Sieve pack for an ion mobility spectrometer

Abstract

An aspect of the disclosure provides a sieve pack for an ion mobility spectrometer, the sieve pack comprising: an inlet, for connection to a pneumatic system of the ion mobility spectrometer to receive a flow of gas from said pneumatic system; a first outlet, for connection to said pneumatic system for providing a cleaned dried flow of gas to said pneumatic system; a first direction changing conduit extending between a first end of the sieve pack and a second end of the sieve pack, wherein the second end is spaced in an axial direction from the first end, wherein: the first direction changing conduit is configured to hold a sieve material for cleaning and drying air; the first direction changing conduit comprises at least one turn around the axial direction; and, the first direction changing conduit is connected to the inlet and the first outlet and configured to carry the flow of gas from the inlet through the sieve material to the first outlet.

Description

[0001]Method and Apparatus Field of Invention The present invention relates to methods and apparatus, and more particularly to methods and apparatus for providing cleaned and / or dried flows of gas, such as air, still more particularly it relates to sieves for cleaning and / or drying flows of such gases in trace detection apparatus such as ion detectors, including ion mobility spectrometers and mass spectrometers. Background In ion mobility spectrometers, and other systems which sample vapours, it may be useful to provide a flow of cleaned dried air. Typically a molecular sieve, often in a so-called sieve pack is used for this purpose. Usually the molecular sieve is provided by a large number of spheres, about 2 mm in diameter, of a zeolite material packed into an outer housing connected in the gas flow path through the detector. Different diameter spheres could be used, and materials other than zeolites. Gas flowing through the detector is circulated through the sieve before being recirculated in the detector. Whilst flowing through the pack, gas follows a tortuous path around the outside of the spheres with some of the gas flowing through the spheres. These packs (which may also be referred to as towers) may comprise a solid block of zeolite held in a closely fitting housing. Such a block may have multiple passages through which gas can flow. A variety of configurations can be used for such a sieve. Over time, the sieve may become less effective. For example, it may accumulate moisture and so be less able to take up moisture from air flowing through it. Excessive moisture in drift gas of ion mobility spectrometers is known to be a problem. Sieve packs may be regenerated by heating them while flowing gas through them. This typically requires the sieve to be removed from the detector and replaced or for the detector to be taken out of Trace detection apparatus may be used in circumstances of reduced supply and / or where personnel are at risk. For example, they may be carried by military personnel, such as soldiers. In these and other circumstances it may be desirable to reduce the frequency with which detection apparatus must be serviced – such as by removing and replacing a sieve pack. One way to reduce the frequency with which a sieve pack must be serviced is simply to make it larger. However this is undesirable because soldiers and other personnel tasked wit carrying such devices may also need to carry a large amount of other kit. Space taken up by a device and its weight are both tightly constrained. Summary An aspect of the present disclosure provides a sieve pack for a trace detector, such as a portable detector which intended to be carried by a single adult human. The sieve pack comprises an inlet, for connection to a pneumatic system of the ion mobility spectrometer to receive a flow of gas from said pneumatic system; a first outlet, for connection to said pneumatic system for providing a cleaned dried flow of gas to said pneumatic system; a first direction changing conduit extending between a first end of the sieve pack and a second end of the sieve pack, wherein the second end is spaced in an axial direction from the first end, wherein: the first direction changing conduit is configured to hold a sieve material for cleaning and drying air; the first direction changing conduit comprises at least one turn around the axial direction; and, the first direction changing conduit is connected to the inlet and the first outlet and configured to carry the flow of gas from the inlet through the sieve material to the first outlet. The at least one turn comprises a change in direction of the direction changing conduit when viewed in plan. For example, viewing the sieve pack “in plan” means viewed along a line of sight that is aligned with the axial direction. The at least one turn of the direction changing conduit when viewed in plan may describe a bend or corner. The bend may comprise at least an arc of a rounded form. The rounded form may be one of: a circle; an ellipse; and a stadium shape. The change in direction may be at least 90 degrees around the axial direction, for example, at least 180 degrees around the axial direction. The change in direction of the direction changing conduit may be configured to direct the flow of gas along a curved path thereby subjecting the gas to a centrifugal force. The direction changing conduit may comprise a series of turns about the axial direction, each turn axially offset from the next so that the gas advances axially by following the series of turns about the axial direction, for example wherein the direction changing conduit The direction changing conduit may be provided by a baffle disposed in a container. The baffle may be configured to provide at least part of a helix. The sieve pack may comprise a plurality of mutually similar baffle modules arranged in the container and fitted together to provide the baffle. Each of the baffle modules may comprise a fin