromatography.

Results of example 1 The average mass of formaldehyde hydrazone in the vials corrected for the blanks was as follows.

Formaldehyde Exposure Average Coefficient of Concentration ppm time min Mass sampled ng Variation 95 1.94 10 121 10.0 2.07 20.5 303 3.8 1.00 30 250 2.6 1.00 10.5 84 2.1 1.94 20 308 4.6 The mass of formaldehyde sampled was proportional to the concentration of formaldehyde and the time of exposure within approximately 10%. The repeatability was acceptable for air analysis being in all cases 10% or less. The method was suitable for the determination of short term exposures of formaldehyde in the region of the current UK exposure limit of 2ppm.

Example 2 An 9mm diameter, 2mm thick disc of porous polypropylene of fractional void volume of 49% and average pore size of 18 micrometers, sold in the United Kingdom by Porvair Ltd under the trade name Vyon T, containing a 1.2mm hole drilled through its centre was placed in an inverted polypropylene screw cap which had a round opening in the top 7mm in diameter. A 9mm separator disc of twill woven polypropylene gauze 100 micrometer aperture size and 0.lmm thread diameter was placed over the porous polypropylene disc. A 9mm disc of twill woven polypropylene gauze 100 micrometer aperture size and 0.lmm thread diameter was dipped into a solution containing 3% by volume methanol, 1% by volume phosphoric acid and 96% by volume of a saturated solution of 2,4 dinitrophenylhydrazine in acetonitrile.The reagent impregnated disc was dried in a stream of air and the impregnating process repeated once. The reagent impregnated disc was placed on the separator disc. The cap and contents were screwed onto a 12mm diameter 32mm high glass vial which contained a screw thread. Seventeen such devices were prepared.

The devices were exposed to formaldehyde test atmospheres containing 0.5 ppm formaldehyde for different exposure periods of 62, 258 and 485 minutes. After exposure the polypropylene gauze discs were displaced into the glass vial by inserting a 1.2mm plastic tube with a stainless steel needle protruding lmm from the tube centre through the hole in the porous polymer disc. The cap and its remaining contents were removed and 0.5ml of a buffered solution of 50% v/v acetonitrile 50% v/v water was added. The vial was sealed with a solid polypropylene cap and the contents swirled until the reagent was visibly dissolved from the polypropylene gauze (approximately 30 seconds). The vial contents were analysed by HPLC to determine the mass of formaldehyde hydrazone sampled. Three unexposed blank sample vials were similarly prepared and analysed.

Results of example 2 The average mass of formaldehyde hydrazone in the vials corrected for the blanks was as follows. The blanks contained an average of 18ng Formaldehyde Exposure Average Concentration ppm time min Mass sampled ng 0.53 485 1547 0.53 258 808 0.54 62 193 The mass of formaldehyde sampled was proportional to the concentration of formaldehyde and the time of exposure.

Example 3 A 9m diameter, 2mm thick disc of porous polypropylene of fractional void volume of 49% and average pore size of 18 micrometers, sold in the United Kingdom by Porvair Ltd under the trade name Vyon T was placed in an inverted polypropylene cap which had a round opening in the top 7mm in diameter. A 7 mm diameter, 2mm thick disc of the same porous polymer was placed on a petri dish and loaded with 30 microliters of a solution containing 39Ó by volume methanol, 1 by volume phosphoric acid and 96% by volume of a saturated solution of 2,4 dinitrophenylhydrazine in acetonitrile. The reagent impregnated disc was dried in a stream of air at 500C.The reagent impregnated disc was inserted into the neck of a 1.8 millilitre (mL) capacity screw threaded glass vial by placing the disc on a firm surface, inverting the vial and pressing the vial opening (approximately 6.8mm in diameter) onto the disc. The outermost surface was left exactly 2mm from the vial opening using a 2mm long spacer pushed into the opening once the disc was in the neck. The polypropylene cap and 9mm porous polymer disc was attached to the vial using the screw thread. Eighteen such vials so prepared were exposed to dynamically generated test atmospheres of formaldehyde of known concentration. Six were exposed to an atmosphere of 1.95ppm v/v (parts per million on a molar basis) formaldehyde for 244 minutes. Six were exposed to an atmosphere of 2.0lppm formaldehyde for 120 minutes.Six were exposed to an atmosphere of 2.08ppm formaldehyde for 31 minutes.

