Patent Publication Number: US-10775283-B2

Title: On-demand vapor generator

Description:
BACKGROUND 
     Ion mobility spectrometry (IMS) refers to an analytical technique that can be used to separate and identify ionized material, such as molecules and atoms. Ionized material can be identified in the gas phase based on mobility in a carrier buffer gas. Thus, an ion mobility spectrometer (IMS) can identify material from a sample of interest by ionizing the material and measuring the time it takes the resulting ions to reach a detector. An ion&#39;s time of flight is associated with its ion mobility, which relates to the mass and geometry of the material that was ionized. The output of an IMS detector can be visually represented as a spectrum of peak height versus drift time. 
     IMS detectors and other detectors often include a vapour generator to supply a dopant chemical to the detector. Vapour generators can also be used to supply a test chemical for use in testing or calibrating a detector, a filter or other equipment. In some applications it is important that the vapour generator can be switched on and off rapidly, and that leakage can be prevented when the detector is switched off. For example, in an IMS detection system, rapid switching of the vapour generator on and off enables rapid switching between different doping conditions, such as different levels of dopant or different dopant substances. Such rapid switching could also enable different regions of the IMS detector to be doped differently by ensuring there was no leakage to undoped regions of the apparatus when the apparatus is switched off. 
     SUMMARY 
     An On-Demand Vapour Generator (OVG) is disclosed. The vapour generator may be configured for use with a detection apparatus, such vapour generators may comprise a vapour source coupled by a flow path to provide vapour through an impeder to an outlet for dispensing vapour to the detection apparatus. The impeder may comprise: a first vapour permeable passage arranged to impede diffusion of the vapour from the source to the owlet. The vapour permeable passage is configured to enable vapour to be driven through a diffusion barrier from the source to the outlet by a pressure difference (e.g. pumped or forced flow as opposed to simply a difference in concentration). The vapour generator may also comprise at least one additional vapour permeable passage to act as a sink, coupled to the outlet by the first vapour permeable passage. The sink can comprise a material adapted to take up the vapour to divert diffusion of vapour away from the outlet. In embodiments, the first vapour permeable passage and the sink are arranged so in response to a pressure difference between the outlet and the vapour source, resistance to driving vapour flow through the first vapour permeable passage to the outlet is less than the resistance to driving vapour flow into the sink. In one or more implementations, the vapour generator includes a vapour chamber configured to produce a vapour and a vapour absorption assembly configured to receive flows of vapour from the vapour chamber, for example via a diffusion barrier. The vapour absorption assembly includes a first vapour-permeable passage having a passage outlet. The vapour absorption assembly may further include one or more second vapour-permeable passages that are closed. When the vapour absorption assembly receives a flow (e.g. a pressure driven flow) of vapour from the vapour chamber, the flow of vapour passes through the first vapour-permeable passage to the passage outlet at least substantially without absorption of vapour from the flow of vapour. However, when a flow of vapour is not received from the vapour chamber, vapour entering the vapour absorption assembly from the vapour chamber passes into the first vapour-permeable passage and the at least one second vapour-permeable passage and is at least substantially absorbed. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identify the figure in which the reference number first appears. The use of the same reference number in different instances in the description and the figures may indicate similar or identical items. 
         FIG. 1  is a schematic block diagram that illustrates an example on-demand vapour generator in accordance with an implementation of the disclosure, wherein the on-demand vapour generator employs a single vapour-permeable passage. 
         FIG. 2  is a schematic block diagram that illustrates another example on-demand vapour generator in accordance with an implementation of the disclosure, wherein the on-demand vapour generator employs a single vapour-permeable passage. 
         FIG. 3  is a schematic block diagram that illustrates an example on-demand vapour generator in accordance with an implementation of the disclosure, wherein the on-demand vapour generator employs a vapour-permeable passage having a passage outlet and one or more vapour-permeable passages that are closed. 
         FIG. 4  is a schematic block diagram that illustrates an example on-demand vapour generator in accordance with another implementation of the disclosure, wherein the on-demand vapour generator employs a vapour-permeable passage having a passage outlet and one or more vapour-permeable passages that are closed. 