wherein fins of adjacent modules may at least partially overlap to provide the baffle. The sieve pack may comprise a second outlet, for connection to said pneumatic system for providing a cleaned dried flow of gas to said pneumatic system; a second direction changing conduit extending between a first end of the sieve pack and a second end of the sieve pack, wherein the second end is spaced in an axial direction from the first end, wherein: the second direction changing conduit is configured to hold a sieve material for cleaning and drying air; the second direction changing conduit comprises at least one turn around the axial direction; and, the second direction changing conduit is connected to the inlet and the second outlet and configured to carry the flow of gas from the inlet through the sieve material to the second outlet. The first direction changing conduit may be helical; and, the second direction changing conduit may be helical. The first direction changing conduit and the second direction changing conduit may be interleaved with each other, for example mutually interleaved, for example intertwined. For example, they may be arranged as a double helix. example, the first direction changing conduit and the second direction changing conduit may each be helical paths and they may be intertwined with each other provide a double helical path. The sieve pack may comprise a first conduit connecting the inlet to the first direction changing conduit and the second direction changing conduit. The first conduit may be axially aligned and it may be straight. It may be arranged to pass through the middle of the sieve pack, so that the two direction changing conduits surround the first conduit in the manner that a spiral staircase surrounds its central pillar. The first conduit and the direction changing conduit may both be substantially filled with a sieve material, for example they may be packed with a sieve material. The first direction changing conduit may be provided by a first baffle disposed in a container. The second direction changing conduit may be provided by a second baffle. The first baffle may comprise at least part of a first helix. second baffle comprises at least part of a second helix. The sieve pack may comprise a plurality of mutually similar baffle modules arranged in the container and fitted together to provide the first baffle and the second baffle. The baffle modules may comprise a centre pillar. The centre pillar may be hollow, for example the pillar may be right cylinder having an axial conduit, aligned with the axis of the right cylinder. Thus, each of the baffle modules comprises a first conduit portion wherein first conduit portions of adjacent modules together provide the first conduit. The axial ends of the centre pillar of the baffle modules may be provided with a complementary fitting for connecting the axial conduit portion of the baffle module to an axial conduit portion of a mutually similar module. These fittings may provide mechanical coupling between adjacent modules. This may allow a plurality of the baffle modules to be assembled together, for example stacked up. When stacked together in this way the central pillars may connect to each other so that the hollow centre of the pillar cylinders provide a single continuous conduit passing axially through the stack of modules. Each of the baffle modules may comprise a first fin and a second fin. The fins may each follow a helical path, circumferentially around the outer surface of the pillar and axially up that outer surface. The baffle modules may be configured so that, when two such baffle modules are assembled together, one on top of the other in a stack with their pillars connected, the upper edge of the first fin of the lower baffle module at least partially overlaps with the lower edge of the first fin of the upper baffle module. Accordingly, when the modules are assembled together the first fins may be configured so that together they provide a continuous helical baffle. This helical baffle may extending radially out around a central pillar made up of the individual central pillars of each of the modules in the stack. The second fin may be arranged in a corresponding way (i.e. in an identical way but rotated 180° about the central pillar from the first fin). Thus, just as the first fins of adjacent baffle modules in a stack provide the first baffle, the second fins of adjacent modules provide the second baffle. In an embodiment there is provided a system comprising: a trace detector module, such as an ion mobility spectrometer, the trace detector module comprising: a pneumatic system configured to receive a flow of gas from said pneumatic system; and, the sieve pack of the present disclosure having an outlet configured for connection to an inlet of the pneumatic system, and an inlet configured for connection to an inlet of the pneumatic system, so that gas received into the sieve pack from the pneumatic system’s outlet passes through the sieve pack and is recirculated back to the pneumatic system’s inlet. In an embodiment there is provided a baffle module for a sieve pack for an ion mobility spectrometer, the baffle module comprising: a first fin wherein the first fin is a portion of a first baffle, wherein arranging a plurality of the baffle modules together so that first fins of adjacent modules at least partially overlap provides the first baffle. The baffle module may comprise: a second fin wherein the second fin is a portion of a second baffle, wherein arranging a plurality of the baffle modules together so that second fins of adjacent modules at least partially overlap provides the second baffle. The baffle module may be arranged so that the first baffle and second baffle provide a first direction changing conduit and a second direction changing conduit. The baffle module may comprise a first conduit portion. The first conduit portion may be provided by a conduit through a central pillar portion of the module. The ends of the