REAL-TIME VAPOUR EXTRACTING DEVICE

Abstract

The invention provides a device for extracting a vaporizable analyte from a sample gas containing the analyte and unwanted aerosol particles; the device comprising a vapour extraction chamber (2), first (3) and second (4) inlets, and first (5) and second (6) outlets; the vapour extraction chamber (2) being provided with or being linked to a heat source for heating the vapour extraction chamber (2) to a desired temperature to facilitate vaporization of analyte present in the sample gas; the first (3) and second (4) inlets being linked to an upstream end of the vapour extraction chamber (2) and the first (5) and second (6) outlets being linked to a downstream end of the extraction chamber (2); the first inlet (3) allowing a sample of gas containing the analyte and unwanted aerosol particles to be introduced into the vapour extraction chamber (2); the second inlet (4) being connected or connectable to a clean gas supply that does not contain the analyte or unwanted aerosol particles; the device being configured such that, in use, a sample gas flow (7) is established through the vapour extraction chamber between the first inlet (3) and the first outlet (5), and a clean gas flow (8) is established through the vapour extraction chamber between the second inlet (4) and the second outlet (6); whereby analyte in vapour form present in the sample gas flow (7) diffuses into the clean gas flow (8), but the clean gas flow (8) reaching the second outlet is substantially free of the said unwanted aerosol particles; the first outlet (5) serving as a waste outlet for the sample gas flow, and the second outlet (6) being connected or connectable to an instrument for analysing analyte that has diffused into the clean gas flow.

Claims

1. A device for extracting a vaporizable analyte from a sample gas containing the analyte and unwanted aerosol particles; the device comprising a vapour extraction chamber, first and second inlets, and first and second outlets; the vapour extraction chamber being provided with or being linked to a heat source for heating the vapour extraction chamber to a desired temperature to facilitate vaporization of analyte present in the sample gas; the first and second inlets being linked to an upstream end of the vapour extraction chamber and the first and second outlets being linked to a downstream end of the extraction chamber; the first inlet allowing a sample of gas containing the analyte and unwanted aerosol particles to be introduced into the vapour extraction chamber; the second inlet being connected or connectable to a clean gas supply that does not contain the analyte or unwanted aerosol particles; the device being configured such that, in use, a sample gas flow is established through the vapour extraction chamber between the first inlet and the first outlet, and a clean gas flow is established through the vapour extraction chamber between the second inlet and the second outlet; whereby analyte in vapour form present in the sample gas flow diffuses into the clean gas flow, but the clean gas flow reaching the second outlet is substantially free of the said unwanted aerosol particles; the first outlet serving as a waste outlet for the sample gas flow, and the second outlet being connected or connectable to an instrument for analysing analyte that has diffused into the clean gas flow.

2. A device according to claim 1 wherein the first outlet is provided with a particle filter to remove particles prior to release into the environment.

3. A device according to claim 1 wherein the first outlet is provided with a filter for removing any volatile compounds remaining in the sample gas stream as it passes to waste through the first outlet.

4. A device according to claim 2 wherein an activated black carbon filter is provided upstream or downstream of a particle filter.

5. A device according to claim 1 comprising a low pulsation clean flow maintaining system for supplying a clean gas flow Q.sub.clean to the second inlet.

6. A device according to claim 1 comprising a flow maintaining system that enables low pulsation gas flows to be generated with Qi/Qi<7%, where Qi is the average magnitude of pulsations in flow rates through each of the first and second inlets and the first and second outlets.

7. A device according to claim 1 wherein the vapour extraction chamber, inlets, outlets and any associated conduits, if present, are configured so as to maintain laminar flow of the sample gas flow and the clean gas flow though the device so that unwanted aerosol particles are preferably directed to the first outlet.

8. A device according to claim 1 which is configured such that sample gas flow and clean gas flow through the device is characterised by a Reynolds number (Re) of <2,300.

9. A device according to claim 1 which is provided with a heat source that can be operated heat the vapour extraction chamber to a temperature T.sub.h of up to 700 C.

10. A device according to claim 1 wherein the vapour extraction chamber is cylindrical in shape.

11. A device according to claim 1 wherein the vapour extracting chamber has a length greater than a length, in centimetres, defined by a non-equality:
L>1.2*(Q.sub.sample+Q.sub.clean)*(T.sub.a/T.sub.h) where Q.sub.sample and Q.sub.clean are the gas flow rates of the sample gas flow and clean gas flow respectively, T.sub.a is the ambient temperature, T.sub.h is the temperature in the vapour extraction chamber, wherein both T.sub.a and T.sub.h are in degrees K and the flow rates are in cm.sup.3/s.