     
    
    
     DETAILED DESCRIPTION 
     One technique of reducing leakage of vapour from a vapour generator when the vapour generator is turned off employs a container of absorbent material that is connected an outlet of a vapour generator via a T-junction. When the generator is turned on, the gas flow through the vapour generator rises to a level that is sufficient to ensure that most of the vapour is carried through the other arm of the T-junction to the outlet. When the vapour generator is off and there is a nominal (e.g., zero (0)) flow, some of the residual vapour produced passes via one arm of the T-junction to the absorbent material. However, some vapour may bypass the absorbent material leading to relatively low absorption efficiency and relatively high levels of escaped vapour. 
     An on-demand vapour generator is disclosed that is suitable for use in a detection system such as an IMS detection system, a gas chromatograph system, a mass spectrometer system, and so forth, to supply a flow of vapour to a detector apparatus (e.g., an IMS detector, a gas chromatograph, a mass spectrometer, and so forth) of the system. In one or more implementations, the vapour generator includes a vapour chamber configured to produce a vapour. The vapour chamber includes a vapour chamber inlet configured to receive a flow of gas into the vapour chamber to generate a flow of vapour, and a vapour chamber outlet configured to allow the flow of vapour to exit the vapour chamber. A vapour absorption assembly receives flows of vapour from the vapour chamber and ports them to the detection apparatus (e.g., to an IMS detector). The vapour absorption assembly includes a vapour-absorbent material configured to absorb the vapour produced by the vapour chamber. A vapour-permeable passage having a passage outlet extends through the vapour-absorbent material and is coupled to the detector assembly. The vapour absorption assembly may further include at least one additional vapour-permeable passage that is closed (e.g., blocked so as to form a “dead end” vapour-permeable passage). When a flow of vapour is not driven (e.g. pumped or drawn) from the vapour chamber (e.g., the on-demand vapour generator is turned off so that there is negligible or no flow), any vapour entering the vapour absorption assembly from the vapour chamber passes into the vapour-permeable passage having the passage outlet and/or the one or more additional dead end vapour-permeable passages and is at least substantially absorbed by the vapour absorbing material. When the vapour absorption assembly receives a flow of vapour (e.g. when the flow of vapour is pumped or drawn) from the vapour chamber, the flow of vapour passes through the first vapour-permeable passage to the passage outlet. As the flow is driven through the passage, more vapour passes to the outlet without being absorbed than when the flow is not driven. 
       FIGS. 1 through 4  illustrate on-demand vapour generators  100  in accordance with example implementations of the present disclosure. As shown, the vapour generator  100  includes an inlet  102  and a vapour outlet  103  connected to an inlet of a detector apparatus  104 . The vapour generator  100  is configured to furnish a readily controllable supply of a dopant vapour to the detector apparatus  104 . In implementations, the vapour generator  100  may supply a flow of vapour to a variety of detector apparatus. For example, in one implementation, the detector apparatus  104  may comprise an IMS detector. However, the vapour generator  100  can be used in conjunction with other detectors such as gas chromatography instruments, and so forth. The vapour generator  100  may also be used for calibration purposes within the instrument. In implementations, the vapour generator  100  and detector apparatus  104  may be part of a detection system (e.g., an IMS detection system)  10 . In such detection systems  10 , the vapour generator  100  and the detector assembly can be housed within a common housing. 
     The vapour generator  100  includes a gas (e.g., air) flow generator  106  such as a fan, a blower, a compressed gas source, and so forth. The flow generator  106  is configured to be switched on or off to provide a flow of gas (air) to its outlet  107  as desired. The flow generator  106  may include various filters or other devices to remove contaminants and water vapour form the gas (e.g., from atmospheric air) before the gas is supplied to the outlet  107 . 
     The outlet  107  of the flow generator  106  is in fluid communication with (e.g., is coupled to) an inlet  108  at one end of a vapour chamber  109 . The vapour chamber  109  may have a variety of configurations, and may comprise any kind of vapour source, for example a permeation source, for example a diffusion source. For example, in the implementation shown, the vapour chamber  109  includes a housing  110  that contains a wicking, absorbent material  111  saturated with a compound in its liquid phase so that the space of the interior  112  within the housing I  10  above the absorbent material  111  is at least substantially filled with a vapour of the liquid at the liquid&#39;s saturated vapour pressure at ambient temperature. The vapour chamber  109  includes an outlet  113  at the end opposite the inlet  108  through which a flow of vapour, comprised of the vapour and gas, can flow out of the vapour chamber  109 . In implementations, the vapour producing liquid comprises acetone. However, vapour-producing substances other than acetone can be used. 