central pillar portion may be provided with complementary fittings for mechanically coupling the end of the central pillar portion to the end of a central pillar portion of an adjacent module. Accordingly, the first conduit portions of adjacent modules may together provide a first conduit of the sieve packs described herein. In an embodiment there is provided a kit of parts for a sieve pack for an ion mobility spectrometer. The kit of parts may comprise a plurality of baffle modules and a container. The plurality of baffle modules may be configured to fit together in a stack, so that the fin(s) of the baffle modules provide helical baffle(s) which helically surround a central pillar. The baffle may be configured to seal against the walls of the container. This may provide a helical conduit defined between the pillar, the baffle(s) and the walls of the container. An internal surface of the container may comprise a groove configured to receive a radial outer edge of the baffle. The upper and or lower edges of the fins may carry a lip so as to overlap with the respective lower / upper edge of the fin of an adjacent module to which it is assembled. The kit of parts may comprise a sieve material, configured to be disposed in such a conduit formed by the baffles. An aspect of the disclosure provides a method of manufacturing a sieve pack the method comprising assembling a plurality of the baffle modules in a container to provide a direction changing conduit of the sieve pack. The method may comprise forming a baffle module by moulding. The trace detector may be sized and shaped to be worn by the carrier, for example it may be detector configured to be carried on a garment of a user, such as a belt or webbing. In an embodiment there is provided an air-purification systems for a trace detection apparatus, the system comprising the sieve pack of the present disclosure configured to clean and / or dry a flow of air for use in the trace detection apparatus. In embodiments the sieve pack of the present disclosure may last longer than prior art sieve packs with greater time intervals between service changes. Embodiments of the disclosure may achieve this by providing a greater path length of gas flow through the sieve than prior art the same size. Embodiments of the disclosure provide a Dual Helix Airflow Sieve pack, which may reduce (for example, minimize) the size and space required for the sieve pack. Embodiments may have been found to achieve a minimum of nine times life over the existing sieve pack size used in the Smiths Detection Lightweight Chemical Detector (LCD) model 3.3 and model 4. Embodiments may provide a variety of volumes of sieve material. In an embodiment, the capacity of a prior art sieve pack may be 19cm3of sieve material. In an embodiment the sieve pack of the present disclosure provides flow of air through a volume 177cm3of material. The Dual Helix design of the present disclosure may better utilize the air flow in order to force the air that is passed through it into a centrifugal flow around the sieve pack pillar in 2 spiraling directions enabling more surface area of sieve to be in contact with the moisture laden air at all times, making the system far more efficient in the drying process. Helical embodiments, and dual helical embodiments may provide particular benefits but other embodiments in which a direction changing conduit provides centrifugal mixing of the gas as it flows through the sieve material also provides benefits. Brief Description of Drawings Embodiments of the disclosure will now be described in detail with reference to the accompanying drawings, in which: Figure 1 shows a schematic view of a sieve pack; Figure 2 shows a schematic view of an ion mobility spectrometer having a pneumatic system connected to a sieve pack, such as that illustrated in Figure 1; Figure 3 shows a portable detector with a sieve pack, such as that described with reference to Figure 1 attached to it; Figure 4 shows a kit of parts for assembly of a sieve pack such as that described with reference to Figure 1; and Figure 5 shows a schematic view of a sieve pack. In the drawings like reference numerals are used to indicate like elements. Specific Description Figure 1 shows a sieve pack 1 having an inlet 6, outlets 2, 4, a housing 10, and a first direction changing conduit 26. The first direction changing conduit 26 is configured to carry air from one end of the sieve pack (the first end 32) to the other end of the sieve pack (the second end 30). The direction from the first end 32 of the sieve pack to the other end of the sieve pack 30 may be referred to herein as the axial direction. Figure 1 comprises three views of the sieve pack 1. The first view, labelled inset A comprises a plan view of the cap of the sieve pack (view from the line A-A in inset B). The second view, labelled inset B, is a schematic side view of the sieve pack, in which the housing of the sieve pack is shown as if cut away or transparent to reveal the structure inside the pack. The third view, labelled inset C, is a schematic cross section through the sieve pack at the point indicated (C-C) in Inset B. Flows of air 20, 22 are illustrated in these views as bold arrows but it will be appreciated that these are not part of the apparatus itself but are included to indicate its function when used. A shown, the sieve pack 1 comprises a cap 12, a container 10, baffles 14, 16, an inlet 6, and two outlets 2, 4. It further comprises a first conduit 18 and two direction changing conduits 26, 28. The direction changing conduits 26, 28 are configured to hold a sieve material for cleaning and drying air. Typically, the direction changing conduits 26, 28 are filled with the sieve material, so that air which flows 20, 22 through them must pass through the sieve material. In the interests of clarity however, Figure 1 does not show any sieve material. In the arrangement shown in Figure a central pillar 24 is shown aligned in the axial direction between the first