12. A device according to claim 1 wherein the vapour extraction chamber has an internal diameter D, in centimetres, defined by a non-equality:
D<0.5*(Q.sub.sample+Q.sub.clean)*(T.sub.a/T.sub.h) where Q.sub.sample and Q.sub.clean are the gas flow rates of the sample gas flow and clean gas flow respectively, T.sub.a is the ambient temperature, T.sub.h is the temperature in the vapour extraction chamber, wherein both T.sub.a and T.sub.h are in degrees K and the flow rates are in cm.sup.3/s.

13. A device according to claim 1 wherein the first outlet is connected to a heat exchanger to reduce the temperature of the waste gas emerging from the waste outlet, and optionally wherein the heat exchanger is linked at a downstream end thereof to an aerosol filter.

14. A device according to claim 1 wherein the first outlet is linked via one or more purification elements to the clean gas supply inlet thereby enabling recycling of the waste gas flow to take place.

15. A combination of a device for extracting a vaporizable analyte from a sample gas containing the analyte and unwanted aerosol particle as defined in claim 1, and an analytical instrument connected thereto.

16. A combination of a device for extracting a vaporizable analyte from a sample gas containing the analyte and unwanted aerosol particle as defined in claim 1 and an analytical instrument connected thereto via an aerosol formation killer device.

17. A combination comprising a plurality of devices as defined in claim 1 connected in parallel or in series, wherein an aerosol formation killer device is connected to a second outlet of any one or more of the plurality of devices.

18. A combination according to claim 16 wherein: (a) the device is a miniature VED device having a vapour extraction chamber of internal diameter D<5 mm and a length L<50 mm which is connected to a handheld IMS; or (b) the device is a VED of an axial symmetry design which is connected to a portable IMS; or (c) the device is a miniature VED having a vapour extraction chamber of internal diameter D<6 mm and a length L<80 mm which is connected to a handheld MS; or (d) the device is a VED of an axial symmetry design having a vapour extraction chamber of internal diameter D<6 mm and length L<290 mm which is connected to a portable MS; or (e) the device is a miniature VED device having a vapour extraction chamber of internal diameter D<5 mm and length L<360 mm which is connected to a handheld GC; or (f) the device is an axial symmetry design having a vapour extraction chamber of inner diameter D<6 mm and length L<110 mm which is connected to a portable/stationary GC.

19. A method for extracting a vaporizable analyte from a sample gas containing the analyte and unwanted aerosol particles; which method comprises passing the sample gas through a device comprising a vapour extraction chamber, first and second inlets, and first and second outlets; the vapour extraction chamber being provided with or being linked to a heat source which heats the vapour extraction chamber to a desired temperature to facilitate vapourisation of analyte present in the sample gas; the first and second inlets being linked to an upstream end of the vapour extraction chamber and the first and second outlets being linked to a downstream end of the extraction chamber; the first inlet allowing a sample of gas containing the analyte and unwanted aerosol particles to be introduced into the vapour extraction chamber; the second inlet being connected to a clean gas supply that does not contain the analyte or unwanted aerosol particles; such that a sample gas flow is established through the vapour extraction chamber between the first inlet and the first outlet, and a clean gas flow is established through the vapour extraction chamber between the second inlet and the second outlet; whereby analyte in vapour form present in the sample gas flow diffuses into the clean gas flow; whereby waste sample gas passes out of the first outlet, and the clean gas flow containing analyte that has diffused into the clean gas flow passes out of the second outlet and is directed to an instrument for analysing the analyte, but wherein the clean gas flow passing out through the second outlet is substantially free of the said unwanted aerosol particles, said unwanted aerosol particles instead remaining predominantly in the sample gas flow and being directed to the first outlet.

20. A method according to claim 19 wherein the device is as defined in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0103] FIG. 1 is a schematic longitudinal sectional view of a VED according to a first embodiment of the invention.

[0104] FIG. 2 is a schematic view of the VED of FIG. 1 but with an aerosol HEPA filter connected in-line with the waste outlet.

[0105] FIG. 3 is a schematic view of the VED of FIG. 1 set up to clean and recycle waste sample gas which is then re-used as a clean gas flow.