     The vapour chamber outlet  113  is in fluid communication with (e.g., is coupled to) an inlet  114  of a vapour absorption assembly  115 , for example via a diffusion barrier. The vapour absorption assembly  115  includes a vapour absorbent  116  configured to absorb the vapour produced by the vapour chamber  109 . A vapour-permeable passage (main flow path)  117  having an outlet (vapour outlet  103 ) extends through the vapour absorbent  116  and is coupled to the detector apparatus  104 . In the illustrated implementations, the vapour absorption assembly  115  includes a single vapour-permeable passage  117 . However, it is contemplated that additional vapour-permeable passages  117  may be provided in parallel to the passage  117  shown. Moreover, a second vapour absorption assembly can be provided between the inlet  108  of the vapour chamber  109  and the flow generator  106  to prevent vapour from the chamber  109  passing to the flow generator  106  in significant quantities when the flow of gas is off (e.g., when the flow generator  106  is turned off). A pneumatic valve can be connected between this second vapour absorption assembly and the vapour chamber. This valve may be maintained closed until gas (air) flow is required. 
     The on demand vapour generator  100  may further include one or more diffusion barriers  105 . In implementations, the diffusion barriers may comprise flow paths with a small cross sectional area that limit the rate of diffusion (and therefore loss) of vapour from the vapour generator  100  when the generator  100  is in the off state (e.g., when no flow of vapour is furnished by the vapour generator  100 ). 
     When the vapour generator  100  is off (e.g., is in the “off” state, that is, when no flow of vapour is provided), the flow generator  106  remains off so that there is no flow of gas (air) through the vapour chamber  109  and the vapour-permeable passage  117 . The vapour-permeable passage  117  is open to the interior  112  of the vapour chamber  109  so that some vapour may drift into the passage  117 . As this drift occurs, the vapour diffuses into the vapour-absorbent material and is absorbed therein. The bore, length, porosity and nature of the vapour absorbent  116  are chosen such that, under zero flow conditions (e.g., no or virtually no flow conditions), the amount of vapour that escapes from the outlet  103  end of the passage  117  is insignificant in the context of the application in which the vapour generator  100  is used. For example, where the vapour generator  100  is used as a dopant source in an TMS detector, the vapour dopant flow in the off state is arranged to be not sufficient to produce any noticeable dopant ion peak by the IMS detector. 
     The vapour generator  100  is turned on to produce a flow of vapour at its outlet  103  by turning on the flow generator  106  to produce a flow of gas (air) into the inlet  108  of the vapour chamber  109 . This flow of gas (air) collects the vapour produced in the vapour chamber  109  and pushes it through the outlet  113  and into the passage  117  of the vapour absorption assembly  115 . The flow velocity in the passage  117  is chosen such that the residence time of the collected vapour in the passage is sufficiently low so that little vapour is absorbed into the vapour absorbent  116 . Thus, a greater proportion of the vapour passes through the vapour-permeable passage  117  to the outlet  103  end of the passage  117  to be delivered to the detector apparatus  104  than when the flow generator is off The flow of vapour can be continuous or pulsed. 
     The vapour generator  100  is configured to be capable of turning off vapour flow very rapidly when not required, such that the vapour does not leak out at a significant rate. In an IMS detection system, this effectively prevents dopant vapour from entering the IMS detector when the system is turned off and is not powered. This can also enable selected regions of IMS detector to be doped with a reduced risk that dopant will leak to undoped regions when the apparatus is turned off In conventional systems, gas flow through the IMS detector can keep undoped regions free of dopant when the apparatus is powered but, when not powered, the gas flow ceases and any slight leakage of dopant will contaminate all regions of the apparatus. This has previously made it very difficult to dope different regions of IMS detector differently except where the apparatus is continuously powered. 
     In  FIGS. 1 through 4 , the flow generator  106  is illustrated as being in fluid communication with (e.g., connected to) the inlet  102  of the vapour chamber  109  to push air into the chamber  109 . However, in other implementations, the flow generator  106  may be connected downstream of the vapour chamber  109  and be arranged to pull air into the chamber  109 . For example, the flow generator  106  may be connected between the outlet  113  of the vapour chamber  109  and the inlet  114  of the vapour absorption assembly  115  (the inlet  114  end of the vapour-permeable passage  117 ), or it could be connected downstream of the vapour absorption assembly  115  (at the outlet  103  end of the passage  117 ). 