end 32 and the second end 30 of the sieve pack 1. The direction changing conduits 26, 28 may comprise first direction changing conduit 26 and a second direction changing conduit 28. The first direction changing conduit 26 comprises at least one turn around the pillar 24. In this example, the first direction changing conduit 26 encircles the pillar 24 and follows a helical path, both around it and axially up it. The housing 10 may be a cylinder. One end 32 of the cylinder is closed and the other end 30 is open but closed by the cap 12. The central pillar 24 is disposed in the cylinder, and the pillar 24 is encircled by two baffles 14, 16. The baffles 14, 16 are helical, and extend radially from the outer surface of the pillar 24 to the inner surface of the housing 10. The respective radial outer edges of the two baffles 14, 16 may each provide sealed join against the inner surface of the housing 10. For example the baffles may sealingly abut the wall of the housing 10, or they may be joined to it. Similarly, the radially inward edge of the respective baffles may each provide sealed join against the outer surface of the pillar 24. The two baffles 14, 16 may be spaced from each other around the pillar. This provides an axial spacing between the two baffles. For example, the two baffles 14, 16 may be arranged at 180° from each other, e.g. so that any given axial position along the pillar, the two baffles are on opposite sides of the pillar from each other. The two baffles 14, 16 may have the same helical pitch as each other, so the axial distance between them is uniform along the length of the baffles. The baffles 14, 16 may each be laminar structures, provided by a helical sheetlike element. The central pillar 24 is hollow, so that a first conduit 18 is provided axially through the pillar 24, from one end 30 of the housing 10 to the other end 32. At the capped end 30 of the housing 10, this conduit is joined to the inlet 6 of the sieve pack 10. At the closed end 32 of the housing, the conduit 18 has openings 34, 36, out from the pillar 24 into the interior of the housing 10 around the pillar 24. These openings 34, 36 may be arranged at the bottom of the housing, and a gap may be provided between the lower edge of the two baffles 14, 16 and the bottom of the housing so that air flowing out from the openings can flow over both surfaces of both baffles. It can be seen that the two baffles 14, 16 thus provide two conduits 26, 28 which are separated from each other by the baffles and which repeatedly change direction around the axial direction defined by the pillar. The two direction changing conduits 26, 28 are both fluid connected to the first conduit 18 by the openings 34, 36 at the closed end 32 of the housing to the first conduit 18, through the pillar 24. It can be seen that the openings 34, 36 may provide a branching or bifurcation of that first conduit 18. The other ends of the two direction changing conduits 26, 28 are connected to the first outlet 2 of the sieve pack and the second outlet 4 of the sieve pack 10 respectively. Specifically, the first direction changing conduit 26 is connected to the first outlet 2 of the sieve pack. The second direction changing conduit 28 is connected to the second outlet 4 of the sieve pack. In operation, such as in an ion mobility spectrometer, the sieve pack inlet 6 can be connected to a pneumatic system of the ion mobility spectrometer. The first outlet 2 can also be connected to a first inlet of said pneumatic system for providing a cleaned dried flow of gas to said pneumatic system. The second outlet 4 can also be connected to a second inlet of said pneumatic system. Air can then be provided to the inlet 6 of the sieve pack from the pneumatic system. The air flows from the inlet axially through the first conduit 18, which in this example is inside the pillar 24. At the closed end of the housing 32, the air flows out through the openings 34, 36 in the pillar 24 to enter the two helical conduits 28 provided around the pillar 24 by the two 14, 16. The air can then flow back up the housing, through the sieve material packed into the housing 10. The two direction changing conduits 14, 16 provide changes in direction of the air, around the axial direction of the air as it flows along the two conduits back up to reach the two outlets. Figure 2 illustrates an example of an ion mobility spectrometer which may be configured to perform a method such as that described above with reference to Figure 1. The ion mobility spectrometer comprises a sampling inlet 202, such as a pinhole inlet for providing samples of air from an inlet of the detector system. In the interests of clarity – the parts of the apparatus “upstream” of the sampling inlet are not shown in Figure 1. A flow of air however may be provided to the sampling inlet, such as from a desorber (not shown) or from ambient surroundings of the of the spectrometer. A pressure pulser, (also not shown) may be operated to draw the samples from this flow of air through the sampling inlet into the reaction region of the detector. At the reaction region 102 an ionisation source 104, such as a corona point is provided for ionising the sample of gaseous fluid in the reaction region 102. The ion mobility spectrometer also comprises an ion shutter 105, a detector 118, and a controller 120. The ion shutter 105 comprises two electrodes 106, 107, which are coupled to the controller 120 to enable a barrier voltage to be provided between the two electrodes 106, 107. When the shutter 105 is “closed” this barrier voltage acts to prevent ions from travelling from the reaction region into a drift region of the IMS, and an open state in which ions can travel into the drift region towards the detector. The ion shutter 105 may comprise a Tyndall- Powell, Bradbury–Nielsen shutter, or other type of shutter. In the example illustrated in Figure 1, the drift region 103 lies between the reaction region 102 and the detector 118. Any appropriate arrangement for