[0106] FIG. 4 is a schematic longitudinal sectional view of a VED according to an embodiment of the invention which has axial symmetry.

[0107] FIG. 5 is a plot of the VED temperature against the concentration of analyte (expressed as a percentage) in the vapour outlet compared to the concentration in the waste outlet obtained from a VED having an axial symmetry geometry. The sample gas flow rate was 0.135 l/min; the vapour extraction flow rate was0.2 l/min; and the waste flow rate0.16 l/min.

DETAILED DESCRIPTION OF THE INVENTION

[0108] The invention will now be illustrated, but not limited, by reference to the specific embodiments shown in the drawings and described below.

[0109] In the specific embodiments below, the operation of the devices may be discussed with reference to air flows (e.g. sample air flow and clean air flow) through the device but it will be understood that other gases may be substituted for air.

[0110] FIG. 1 illustrates a real-time VED device 1 according to a first embodiment of the invention. The VED device comprises a vapour extraction chamber 2 which can be heated (heating element not shown) to maintain an elevated temperature at a predefined level. The vapour extraction chamber 2 has two inlets 3 and 4 and two outlets 5 and 6.

[0111] Inlet 3 (the first inlet) is connected by inlet conduit 3c (the first inlet conduit) to the upstream end of the vapour extraction chamber 2. Inlet 4 (the second inlet) is connected to the upstream end of the vapour extraction chamber 2 by inlet conduit 4c (the second inlet conduit).

[0112] Outlet 5 (the first outlet) is connected by outlet conduit 5c (the first outlet conduit) to the downstream end of the vapour extraction chamber. Outlet 6 (the second outlet) is connected by outlet conduit 6c (the second outlet conduit) to the downstream end of the vapour extraction chamber 2.

[0113] In use, the vapour extraction chamber is heated to a desired temperature in order facilitate evaporation of analyte compounds of interest. A sample of air (or another sample gas) containing aerosol particles and vapour is introduced through the first inlet 3 into the conduit 3c. A stream of clean air (or another clean gas) without aerosol particles is introduced through the second inlet 4 into the conduit 4c. At the upstream entrance to the vapour extraction chamber 2, the sample air flow coming from the conduit 3c and the clean air flow coming through conduit 4c are joined together to form a non-uniform (but preferably laminar) joint flow containing a sample flow section and a clean air section where the two air masses move in parallel and in close proximity. During their passage through the vapour extraction chamber, volatile analyte compounds in the sample flow section that enter the chamber 2 in the form of particles 7 are vaporized and at least a proportion of the vaporized analyte compounds diffuse into the clean air section 8 of the joint flow. Non-volatile residual particles 7 are carried out to the waste flow outlet 5 via the conduit 5c and are either released directly into the atmosphere or (more preferably) are first passed through a high capacity HEPA filter 9 (see FIG. 2) before released into the atmosphere.

[0114] The clean air section 8 of the joint flow containing volatile analyte compounds that have diffused from the sample air-flow section passes along the conduit 6c to the second outlet 6 from which it is directed to an instrument (e.g. an IMS) that analyses analyte compounds of interest.

[0115] In this way the vapour is extracted from aerosol particles in the sample flow and the vaporized analytes can then to be analysed with a vapour quantifying analytical instrument. At the same time, non-volatile residual particles 7 are released through the waste outlet 5. Thus, non-volatile residual particles do not pass into the analytical instrument to any significant extent and therefore damage to the instrument that might otherwise have been caused by such particles is avoided.

[0116] A further advantage of the device of the invention is that there is no heated in-line filter that can eventually become clogged. A high-capacity HEPA filter, if used in the waste flow, requires a long time to be completely clogged and, in any event, the extent of loading of the filter does not affect the performance of the VED because analytes do not enter the waste flow to any significant extent.

[0117] A first important factor governing the performance of the VED device of the invention is the flow regime. This factor can be referred to as a VED laminarity criterion. Thus, for efficient performance, the gas flow in the vapour extraction chamber should be substantially laminar to stop aerosol particles becoming entrained in the clean gas (e.g. air) flow. Typically, the Reynolds number (Re) is <2,300. Preferably the Reynolds number (Re) is <2,000 and more preferably the Reynolds number (Re) is less than 1,700.

