GB2623365A

GB2623365A - An analytical apparatus for continuous sampling and analysis of airborne particulate matter

Info

Publication number: GB2623365A
Application number: GB2215214.4A
Authority: GB
Inventors: Davis Stephen; Green Bob; Ogden James; Marshall Josh
Original assignee: Markes International Ltd
Current assignee: Markes International Ltd
Priority date: 2022-10-14
Filing date: 2022-10-14
Publication date: 2024-04-17
Also published as: GB202215214D0

Abstract

An analytical apparatus for continuous sampling and analysis of Volatile Organic Compounds on airborne particulate matter comprises a plurality of particulate sample tubes 20, each comprising a particulate filter, and a thermal desorption unit 16 having a sampling station 18 that receives a sample tube and locates it in the sampling line, and desorbs VOCs from particulate matter collected on the particulate filter. An automated sample tube changer 30 is operated to remove sample tubes from the sampling station for storage in a tube holder and insert sample tubes from the holder into the sampling station. An analyser receives and analyses samples desorbed from the particulate sample tubes. The automated sample tube changer 30 is operated to remove a used sample tube from the sampling station following VOC desorption and replace the tube with a subsequent sample tube from the tube holder on a continuous basis.

Description

AN ANALYTICAL APPARATUS FOR CONTINUOUS SAMPLING AND ANALYSIS OF AIRBORNE PARTICULATE MATTER

FIELD OF INVENTION

[1] The present invention relates to an analytical apparatus and, more particularly, to an automated apparatus for the continuous sampling and analysis of Volatile Organic Compounds and Semi Volatile Organic compounds on particulate matter.

BACKGROUND

[2] The measurement of volatile compounds in the environment, such as Volatile Organic Compounds (VOC) and Semi Volatile Compounds (SVOC), is of considerable interest given their potential toxic impact to human health and the effect on climate change. These volatile compounds are present in air either as free chemicals or attached to particulate matter such as that found in diesel exhaust. Particulate matter can be characterised by size, with fine particulate matter (PM2.5) for example being a class of particulate matter 2.5 micron or smaller that is readily inhaled and are small enough to pass through the lungs into the blood stream. The VOC and SVOC attached to PM2.5 are referred to as secondary aerosols. In addition to adsorbing volatile compounds, particulate matter also contributes directly to the volatile compound content in air by virtue of being partially derived from burnt organic material. Given this particulate matter effects the amount of free volatile compound in the air and is sufficiently small to be inhaled, the volatile compound content is of great interest.

[3] Determination of the volatile compound content of particulate matter involves fractionation of the particulate matter according to size followed by trapping of the analyte particulate matter on a filter. The volatile compounds are then removed from the particulate matter for analysis either by solvent extraction or thermal desorption. In the latter, heat is applied to the filter and the evolved volatile compounds are analysed by a technique such as chromatography coupled with mass spectrometry which allows for the identification of the volatile compounds.

[004] Thermal Desorption Gas Chromatography Mass Spectroscopy (TD-GCMS) is used for analysing the VOC and SVOC content of particulate matter. TD-GCMS involves sampling air onto discrete filters. The filters are then heated to thermal desorb the VOC and SVOC content, for GCMS analysis. This approach necessitates an offline discontinuous batch process, in which the filters are periodically replaced within the sampler and then analysed at a remote location. Analysis also requires each filter to be manually loaded into an analyser by an operator, requiring the operator to be present to load each subsequent filter in turn after each analysis. Continuous on-line analysis has been attempted, but involves a single filter being repeatedly regenerated by thermal desorption prior to its re-use to collect the next sample.

is [005] Reliance on a single filter has the principal drawback of carryover from one sample to the next due to the incomplete desorption of an entire sample during the thermal desorption stage. Any particulate matter left on the filter from the previous samples can then also act as sorbent material, capturing free VOC content from the air sample during the sampling phase. This results in an erroneous result as the current sample would no longer then consist of just secondary aerosol on the particulate matter on this sample but will also show a contribution from captured free VOC. In addition, this approach is limited to the measurement of VOC and SVOC on particulate matter and does not include collection and analysis of free VOC and SVOC.