detecting the arrival of ions may serve as the detector 118. Examples of detectors include a collector electrode, for example a Faraday cup for detecting the arrival of ions. A voltage profile may be provided in the drift region 103 using a series of drift electrodes 103a, 103b spaced apart along the drift region. Although not illustrated in Figure 1, a repeller plate or other electrode may be arranged for extending this voltage profile into the reaction region 102. Between the reaction region 102 and the detector 118 the profile voltage varies spatially (e.g. as a function of displacement along the cell in the drift direction) to provide an electric field that moves ions along the cell 100 towards the detector 118. The electric field may be uniform and / or known along the drift region 103 and / or the reaction region 102. A guard grid 123 may be interposed between the drift region and the detector 118 so as to shield the detector from approaching ions, to prevent “image effect” from approaching ions detracting from timing accuracy. The controller 120 comprises a programmable processor, an input / output interface such as an ADC / DAC (not shown in the drawings) which is able to control the provision of appropriate electrical control signals and / or power supply to the desorber, the ionisation source, and the ion shutter. It is also configured to receive detection signals from the detector 118 indicating the arrival of ions at the detector. At the same end of the IMS cell 100 as the detector 118 and the guard grid 123, there is provided a drift gas inlet 122. At the other end of the IMS cell, toward the reaction region 102, there is provide a drift gas outlet. The drift gas outlet is coupled by conduits of the pneumatic system 126 to a connection interface 127 of the pneumatic system 126. The interface 127 of the pneumatic system may comprise connectors for allowing the inlet of the sieve pack to be connected, via a conduit of the pneumatic system, to receive gas from the drift gas outlet 125 of the IMS cell 100. Where more than one such cell is present the drift gas outlet(s) of the respective cells may all be coupled to the relevant connector for providing gas into the inlet of the sieve pack. The interface 127 of the pneumatic system may also comprise connectors for connecting the outlet(s) of the sieve pack to a conduit of the pneumatic system which is connected to provide gas to the drift gas inlet 122 of the IMS cell 100. The interface 127 may also comprise mechanical coupling means, such as a docking recess and / or retaining mechanism, to hold the sieve pack 1 in engagement with the connectors. The interface may be configured to provide a sealed connection between the conduits of the pneumatic system and the respective inlets 2, 4 and outlet 6 of the sieve pack 1 so that gas can flow between the pneumatic system and the sieve pack 1 without leakage. In operation, an air mover of the pneumatic system, such as a fan a pump or blower, is operated to provide a circulating flow of drift gas into the IMS cell from the inlet 122 and out of the IMS cell through the outlet 125. The flow of gas then passes from the outlet 125 to the inlet 6 of the sieve pack via the interface 127. In the sieve pack, the flow of gas passes axially down the first conduit 18 (e.g. inside the centre of the pillar 24) to the closed end of the sieve pack. The flow then bifurcates, e.g. at the closed end of the sieve pack, and different parts of the flow of air from the first conduit flow into the different direction changing conduits 26, 18 on the different sides of the baffles 14, 16 in the sieve pack. The flow of air which passes on top of the first baffle 14 (beneath the second baffle 16) flows through the sieve material which is packed into the first direction changing conduit 26 defined above the first baffle. This carries the flow of air around a series of bends, so that it corkscrews around the axial direction, back up the sieve pack axially, in the opposite direction to the flow down the first conduit. Pushing the air through a series of changes of direction may cause a centrifugal mixing of the gas and may reduce the occurrence of “dead space” in the sieve material. These changes of direction may be provided by the bends of a helix of curved form, such as the elliptical cross-section form of the sieve pack illustrated in Figure 1. The flow of air which passes beneath the first baffle (on top of the second baffle) flows through the sieve material which is packed into the second direction changing conduit defined above the second baffle. As with the first direction changing conduit, this carries the flow of air around a series of bends, so that it corkscrews around the axial direction, back up the sieve pack. It can be seen that these two conduits are intertwined with each other, which provides a very compact and efficient spatial arrangement of flow paths through the sieve material. The sieve pack may be elliptical as illustrated in Figure 1. It will however be appreciated that other shapes of curved form may also be used – such as circular forms and stadium shaped forms. Structures having corners may also be used. Curved forms may be more efficient. As illustrated in Figure 3, the sieve pack 1 may be connected to a portable detector system 1000, which may comprise a detector cell such as the ion mobility spectrometry cell 100 illustrated in Figure 2. The portable detector system may also comprise a pneumatic system 126, such as that described with reference to Figure 2, or another type of pneumatic system. An interface, such as interface 127, may be provided by the portable detector system to enable the sieve packs of the present disclosure to be removably connected and reconnected to the pneumatic system. Such a detector system may be configured to be carried on a garment worn by a human user of the device, such as a belt or webbing. Such systems may also be attached to vehicles, UAVs, and other carrying devices. Figure 4 