[0118] The VED laminar criterion can be tested by measuring a fraction of the aerosol particles in the second outlet when the VED is not heated. Thus, it is important to maintain the non-uniformity of the gas flow, and the spatial separation of the two streams (aerosol laden stream and the initially clean air stream) forming the gas flow, along the length of the vapour extraction chamber 2.

[0119] The vapour extraction is based upon the difference in diffusion of aerosol particles and analytes. Diffusion coefficients of aerosol particles normally are many orders of magnitude lower than diffusion coefficients of analyte molecules.

[0120] This difference ensures that non-volatile residual particles 7 remain in the sample flow section of the non-uniform flow inside the chamber 2 and are carried out through the waste flow conduit 5c and the waste outlet 5. Because the extraction process involves diffusion from one gas stream to another, it is important to avoid turbulent mixing of the two gas streams.

[0121] The establishment of laminar flow and the avoidance of turbulent flow and mixing can be assisted by ensuring that the surfaces of the interior of the device that are in contact with the gas flows are as smooth as possible and that sharp edges and other formations that lead to turbulence are avoided. The manner in which this can be achieved will be readily apparent to the skilled person.

[0122] A second important factor influencing the performance of the VED is the length of the vapour extraction chamber 2. The length of chamber 2 should be great enough to enable vapour to be evaporated from aerosols efficiently. It should be noted that the efficiency of evaporation is influenced by the temperature of the chamber T.sub.h. For a given analyte, the minimum necessary length of the chamber and the optimal heating temperature T.sub.h can be determined empirically by trial and error experimentation.

[0123] A further factor influencing the performance of the VED can be defined as the buoyancy restriction or buoyancy criterion. Inside the vapour extraction chamber 2 the central section is cooler than the section near the internal surface wall bounding the chamber. This temperature difference generates convection flows due to expansion of the gas when the temperature is increasing. In order to prevent buoyancy arising from the temperature difference from causing mixing of the two sections of the non-uniform flow in chamber 2, a restriction may be placed on the maximal diameter D of the vapour extracting chamber 2.

[0124] An example of the VED buoyancy criterion (which is an indicative criterion) is D <0.5*(Q.sub.sample+Q.sub.clean)*(T.sub.a/T.sub.h). For each geometry and operation regime, the minimum diameter of the chamber and the optimal heating temperature T.sub.h can be determined empirically by trial and error experimentation.

[0125] FIG. 2 illustrates a VED of the type shown in FIG. 1 but wherein an aerosol filter 9 is connected by tubing 10 to the waste flow outlet 5 to reduce contamination of the environment with particulate matter in the sample gas flow. The aerosol filter can be a HEPA filter or any other filter of sufficient capacity.

[0126] FIG. 3 shows an arrangement in which the waste gas flow 7 is cleaned and recycled to be used as the clean gas (e.g. air) supply. The waste flow 7 containing non-volatile residual particles is directed through the outlet 5 to the first aerosol filter 9 via tubing 10. A pump 11 directs the flow of filtered air to the second filter 12 and finally to an activated black carbon filter 13. In this arrangement, the waste flow initially is cleaned with the first filter 9 that removes residual non-volatile particles from the flow, the second filter 12 removes particles that might be generated by the pump 11 and finally the activated black carbon filter 13 removes traces of analytes from the waste flow. The cleaned air flow the enters the clean air inlet 4. A valve 14 attached to a bleed line (shown with an arrow) is provided so that adjustments to the flow rates of air being recycled can be made and removal of the non-volatile residual aerosol particles can be optimised. The optimal flow rates can be determined by trial and error.

[0127] FIG. 4 illustrates a VED according to another embodiment of the invention. In this embodiment, the VED has an axial symmetry.

[0128] In this embodiment, a sample air flow containing aerosol particles enters the first inlet 3 and passes along a short region (the first inlet conduit) of restricted width which opens out into the main body of the vapour extracting chamber 2. Non-volatile residual particles are carried straight along the chamber 2 with the waste air flow, through a further short region of restricted width (the first outlet conduit) at the downstream end of the chamber 2 to the waste flow outlet 5 (the first outlet).