[006] It is therefore desirable to provide an improved analytical apparatus for the continuous sampling and analysis of VOCs and SVOCs on airborne particulate matter, and/or a method of sampling and analysing the same, which addresses the above-described problems and/or which offers improvements generally.

SUMMARY

[7] According to the present disclosure there is provided an analytical apparatus for the continuous sampling and analysis of VOCs and SVOCs as described in the s accompanying claims.

[8] In an aspect of the disclosure there is provided an analytical apparatus for the continuous sampling and analysis of Volatile Organic Compounds and Semi Volatile Organic compounds on airborne particulate matter, comprising a sampling line for receiving and channelling a sample airflow. A plurality of particulate sample tubes are provided, each including a sampling chamber containing a particulate filter for collecting particulate matter from a sample airflow during a sampling phase. The apparatus further includes a thermal desorption unit having a sampling station configured to receive a particulate sample tube and locate the sample tube in the sampling line such that the sample airflow passes through the particulate sample tube, and to desorb volatile and semi volatile organic compounds from particulate matter collected on the particulate filter during a desorption phase. A sample tube holder is configured to receive said plurality of particulate sample tubes. An automated sample tube changer is operative to remove sample tubes from the sampling station for storage in the sample tube holder and insert sample tubes from the sample tube holder into the sampling station. A controller is operative to control the apparatus, including the thermal desorption unit and the automated sample tube changer. An analyser is connected to the sampling station to receive and analyse samples desorbed from the particulate sample tubes. The automated sample tube changer is operated by the controller to remove a used particulate sample tube, that is to say a sample tube from which a sample has been desorbed, from the sampling station following desorption and replace the particulate sample tube with a subsequent further particulate sample tube selected from the sample tube holder for continued sampling. The term sampling station is used to refer to a location within the apparatus for receiving the sample tube such that the tube axis of the sample tube is aligned with the sampling line and sample airflow is able to be drawn through the sample tube.

[009] The use of an automated sample tube changer to remove the first particulate sample tube and replace it with a second particulate sample tube enables sampling to be immediately continued with minimal disruption. The automated sample tube holder comprises a carousel that can contain up to 100 sample tubes. Therefore, the sample tube changer can continuously replace the sample tubes to enable 100 sampling phases. Furthermore, the automated sample tube changer may contain multiple carousels. Therefore, hundreds of sampling and analysis cycles may be completed before manual intervention is required to replace the carousels. Therefore, a substantially continuous, online sampling analysis process is achieved, that is able to provide live, online data on the VOC and SVOC content of particulate matter in the atmosphere at the sampling location. In addition, it is not required to transport the samples to a remote laboratory as the analysis is conducted instantaneously in situ.

[0010] The apparatus is preferably operated to analyse the sample desorbed from the first particulate sample tube during the sampling phase of the subsequent particulate sample tube.

[0011] The sampling line may have an inlet end and a particulate matter separator is located at the inlet end to remove particulate matter above a pre-selected size threshold from the sample airflow. The separator is preferably operated to remove all particulate matter over 2.5 microns. Therefore, analysis is limited to PM2.5, to provide data relating to those particulates with most potential for deep lung inhalation and transmission to the blood stream.

[0012] Preferably the particulate matter separator comprises a cyclone separator, and the apparatus includes a pump operated to at a flow rate selected to cause the required size separation.

S

[0013] The apparatus preferably includes a second sampling pump arranged to draw a sample airflow through the sample line. The sampling pump has a lower flow rate than the higher flow rate pump used to operate the cyclone separator.

s [0014] In a first mode of operation the filtered sample airflow may be diverted from the analyser and vented during the sampling phase.

[0015] In an alternative mode of operation, the analyser may receive the sample airflow filtered by the sample tube during the sampling phase to analyse free volatile and semi volatile organic compounds within the sample airflow while the particulate sample is collected. In this way, simultaneous continuous sampling of particulate carried and free VOCs and SVOCs is achieved.

[0016] The apparatus may include devices for monitoring the flow rate and/or total flow through the sample tube and/or the duration of the sampling phase. This data may be communicated to a processor or data storage device.

[0017] The analyser preferably comprises a focussing trap and the desorbed sample from the particulate sample tube is collected on the focussing trap and then released in a second desorption stage prior with a lower flow volume for analysis. Similarly, the filtered sample airflow may be collected on the trap and released for analysis prior to desorption of the particulate sample.