Three vials prepared in the same way were retained as analytical blanks. After exposure the diffusion element and caps were removed from the vials and a solid polypropylene cap lined with a 9mm disc of aluminium foil was used to seal the vials for overnight storage. The contents of the vials were extracted by pushiny the absorbent layer disc into the vial cavity, adding 0.5ml of a buffered solution of 50% v/v water and 50% v/v acetonitrile sealing the vial with a solid cap ad agitating at 400C for 30 minutes. The vial contents were analysed for formaldehyde hydrazone by HPLC.

Results of example 3 The average mass of formaldehyde hydrazone in the vials corrected for the blanks was as follows. The blanks contained an average of 18 nanogrammes (ng).

Formaldehyde Exposure Average Coefficient of Concentration ppm time min Mass sampled ng Variation 90 1.95 244 1720 3.9 2.01 120 866 10.7 2.08 31 227 9.2 The mass of formaldehyde sampled was proportional to the concentration of formaldehyde and the time of exposure. The repeatability was acceptable for air analysis being in all cases less than 11%.

Claims

1. A desorbable vapour sampling apparatus for the sampling and analysis of at least one component of a gas consisting of a porous diffusion layer, an absorbing layer and a solvent extraction chamber charachterised by the solvent extraction chamber being the cavity of a glass vial the absorbing layer being between the diffusion layer and the solvent extraction chamber and outside of the solvent extraction chamber during sampling the sampling rate of the apparatus as determined by the distance from the sampled gas to the absorbing layer being independent of the dimensions of the extraction chamber the position of the absorbing layer during sampling and during solvent extraction being different

2.An apparatus as claimed in claim 1 with a solid member which contains an end wall with an opening which defines the diffusive access into the device and side walls which contain a means of removably attaching a glass vial in a manner which forms a secure seal. The solid member is attached to a glass vial and holds in place a porous diffusion layer and an absorbing layer between the end wall of the member and the vial opening.

3. An apparatus as claimed in claim 1 with a solid member which contains an end wall with an opening which defines the diffusive access into the device and side walls which contain a means of removably attaching a glass vial in a manner which forms a secure seal. The solid member is attached to the glass vial and holds in place a porous diffusion layer between the end wall of the member and the vial opening. An absorbing layer is disposed within the vial neck which has a smaller internal diameter than the vial cavity.

4. An apparatus as claimed in any of claims 1-3 where the porous diffusion layer is separated from the absorbing layer by an air diffusion gap or a polymer gauze or a combination of the the two.

5. An apparatus as claimed in any of claims 1-4 where the porous diffusion layer is a porous polymer disc or a porous polymer disc with a small hole through the disc.

6. An apparatus as claimed in any of claims 1-5 where the absorbing layer is a reagent immobilised within the filaments of a polymer gauze or within the pores of a porous polymer disc.

7. An apparatus as claimed in any of claims 1-6 where the vial has a neck and a cavity and the absorbing layer is a reagent bearing polymer gauze in the shape of a disc of diameter greater than the internal diameter of the vial neck and less than the internal diameter of the vial cavity.

8. An apparatus as claimed in claims 1-7 where the vial is suitable for use in an autosampler.

9. An apparatus as claimed in any of claims 1-8 provided with a means of attaching it to clothing.

10. An apparatus as in any of claims 1-9 where the absorbing layer contains acidified 2,4 dinitrophenylhydrazine.

11. An apparatus as claimed in claim 10 where the apparatus is used in the determination of formaldehyde in air

12. An analytical method for the sampling and solvent extraction of at least one component of a gas using an apparatus of a design as in any of claims 1-11 characterised by the internal relocation of the absorbing layer from its fixed sampling position into the extraction chamber such that the absorbing layer is then disconnected from any part of the apparatus, followed by the addition of a solvent such that the solvent has free access to the absorbing layer followed by the analysis of an aliquot of the extracted sample.

13. An analytical method as claimed in claim 12 where a rod or a flat ended rod equipped with a fine protuberance is used to push the absorbing layer from its sampling position into the vial cavity.

14. An analytical method as claimed in claim 13 where the solid member and diffusion layer is removed from the vial after exposure and a rod is used to push the absorbing layer into the vial cavity where a solvent is added to extract the sample, an aliquot of which is analysed.

15. An analytical method as claimed in claim 13 where a rod or a flat ended rod equipped with a fine protuberance is pushed through the diffusion layer and pushes the absorbing layer into the vial cavity where a solvent is added to extract the sample, an aliquot of which is analysed.

16. An analytical method as in claims 12-15 where the method is for the determination of formaldehyde in air.