[0129] The clean air flow enters the device through the clean air inlet (the second inlet) 4 and passes through the axial symmetry conduit 15 (the second inlet conduit) that has a circular slot 16 providing communication with the vapour extraction chamber and enabling the formation of an axially symmetrical flow of the clean air around the sample flow. The clean air flow and sample flow come together into a non-uniform axially symmetrical flow containing a sample flow 7 in the centre and a sheath of clean air flow around it. Provided that the two air flows are laminar according to the VED laminarity criterion, and there is no turbulence or convection mass transfer in the chamber 2, the aerosol particles 7 remain in the central section of the non-uniform flow, but volatile compounds evaporated from the particles move into the clean air flow 8 by Brownian diffusion. At the downstream end of the chamber 2, the non-uniform flow is split into two axially symmetrical flows: the central flow with non-volatile residual particles 7 and the clean air flow laden with vapour 8 that is directed through the circular slot 17 into the axial symmetry air conduit 18 (second outlet conduit) and finally to the vapour outlet 6 (second outlet) which is connected to an analytical instrument for analysing the analyte in the vapour-laden clean air flow. The splitting of the air flows at the downstream end of the chamber 2 prevents non-volatile residual particles entering the analytical instrument and damaging it. The VED shown in FIG. 4 is also a real-time device and provides rapid analysis of vaporizable analytes in air and other analytes with a device that operates preferably with vapour samples.

EXAMPLES

[0130] A number of different designs of the VED device have been investigated, and tests have been carried out at temperatures varying from 20 C. to 300 C. and at flow rates of 0.1 l/min<{Q.sub.sample, Q.sub.clean, Q.sub.waste, Q.sub.vapour}<1.5 l/min. Several different types of geometries of VED were manufactured and tested. Two examples are described below.

Example 1

[0131] An axial symmetry VED device similar to that shown in FIG. 4 has been manufactured from a stainless-steel cylinder of ID=5 mm and length L=120 mm. All the inlets and outlets were equipped with Swage locks and copper tubing was used to connect the VED to the measuring instruments (e.g. an lonscan 400 instrument). The waste air flow leaving the VED chamber was cooled using coiled copper tubing 100 mm in length and filtered with two Mitsubishi aerosol filters connected to a SPF30 pump as shown in FIG. 3. The circular slot 16 was 1.5 mm wide and the axial symmetry conduits 15 were of 10 mm10 mm cross-section.

[0132] FIG. 5 shows the results of tests carried out to determine the distribution of non-volatile particles of tris(2-ethylhexyl) phosphate between the waste air flow and clean air flow. Thus, tris(2-ethylhexyl) phosphate with a particle number concentration of 1.210.sup.6 cm.sup.3 was introduced into the sample inlet 3. This level of concentration is typical for a heavily polluted atmosphere. The efficiency of particle removal from the vapour outlet 6 was evaluated as the ratio of the number concentration of particles measured in the vapour outlet 6 to the number concentration of particles measured in the sample inlet 3 (see FIG. 4).

[0133] The results in FIG. 5 show that increasing the heater temperature of the VED (T.sub.h) did not result in an increase in the number of particles reaching the vapour outlet. The percentage of particles reaching the vapour outlet remained very low (0.2%) throughout the temperature range from 20 C. to 100 C. Thus, the results show that, in a VED having the dimensions (e.g. extraction chamber ID) given in Example 1, the VED buoyancy criterion between the sample gas flow and clean gas flow was satisfied and the extent of mixing of the two gas flows was minimal.

[0134] The results demonstrate a considerable reduction in contamination of the vapour outlet by particles. Importantly the analyte (tris(2-ethylhexyl) phosphate) vapour concentration in the vapour outlet was 24 times greater than the vapour concentration in the sample flow. Therefore, the VED of the invention provides an improved sensitivity of analyte detection and prevents damage to analytical instruments by particulate matter in air samples.

Example 2

[0135] Another example of an axial symmetry VED similar to that described in Example 1 was manufactured from a stainless-steel cylinder of ID=30 mm and a length L=120 mm. In Example 1, the ID was 5 mm. The other dimensions were as described in Example 1. For the larger ID, the number concentration of particles measured in the vapour outlet increased at the onset of heating to unacceptable levels close to 50% thereby demonstrating that mixing of the two gas streams can occur if the diameter of the VED is too great.

[0136] Using the template established by the specific embodiments and examples set out above, the optimal configuration (e.g. width and length) and the optimal operating conditions can readily be determined by routine trial and error.

[0137] It will be appreciated that numerous modifications and alterations can be made to the VED devices illustrated in the drawings and described in the specific examples above without departing from the principles of the invention as defined in the claims appended hereto.