[0018] The analytical apparatus may comprise a second sampling line including a particulate filter and a second analyser arranged to receive a sample airflow from the second sampling line and analyse free volatile and semi volatile organic compounds within the sample airflow. The apparatus is therefore able to provide continuous online analysis of free volatiles in addition or alternatively to the sampling and analysis of particulate matter.

[0019] The analytical apparatus includes a sample valve configured to control the flow of sample airflow to the sample tube. The sample valve may be operated by the controller. The sample valve is opened during the sampling phase and closed during the desorption phase. A purge valve may control the flow of an inert purge gas to the sample tube. The purge valve is closed during the sampling phase and opened during the desorption phase.

[0020] The analyser may comprise a gas chromatograph and a mass spectrum analyser.

[0021] The analytical apparatus may further comprise data storage and/or a processor for processing the analysis data. The apparatus may further comprise a data communication system for communicating the online analysis data.

[0022] The filter of the sample tubes is preferably configured to retain particles of a desired size.

[0023] An electronic device may be included to record one or more parameters associated with the sampling such a duration of sampling, total flow.

[0024] In an embodiment one or more of the sample tubes may include an adsorbent suited to trap free VOC and SVOC.

[0025] Multiple tubes are preferably arranged in the sample tube holder such that each tube may be presented in turn to the sampling path for collection of successive samples with each tube having an associated sampling phase of a pre-determined duration and the combined sampling phases provide an effectively continuous representation of the VOC SVOC content of the PM in the sampled stream.

[0026] The sample airflow that passes through the filter of the sample tube may be passed through the focussing trap of the thermal desorber such that any free VOC and SVOC not attached to the PM are retained on the trap.

[0027] The trap may be thermally desorbed prior to desorption of the sampling tube.

[0028] The free VOC and SVOC may be retained on the trap until the sample tube is desorbed and combined with the desorbed sample from the sample tube such that both samples are held on the trap and thermally desorbed simultaneously to the analyser as one released sample to provide a composite mass balance measurement of total VOC/ SVOC from the sample airflow.

[0029] The desorbed eluent (samples) from multiple tubes may be passed to a secondary trap, which may be the focussing trap, to combine the VOC and SVOC prior to thermal desorption to combine samples from multiple tubes. For example, samples tubes designed to sample exclusively either PM or free VOC SVOC may be combined prior to an [0030] The system may comprise means to perform a blank analysis on the tubes (without sampling) to detect residual volatiles in the sample tubes and establish if they can be reused for subsequent sampling.

[0031] In another aspect of the disclosure there is provided a method of sampling and analysing volatile and semi volatile organic compounds on particulate matter. The method comprises: a) inserting a particulate sample tube containing a particulate filter within a sampling station of a thermal desorption unit; b) passing a sample airflow through the particulate sample tube in a sampling phase to collect particulate matter from the sample airflow; c) heating the particulate sample tube in a desorption stage to desorb volatile and semi volatile organic compounds from the particulate matter collected on the filter; d) passing the desorbed sample to an analyser; e) operating an automated sample tube changer to remove the particulate sample tube from the sampling station and replace the particulate sample tube with a further sample tube; and f) repeating steps b to e for the further sample tube.

[0032] The method may further comprise analysing the sample desorbed from the particulate sample tube during the sampling phase of the subsequent particulate sample tube.

[0033] The automated sample tube changer may comprise a sample tube holder such as a carousel containing a plurality of particulate sample tubes, and steps b-e are repeated for a selected number of said plurality of particulate sample tubes. That is to say, after each sample tube is removed from the sampling station by the automated sample tube changer it is replaced by a subsequent sample tube selected by the controller from the sample tube holder until a predetermined number of sampling tubes is used, which may be the total number of sample tubes contained in the sample tube holder and may be less.

[0034] The incoming sample airflow may be separated prior to the sample tube to remove particulate matter above a threshold size. Separation refers to the fractionation of the particulate matter according to size to allow only particles of a certain size and smaller to pass into the sampling line.

[0035] During the sampling phase the sample airflow filtered by the sample tube may be directed to the analyser to analyse free volatile and semi volatile organic compounds within the sample airflow. The airflow may be directed to the analyser by opening a trap valve and closing a vent valve.