     In the implementations shown in  FIGS. 3 and 4 , the vapour absorption assembly  115  is illustrated as further including one or more additional vapour-permeable passages (region) that are closed (e.g., blocked) so as to form “dead end” vapour-permeable passages (four (4) dead end vapour-permeable passages  317 A-D, collectively  317 , are illustrated). As shown, the dead end vapour-permeable passages  317  may thus extend only partially through the vapour absorbent  116 , and do not include outlets. 
     When the vapour absorption assembly  115  receives a flow of vapour from the vapour chamber  109  (e.g., the flow generator  106  is turn on), the flow of vapour passes through the primary vapour-permeable passage  117 , which functions as a main flow path, to the passage outlet  103  at least substantially without absorption of vapour from the flow of vapour by the vapour absorbent  116 . However, when a flow of vapour is not received from the vapour chamber (e.g., the flow generator  106  is turned off so that there is negligible or no flow of vapour), vapour entering the vapour absorption assembly  115  from the vapour chamber  109  passes into the vapour-permeable passage  117  and/or the dead end vapour-permeable passages  317  and is at least substantially absorbed by the vapour absorbent  116 . 
     When the vapour generator  100  is in the off-state (e.g., when no flow of vapour is supplied), vapour diffusing out of the vapour chamber  109  enters the vapour absorption assembly  115  as before, but now passes down both the vapour-permeable passage  117  (main flow path) and the dead end vapour-permeable passages  317 . As a result, the area of absorption provided f©r the vapour (and therefore the extent of absorption) is greatly increased. However, when the vapour generator  100  is in the on-state (e.g., when a flow of vapour is supplied), the dead end vapour-permeable passages  317  act as dead volumes with essentially no gas exchange and do not contribute to the absorption of vapour from the flow of vapour. Therefore, there is no significant change in the concentration of vapour exiting the vapour generator  100  with the the dead end vapour-permeable passages  317  from implementations that include only the vapour-permeable passage  117  without the dead end vapour-permeable passages  317 . 
     In implementations, the addition of dead-end vapour-permeable passages  317  allows the width of the temperature range over which the on-demand vapour generator  100  can be operated to be increased. As temperature increases, the activity of permeation and diffusion sources rise, the rate of diffusion rises, and the ability of absorbent materials (e.g. activated charcoal) to capture chemicals often decreases. Consequently, a greater concentration of vapour, at a higher rate, is delivered to the vapour absorption assembly  115  of the vapour generator  100 . This increase will be compounded by the reduction in absorption capacity/rate, leading to the vapour absorption assembly  115  being less capable of dealing with the vapour. Leakage in the off-state may therefore increase. Therefore, when the vapour-permeable passage  117  of the vapour absorption assemblies  115  shown in  FIGS. 1 and 2  (without dead end vapour-permeable passages  317 ) are designed to be of suitable length to allow an adequate concentration of vapour to exit the vapour generator  100  in the on-state at extremely low temperatures, the passages  117  may not be adequately long to absorb all vapour in the off-state at extremely high temperatures. The addition of dead end vapour-permeable passages  317  to the vapour absorption assembly  115 , as shown in  FIGS. 3 and 4 , increases the off-state absorption while not decreasing the on-state vapour concentration exiting the vapour generator  100 . Accordingly, the addition of dead end vapour-permeable passages  317  to the vapour absorption assembly  115  makes it possible to reduce the leakage of vapour over a greater range of temperatures without limiting the ability of the vapour generator  100  to supply adequate vapour at extremely low temperatures. Moreover, the additions of dead end vapour-permeable passages  317  makes it possible to further increase the concentration of the vapour leaving the vapour generator  100  without compromising the ability of the vapour generator  100  to restrict the leakage of vapour in the off-state. 