shows a sieve pack of the present disclosure in the form of a disassembled kit of parts 2000. The kit of parts 2000 comprises a plurality of mutually similar baffle modules 2002, 2002’, 2002’’, 2002’’’ each comprising fins 2004, 2006 for defining a baffle, and a central pillar portion 2024. The central pillar portion 2024 may be a cylinder which may be hollow, to provide a conduit 2018 which extends axially through the pillar portion. The radial outer surface of the central pillar 2024 portion carries the fins,2004, 2006 which extend radially outward from the pillar portion 2024. The baffle modules 2002 may each comprise two such fins, which may be provided opposite each other on the central pillar portion 2024. Each of the fins may be a part of a helical form, such as at least part of a turn of a helix. Thus, when the baffle modules 2002 are axially assembled together, the first fins of the baffle modules cooperate to together form the first baffle. The second fins similarly cooperate to form the second baffle. The two baffles may form the two direction changing conduits as described above with reference to Figure 1. The axial ends of the pillar portion of each module 2002 may each comprise a fitting 3000, 3002 configured to sealingly engage with an axial end of another module 2002’’. These fittings, and the fins, may be arranged so that when the modules are stacked axially together and the fittings are connected to seal the walls of the central conduit 18, the fins 2004, 2006 of adjacent baffle modules adjoin one another to provide the first baffle 14 and the second baffle 16 respectively (as illustrated in Figure 1. The joins between adjacent fins may be sufficiently close as to provide a single continuous baffle. The baffle need not be sealed at the joins however provided that the flow is guided sufficiently along the respective conduits without too much mixing between the conduits. It may be better however for the fins to meet and optionally also overlap a little or otherwise to provide a substantially sealed joint between adjacent fins 2004, 2006. The baffle module 2002 may comprise a first conduit portion 2018. The first conduit portion may be provided by a conduit through the central pillar portion 2024 of the module 2002. The fitting provided at the ends of the central pillar portion may be complementary (such as having a projection and / or a recess for mating with another mutually similar fitting). The fittings 3000, 3002 may be configured for mechanically coupling the end of the central pillar portion to the end of a central pillar portion of an adjacent module. Accordingly, the first conduit portions of adjacent modules may together provide a first conduit of the sieve packs described herein. The kit of parts 2000 may further comprise a container 10. The plurality of baffle modules 2002 may be configured to fit together in a stack, so that the fin(s) of the baffle modules provide helical baffle(s) which helically surround central pillar as described above. The radial extent of the baffle formed by the fins may be selected to match the container 10 – for example so that the baffle seals against the walls of the container. This may provide a helical conduit defined between the pillar, the baffle(s) and the walls of the container. The kit of parts may comprise a sieve material, configured to be disposed in such a conduit formed by the baffles. To assemble a sieve pack according to the present disclosure, these modules may be assembled together into a stack, so that the pillar portions form the central pillar of the sieve pack and the respective fins adjoin to provide respective baffles. The stack of baffle modules can then be provided into the container. The container can be packed with a sieve material to fill all or substantially all of the space in the container which is not occupied by the baffles. For example the sieve material may fill the direction changing conduit and the first conduit, which in Figure 1 and Figure 5 goes through the central pillar 24. A cap 12 may then be secured to the container to enclose the stack of baffle modules and couple a first outlet to the first direction changing conduit, a second outlet to the second direction changing conduit, and an inlet to the first conduit. The kit of parts 2000 illustrated in Figure 4 may be configured so that, when assembled together, it provides the sieve pack as described with reference to Figure 1 and / or Figure 5. For example, in Figure 4 it can be seen that the baffle modules which make up the baffle may comprise an entire helical turn, so that the fin of each baffle module circumscribes the entire circumference of the central pillar. However, this is not necessary. In other embodiments, a lesser amount of a helical turn may be provided by each baffle module as described below with reference to Figure 5. Figure 5 shows a section view through a further sieve pack according to the present disclosure. This is another example of a sieve pack 10 of the type illustrated in Figure 1 and which may be assembled from a kit of parts 2000 such as that illustrated in Figure 4. The sieve pack 1’ in Figure 5 is labelled with identical reference numerals as the sieve pack 1 shown in Figure 1. In addition to the features described above with reference to Figure 1, the inlet 6’ and the outlets 2’, 4’ may be provided as plain conduits through the The internal surface of the container 10’ may comprise a groove into which the outer edges (radial outer tips) of the baffles which may up the direction changing conduits may fit. For example, this groove may be helical. The groove may be arranged so that the helical baffle can be screwed down into the container by threading it into the groove. a central pillar 24, assembled from a plurality of baffle modules similar to those described above with reference to Figure 4. However, unlike the baffle modules shown in Figure 4, the fins 2004, 2006 of the baffle modules 2002’’’’ each only occupy less than a complete helical