[0036] The method may further comprise collecting a second sample airflow through a second sampling line, filtering particulate from the second sampling line and directing the second sample airflow to an analyser to analyse free volatile and semi volatile organic compounds within the second sample airflow.

[0037] The analyser may be a second analyser and the steps of collecting and analysing the second sample airflow may be conducted simultaneously with or independently of the steps of collecting and analysing the first sample airflow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The present disclosure will now be described by way of example only with reference to the following illustrative figures in which: Figure 1 shows a schematic of an analytical apparatus according to an embodiment of the disclosure; Figure 2 is a particulate sample tube according to an embodiment of the disclosure; and Figure 3 is a schematic of an analytical apparatus according to another

embodiment of the disclosure.

DESCRIPTION OF EMBODIMENTS

[0039] The following description presents exemplary embodiments and, together with the drawings, serves to explain principles of the disclosure. The scope of the disclosure is not intended to be limited to the precise details of the embodiments or exact adherence with all method steps. Variations will be apparent to a skilled person and are deemed also to be covered by the description. Terms for features used herein should be given a broad interpretation that also encompasses equivalent functions and features. In some cases, several alternative terms (synonyms) for structural features have been provided but such terms are not intended to be exhaustive.

[0040] Descriptive terms should also be given the broadest possible interpretation; e.g. the term "comprising" as used in this specification means "consisting at least in part of" such that interpreting each statement in this specification that includes the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" are to be interpreted in the same manner. Directional terms such as "vertical", "horizontal", "up", "down", "upper" and "lower" are relative terms that may be used for convenience of explanation usually with reference to the illustrations and are not intended to be ultimately limiting if an equivalent function can be achieved with an alternative dimension and/or direction.

[0041] The description herein refers to embodiments with particular combinations of configuration steps or features. However, it is envisaged that further combinations and cross-combinations of compatible steps or features between embodiments will be possible. The description of multiple features in relation to any specific embodiment is not an indication that such features are inextricably linked, and isolated features may function independently from other features and not necessarily require implementation as a complete combination.

[0042] Referring to figure 1, an analytical apparatus 1 for continuous online sampling and analysis of VOC and SVOC from particulate matter is schematically illustrated. The analytical apparatus 1 is configured to be located at a sampling location and provide continuous in-situ sampling and analysis of collected samples. The analytical apparatus comprises a first input line 2. The first input line 2 has a high efficiency cyclone separator 4 located at its inlet 6, configured to separate particles larger than 2.5 microns from the incoming airflow. The cyclone separator 4 is located at a sampling location, which for example may be a rooftop for the purpose of air quality monitoring. Air is drawn into the cyclone separator 4 by a downstream high flow pump 8. The flow rate of the pump 8 is selected to create the PM2.5 cut off within the cyclone separator 4.

[0043] A sample splitter 10 is located between the cyclone separator 4 and the high flow pump 8. The sample splitter 10 directs a first excess fraction of the incoming sample airflow to an exhaust vent 12. A second fraction of the incoming sample airflow is directed by the sample splitter 10 to a sample supply line 14. A dryer 15 is located along the sample supply line 14, that includes a moisture removal membrane arranged to remove moisture from the sample airflow. Moisture drawn from the airflow through the membrane is removed by a secondary airflow. After the moisture removal stage, the sample airflow is passed to a thermal desorption unit 16.

[0044] The thermal desorption unit 16 includes a sampling station 18 where a sample tube 20 is located in the flow line. The sampling station 18 includes a supply inlet end 21 and an outlet end 23. A heater is located at the sampling station 18 such that it is adjacent to or partially surrounds the sample tube 20. The supply line 14 connects to the inlet end 21 of the sampling station 18. The supply line 14 includes a heated section 22 extending from the dryer 15 that connects to the sampling station 18. A sample valve 24 controls the flow of incoming sample air to the sampling station 18. The sample valve 24 is configured to selectively block or permit the flow of sample air from the dryer to the sample tube 20 located in the sampling station 18.

[0045] A purge line 19 connects an inert gas source to the inlet end 21 of the sampling station 18. The secondary supply line may have a branched connection with the sampling line 14. A control valve 25 controls the flow of gas from the inert gas source to the sampling station 18. Closure of the control valve 25 prevents the flow of inert gas to the sampling station 18. A trap line 27 connects the outlet end 23 of the sampling station 18 to the thermal desorption trap 29 of the thermal desorption unit 16, which is described in further detail below.

[0046] A low flow rate sampling pump 26 is also located on the outlet side of the sampling station 18. A vent line 28 connects the sampling station 18 to the sampling pump 26 via a branched connection with the trap line 27. A vent filter 30, vent valve 32 and flow controller 34 are located along the vent line 28 between the sampling station 18 and the sampling pump 26. The vent filter 30 is provided to prevent particulate matter flowing to the sampling pump 26. The vent valve 32 is operable to close the vent line 28 to prevent the flow of secondary sample through the vent line 28. The flow controller 34 measured the flow rate through the vent line 28. Airflow through the sampling pump 26 s is vented to an exhaust 31.

[0047] The thermal desorption unit 16 includes an autochanger 30 comprising a sample tube holder in the form of a carousel including a plurality of sample tube holders. The autochanger 30 is loaded with sample tubes such that at least one storage location on the carousel is vacant. The autochanger is operable to transfer sample tubes 20 from the sampling station 18 into a vacant storage location on the carousel, and to load further sample tubes 20 selected from the carousel into the sampling station. The carousel is configured to move each sample tube holder to the required position for unloading or receiving a sample tube from the sampling station 18. An actuator (not shown), is provided to remove sample tubes 20 from the carousel and to return sample tubes to the carousel. The actuator is operated by a controller which is programmed with the location in the carousel of each sample tube. The sample tube holder comprise receptacles located in the carousel tray that are each configured to releasably hold an individual sample tube 20. Each sample tube 20 has a unique identifier, and the position of each sample tube 20 within the carousel tray is registered with the controller.

[0048] Referring to Figure 2, the sample tube 20 is a custom sample tube configured for the collection and retention of PM2.5 particulate matter. The sample tube 20 comprises a rigid tubular body 36 formed of steel and having a tube axis A-A. The inner surface of the sample tube 20 includes an inert coating. The tube includes an inlet end 40 and an outlet end 42. The tubular body 36 of the sample tube 20 defines a fluid passage extending from the inlet 40 to the outlet. A particulate filter assembly 44 is located within the tubular body 36. The particulate filter assembly 44 comprises a first cylindrical spring located with the tubular body 36. The first hollow cylindrical spring 46 is biased radially outwards to hold itself in position within the sample tube 20 and defines a first stop element. A first gauze member 48 abuts the first spring. A series of filter membranes 50 are packed against the first gauze member 48 by a second gauze member 52. A second cylindrical spring 54 holds the second gauze 52 in position such that the filter membranes 50 are clamped between the cylindrical springs 46, 54 and the first and second gauzes 48, 52. The filter membranes 50 comprises discs formed from particulate filter paper, although other suitable filter materials may be used. The filter grade is selected to trap PM2.5 material. The carousel may also be loaded with sorbent sample tubes comprising a sorbent media located within the sample tube, in place of the filter media. The sorbent media is selected to absorb VOCs and SVOCs from the airflow.

[0049] In a first mode of operation the sampling apparatus 1 is used to perform continuous particulate sampling. A particulate filter sample tube 20 is automatically selected from the carousel and loaded into the sampling station 18 by the autochanger. The sample tube 20 is loaded such that the tube axis A-A is aligned with the airflow path.

The high flow pump 8 is operated to draw airflow into the input line 2 via the cyclone separator 4. The cyclone separator 4 removes any particulate matter larger than 2.5 microns, allowing only the PM2.5 into the input line 2. Varying the rate at which the high flow pump 8 operates varies the size range of particulate matter selected by the cyclone separator 4.

[0050] A first fraction of the sample airflow is vented to the exhaust 12 via the splitter 10 and high flow pump 6. The sampling pump 26 is operated to draw a second fraction of the sample airflow from the splitter 10 to the sampling station 18. Moisture from the sample airflow is removed by the dryer 15 and the sample airflow passes along the heated line 22 to the sampling station 18 via the open sample valve 24. The purge valve is closed at this stage. A trap valve also prevents flow of the sample airflow to the trap 29.