turn, for example a quarter of a turn of the helix, so that each fin 2004, 2006, spans 90° of the outer circumference of the central pillar and four such modules, assembled axially together are necessary for the baffle formed from their fins to form a complete (360°) turn of the helix. The upper and / or lower edge of each fin 2004, 2006 may overlap with the fin of the adjacent module. The edges may be provided with a lip or other engagement structure which may provide a seal against the adjacent edge of the fin of the adjacent baffle module, so that the fins together form a continuous baffle. Other embodiments are envisaged. For example, the first conduit need not be centrally disposed and may be provided along a wall of the container or at some other position. In some embodiments the inlets and the outlets are at opposite ends of the container and so no “first conduit” is necessary at all. As another example, the system has been described as having two flow paths through the sieve pack, so as to provide a dual flow of cleaned and / or dried air. However, in some embodiments a different number of pats may be provided. For example a single path, such as may be provided by a single helix rather than a double helix. Thus, a single direction changing conduit maybe present. Equally, a greater number of conduits may be provided. Embodiments of the disclosure provide a modification of the air- flow system used to maintain dry and clean air in detector systems. They may be used in ion mobility spectrometry based detectors, but may also be used in other trace chemical detectors in which a supply of cleaned dried air is required. The pneumatic system of the system may be used to supply gas into the detection cell, such as the drift gas of the IMS described with reference to Figure 1. However, cleaned and / or dried gas from the sieve pack may be used in other pneumatic systems of a detector. For example, in those embodiments in which a desorber is present, an air mover may provide a supply of cleaned dried air from the outlet of the sieve pack past the desorber and to the sampling inlet for this purpose. Flows of cleaned and or dried air from the outlet of sieve pack may also be used to purge the detector and or may be supplied to other parts of a variety of different types of trace detectors. Embodiments of the present disclosure however find particular application where there is a need to provide a highly compact (space saving) sieve pack with a long service interval. The molecular sieves described herein may be of any appropriate type, such as those used in IMSs and other detection apparatus to remove unwanted chemicals from gas supplied to the detection apparatus. For example, the molecular sieves may comprise pieces of an absorbent and / or adsorbent material such as zeolite packed together into a housing. For example the pieces may be spheres, e.g. about 2 mm in diameter. The housing may be connected in to a gas flow path so that gas flowing through the pack follows a tortuous path around the outside of the pieces of material with some of the gas flowing through the material. Other types of molecular sieve material may be used, such as those in which the sieve is formed of a solid block of molecular sieve material provided with a multiplicity of gas passages extending through it. In such sieves the gas flow through the sieve may primarily be confined to flow through the interior of the block. The block may comprise zeolite. Any feature of any one of the examples disclosed herein may be combined with any selected features of any of the other examples described herein. For example, features of methods may be implemented in suitably configured hardware, and the configuration of the specific hardware described herein may be employed in methods implemented using other hardware. It will be appreciated from the discussion above that the embodiments shown in the Figures are merely exemplary, and include features which may be generalised, removed or replaced as described herein and as set out in the claims. With reference to the drawings in general, it will be appreciated that schematic functional block diagrams are used to indicate functionality of systems and apparatus described herein. It will be appreciated however that the functionality need not be divided in this way, and should not be taken to imply any particular structure of hardware other than that described and claimed below. The function of one or more of the elements shown in the drawings may be further subdivided, and / or distributed throughout apparatus of the disclosure. In some embodiments the function of one or more elements shown in the drawings may be integrated into a single functional unit. In some examples the functionality of the controller may be provided by a general purpose processor, which may be configured to perform a method according to any one of those described herein. In some examples the controller may comprise digital logic, such as field programmable gate arrays, FPGA, application specific integrated circuits, ASIC, a digital signal processor, DSP, or by any other appropriate hardware. In some examples, one or more memory elements can store data and / or program instructions used to implement the operations described herein. Embodiments of the disclosure provide tangible, non-transitory storage media comprising program instructions operable to program a processor to perform any one or more of the methods described and / or claimed herein and / or to provide data processing apparatus as described and / or claimed herein. The controller may comprise an analogue control circuit which provides at least a part of this control functionality. An embodiment provides an analogue control circuit configured to perform any one or more of the methods described herein. The above embodiments are to be understood as illustrative examples. Further embodiments are envisaged. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in theaccompanying claims.