[0051] The flowrate of the sampling pump 26 is selected to achieve a flowrate though the sample tube 20 suitable for sample deposition. As the sample airflow is drawn through the particulate matter deposits on the filter membranes 50. The airflow is measured by the flow controller 21, which controls the air flow to the sampling pump to achieve a predetermined flow rate. The sample airflow is passed through the sample tube for a predetermined sampling period selected to achieve a measurable sample size and prevent filter saturation. At the end of the sampling period the sampling pump 26 is stopped, and the sample valve 24 is closed to prevent further flow of sample airflow into the tube.

[0052] When the sampling stage is complete a pre purge is initiated prior to desorption to flush resident gas from the sample tube 20 containing gas phase VOC and SVOC from the sample airflow. The purge valve 25 is opened to permit a flow of inert gas through the sample tube, which is vented via the exhaust 31. The desorption stage is initiate flowing the pre purge. During the desorption stage the sample tube 20 is heated by the oven to a temperature at which the volatile VOCs and SVOCs components are desorbed from the particulate matter and released into the gas phase. The inert gas flow from the purge line is 19 drives the desorbed secondary sample from the sample tube 20. The vent valve 32 is closed and the trap valve is opened, allowing the secondary sample (i.e. the desorbed VOC and SVOC) to flow along the trap line 27 to the thermal desorption trap.

[0053] The purge stage continues for a predetermined purge period to ensure the majority of volatile components are released from the particulate matter. At the end of the purge stage the purge valve 25 is closed and the thermal desorption trap is rapidly heated and purge to drive the sample to for analysis, for example by GCMS.

[0054] When the sample tube 20 has been desorbed and purged, the sampling and desorption phases are complete. The purge valve 25 is closed to discontinue the flow of the purge gas through the sample tube 20. The autochanger is operated to remove the sample tube 20 and store the sample tube 20 in a vacant sample tube holder within the carousel. The controller selects a further, unused sample tube 20 from the carousel and operates the autochanger to insert the new sample tube 20 into the sampling station 20 and connect the sample tube to the flow line. The sampling and desorption process is then recommenced using the new sampling tube. Analysis of the released sample from the previous sample tube 20 is performed while a sample is collected on the subsequent sample tube 20. This process is continued until a predetermined sampling run has been completed and/or all the sample tubes in the carousel have been used. At this stage, an operator can replace the tubes in the carousel with unused sample tubes for continued sampling. s

[0055] In a second mode of operation, the analysis apparatus 1 is operated to sample and analyse gas phase volatiles from the atmosphere in addition to sampling volatiles from the particulate matter. In the second mode of operation, the sampling process is operated substantially as described above. During the sampling stage, the sample airflow is drawn through the sample tube 20 and onto the trap 29 by a secondary sampling pump downstream of the trap 29, rather than being drawn through the primary sampling pump 26 and exhausted. Prior to completion of the particulate sampling phase, the trap 29 is purged to drive the trapped gas phase volatiles to the analyser. When the trap has been purged, the sample tube 20 may be desorbed and purged for analysis of the volatile is compounds desorbed from the particulate matter. In this way, online data is generated of the gas phase volatiles and the volatiles present on the particulate matter using the same thermal desorption apparatus.

[0056] In another embodiment as shown in Figure 3, the system comprises a second thermal desorption unit 60 and a second input line 62. The second thermal desorption unit 16 comprises an integral focussing trap without an intermediate sampling station. A sample airflow is drawn into the system via the second input line 62. A filter 64 is located along the second input line 62 to remove particulate matter from the airflow. The sample airflow is then supplied directly to the focussing trap of second thermal desorption unit 60. The VOCs and SVOCs in the airflow are absorbed directly onto the focussing trap. The sample airflow is continued for a sampling period, and the sample is then rapidly desorbed from the focussing trap and driven to an analyser by a purge gas flow. This sampling and analysis of gas phase volatile compounds may be conducted continuously and simultaneously with the particulate matter sampling and analysis.

[0057] While the above embodiments are described for the sampling and analysis of VOC and SVOC, the invention is not limited to this application and may be utilised for the sampling and analysis of other target compounds of sufficient volatility to be desorbed from a filter or other means of sample collection.

Claims

CLAIMS1. An analytical apparatus for the sampling for airborne particulate matter, comprising: a sampling line for receiving and channelling a sample airflow; a plurality of particulate sample tubes, each including a sampling chamber containing a particulate filter for collecting particulate matter from a sample airflow during a sampling phase; a thermal desorption unit having a sampling station configured to receive a particulate sample tube and locate the sample tube in the sampling line such that the sample airflow passes through the particulate sample tube, and to desorb volatile and semi volatile organic compounds from particulate matter collected on the particulate filter during a desorption phase; a sample tube holder configured to receive said plurality of particulate sample tubes; an automated sample tube changer operative to remove sample tubes from the sampling station for storage in the sample tube holder and insert sample tubes from the sample tube holder into the sampling station; a controller; and an analyser connected to the sampling station to receive and analyse samples desorbed from the particulate sample tubes; wherein the automated sample tube changer is operated by the controller to remove a used particulate sample tube from the sampling station following desorption and replace the particulate sample tube with a further particulate sample tube selected from the sample tube holder for continued sampling.
2. An analytical apparatus according to claim 1wherein the sampling line has an inlet end and a particulate matter separator is located at the inlet end to remove particulate matter above a pre-selected size threshold from the sample airflow.
3. An analytical apparatus according to claim 2 wherein the particulate matter separator comprises a cyclone separator.
4. An analytical apparatus according to claim 1 further comprising a pump arranged s to draw the sample airflow through the sample line.
5. An analytical apparatus according to any preceding claim wherein in a first mode of operation the filtered sample airflow is diverted from the analyser and vented during the sampling phase.
6. An analytical apparatus according to any preceding claim wherein in a second mode of operation the analyser receives the sample airflow filtered by the sample tube during the sampling phase to analyse free volatile and semi volatile organic compounds within the sample airflow.
7. An analytical apparatus according to any preceding claim further comprising a flow meter arranged to measure the flow rate of the sample airflow through the sample tube.
8. An analytical apparatus according to any preceding claim wherein the analyser comprises a focussing trap and the desorbed sample from the particulate sample tube is collected on the focussing trap and then released in a second desorption stage prior to analysis.
9. An analytical apparatus according to any preceding claim further comprising a second sampling line including a particulate filter and a second analyser arranged to receive a sample airflow from the second sampling line and analyse free volatile and semi volatile organic compounds within the sample airflow.
10. A method of sampling and analysing volatile and semi volatile organic compounds on particulate matter, the method comprising: a) inserting a particulate sample tube containing a particulate filter within a sampling station of a thermal desorption unit; b) passing a sample airflow through the particulate sample tube in a sampling phase to collect particulate matter from the sample airflow; c) heating the particulate sample tube in a desorption stage to desorb volatile and semi volatile organic compounds from the particulate matter collected on the filter; d) passing the desorbed sample to an analyser; e) operating an automated sample tube changer to remove the particulate sample tube from the sampling station and replace the particulate sample tube with a subsequent sample tube; and f) repeating steps (b) to (e) for the further sample tube.
11. A method according to claim 10 further comprising analysing the sample desorbed from the particulate sample tube during the sampling phase of the subsequent particulate sample tube.
12. A method according to claim 10 or 11 wherein the automated sample tube changer comprises a sample tube holder containing a plurality of particulate sample tubes, and steps b-e are repeated for a selected number of said plurality of particulate sample tubes.
13. A method according to any one of claims 10 to 12 further comprising separating the incoming sample airflow prior to the sample tube to remove particulate matter above a threshold size.
14. A method according to one of claims 10 to 13 further comprising during the sampling phase directing the sample airflow filtered by the sample tube to the analyser to analyse free volatile and semi volatile organic compounds within the sample airflow.
15. A method according to one of claims 10 to 14 further comprising collecting a second sample airflow through a second sampling line, filtering particulate from the second sampling line and directing the second sample airflow to an analyser to analyse free volatile and semi volatile organic compounds within the second sample airflow.
16. A method according to claim 15 wherein the analyser is a second analyser and the steps of collecting and analysing the second sample airflow may be conducted simultaneously with or independently of the steps of collecting and analysing the first sample airflow.