Claims

CLAIMS:

1. A sieve pack for an ion mobility spectrometer, the sieve pack comprising: an inlet, for connection to a pneumatic system of the ion mobility spectrometer to receive a flow of gas from said pneumatic system; a first outlet, for connection to said pneumatic system for providing a cleaned dried flow of gas to said pneumatic system; a first direction changing conduit extending between a first end of the sieve pack and a second end of the sieve pack, wherein the second end is spaced in an axial direction from the first end, wherein: the first direction changing conduit is configured to hold a sieve material for cleaning and drying air; the first direction changing conduit comprises at least one turn around the axial direction; and, the first direction changing conduit is connected to the inlet and the first outlet and configured to carry the flow of gas from the inlet through the sieve material to the first outlet.

2. The sieve pack of claim 1, wherein the at least one turn comprises a change in direction of the direction changing conduit when viewed in plan.

3. The sieve pack of claimwherein the at least one turn of the direction changing conduit when viewed in plan describes a bend or corner.

4. The sieve pack of claim 3 wherein the bend comprises at least an arc of a rounded form.

5. The sieve pack of claim 4, wherein the rounded form is one of: a circle; an ellipse; a stadium shape.

6. The sieve pack of any of claims 2 to 5, wherein: the change in direction is at least 90 degrees around the axial direction, for example, at least 180 degrees around the axial direction. The sieve pack of any of claims 2 to 6, wherein: the change in direction of the direction changing conduit is configured to direct the flow of gas along a curved path thereby subjecting the gas to a centrifugal force.

8. The sieve pack of any of claims 1 to 7, wherein the direction changing conduit comprises a series of turns about the axial direction, each turn axially offset from the next so that the gas advances axially by following the series of turns about the axial direction, for example wherein the direction changing conduit is 9. The sieve pack of any of claims 1 to 8, wherein: the direction changing conduit is provided by a baffle disposed in a container.

10. The sieve pack of claim 9, wherein: the baffle comprises at least part of a helix.

11. The sieve pack of claim 10, comprising: a plurality of mutually similar baffle modules arranged in the container and fitted together to provide the baffle.

12. The sieve pack of claimwherein:each of the baffle modules comprises a fin wherein fins of adjacent modules at least partially overlaps to provide the first baffle.

13. The sieve pack of any of claims 1 to 12, comprising: a second outlet, for connection to said pneumatic system for providing a cleaned dried flow of gas to said pneumatic system; a second direction changing conduit extending between a first end of the sieve pack and a second end of the sieve pack, wherein the second end is spaced in an axial direction from the first end, wherein: the second direction changing conduit is configured to hold a sieve material for cleaning and drying air; the second direction changing conduit comprises at least one turn around the axial direction; and, the second direction changing conduit is connected to the inlet and the second outlet and configured to carry the flow of gas from the inlet through the sieve material to the second outlet.

14. The sieve pack of claim 13, wherein: the first direction changing conduit is helical; and, the second direction changing conduit is helical.

15. The sieve pack of any of claims 13 to 14, wherein: the first direction changing conduit and the second direction changing conduit are mutually interleaved.

16. The sieve pack of claim 15 as dependent on claim 14, wherein: the first direction changing conduit and the second direction changing conduit are mutually interleaved to provide a double helix.

17. The sieve pack of claim 16, comprising:a first conduit connecting the inlet to the first direction changing conduit and to the second direction changing conduit.

18. The sieve pack of any of claims 13 to 17, wherein: the first direction changing conduit is provided by a first baffle disposed in a container; and, the second direction changing conduit is provided by a second baffle disposed in the container.

19. The sieve pack of claim 18 as dependent on any of claims 14 to 17, wherein: the first baffle comprises at least part of a first helix; and, the second baffle comprises at least part of a second helix.

20. The sieve pack of claim 19, comprising: a plurality of mutually similar baffle modules arranged in the container and fitted together to provide the first baffle and the second baffle.

21. The sieve pack of claim 20, wherein: each of the baffle modules comprises a first fin and a second fin wherein: first fins of adjacent baffle modules at least partially overlap to provide the first baffle; and, second fins of adjacent baffle modules at least partially overlap to provide the second baffle.

22. The sieve pack of any of claims 20 to 21 as dependent on claim 17, wherein: each of the baffle modules comprises a first conduit portion wherein first conduit portions of adjacent modules together provide the first conduit.A system comprising: ion mobility spectrometer comprising: a pneumatic system configured to receive a flow of gas from said pneumatic system; and, a sieve pack of any of claim 1 to 22 connectable to the pneumatic system of the sieve pack.

24. A baffle module for a sieve pack for an ion mobility spectrometer, the baffle module comprising: a first fin wherein the first fin is a portion of a first baffle, wherein arranging a plurality of the baffle modules together so that first fins of adjacent modules at least partially overlap provides the first baffle.

25. The baffle module of claim 24, comprising: a second fin wherein the second fin is a portion of a second baffle, wherein arranging a plurality of the baffle modules together so that second fins of adjacent modules at least partially overlap provides the second baffle.

26. The baffle module of claim 25, wherein: the first baffle and second baffle provide a first direction changing conduit and a second conduit, the baffle module comprising: a first conduit portion wherein first conduit portions of adjacent modules together provide a first conduit.

27. A kit of parts for a sieve pack for an ion mobility spectrometer comprising: a plurality of baffle modules of any of claims 24 to 26; and, a container. The kit of parts of claim 27 comprising: a sieve material in a conduit formed by the baffles.

29. A method of manufacturing a sieve pack the method comprising assembling a plurality of the baffle modules of any of claims 24 to 26 in a container to provide a direction changing conduit of the sieve pack.

30. The method of claim 29 comprising forming a baffle module by moulding.

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