     In implementations, addition of dead end vapour-permeable passages  317  to the vapour absorption assembly  115 , as shown in  FIGS. 3 and 4 , may facilitate shortening of the main flow path (e.g., shortening of the vapour-permeable passage  117 ) to allow higher vapour concentrations to be produced by the vapour generator  100  in the on-state without limiting the ability of the generator  100  to limit leakage in the off-state. Moreover, in situations where the detection system  10  is to be operated over a range of temperatures, the addition of dead end vapour-permeable passages  317  to the vapour absorption assembly  115  enhances the ability of the vapour generator  100  to furnish an adequate concentration of vapour exiting the vapour generator  100  in the on-state at low temperature by having a short main flow path (when the activity of the source is lower than at high temperature), while simultaneously restricting the leakage of the vapour generator  100  in the off-state to acceptable levels at higher temperatures (when the activity of the source and the rate of diffusion are higher than at low temperatures), 
     The dimensions, layout and configuration of the vapour absorption assemblies  115  of the on-demand vapour generators  100  shown in  FIGS. 1 through 4 , including the the vapour-permeable passage  117  (main flow path) and/or the dead end vapour-permeable passages  317  may vary depending on a variety of factors including, but not limited to: the activity of the vapour source (vapour chamber  109 ), the required concentrations to be provided, the flows used in the on-state of the vapour generator  100 , the acceptable level of release when in the off-state and the conditions (e.g. temperature) under which the vapour generator  100  be operated. Accordingly, any dimensions, layouts, or configurations presented herein are for illustrative purposes, and are not necessarily meant to be restrictive of the disclosure. 
     In implementations shown in  FIGS. 1 and 3 , the vapour-permeable passage  117  and/or the dead end vapour-permeable passages  317  of the vapour absorption assembly  115  comprise machined bores formed in a block  118  of an absorbent material such as carbon (e.g., activated charcoal) or a sintered material, such as a molecular sieve material, which could be of zeolite. In other implementations, the vapour-permeable passage  117  and dead end vapour-permeable passages  317  may be formed by molding the block  118  about a core structure that is subsequently removed. The absorbent material is configured to be absorbent of the vapour (e.g., of acetone vapour, and so forth). For example, the material may itself be formed of an absorbent material, such as carbon (e.g., activated charcoal), or the material itself may be a non-absorbent material rendered absorbent via impregnation with a suitable substance. In this manner, the vapour (e.g., acetone vapour, and so forth) may be absorbed by the vapour absorbent  116  generally along the length of the vapour-permeable passage  117  and within the dead-end vapour-permeable passages. 
     In the implementation shown in  FIGS. 2 and 4 , the vapour-permeable passage  117  and/or the dead end vapour-permeable passages  317  comprise lengths of tube  219  having a vapour-permeable outer wall or membrane  220  that are at least substantially enclosed within an outer housing  221  formed of a vapour-impermeable material. For example, as shown, the tube  219  forming, the vapour-permeable passage  117  may extend axially along the center of the housing  221 , while tubes  219  forming the dead end vapour-permeable passages  317  are arrayed around the central tube. As shown, the tube  219  that forms the vapour-permeable passage  117  includes a first end coupled to the inlet  114  and a second end coupled to the vapour outlet  103 . Similarly, the tubes that form the dead end vapour-permeable passages  317  include first ends that are coupled to the inlet  114 . However, the second ends of these tubes are blocked and do not extend from the housing  221 . The bore, length, wall thickness and material of the tubes  219  may be chosen such that, under zero flow conditions, the amount of vapour that escapes from the outlet  103  end of the tube  219  is insignificant in the context of the application in which the vapour generator  100  is employed. In one example, the tube  219  forming the vapour-permeable passage  117  shown in  FIG. 2  is approximately one hundred millimeters (100 mm) long with an external diameter of approximately one millimeter (1 mm), and an internal diameter of approximately one half millimeter (0.5 mm). However, tubes  219  having other sizes are contemplated. The volume between the outside surface of the tubes  219  and the inside surface of the housing  221  is at least substantially Filled with a material  221  that readily absorbs the vapour produced by the vapour chamber  109 . In implementations, the material  221  may comprise activated charcoal granules that are effective to absorb vapour, such as acetone vapour, or the like. Thus, the tubes  219  may be surrounded on all sides by the absorbent charcoal granules. In implementations, the tubes  219  may be formed of an elastomeric plastic, such as silicone rubber, and so forth. 
     In implementations, the on-demand vapour generator  100  may further include a pneumatic valve connected to block flow of vapour from the vapour chamber  109  to the absorbent passage until vapour flow is employed. The pneumatic valve would have the advantage of preventing continual adsorption of the vapour into the vapour absorbent  116 , thus lengthening the life of both the vapour chamber  109  and the absorbent material of the vapour absorbent  116 . The vapour-permeable passage  117  and/or the dead end vapour-permeable passages  317  may thus trap vapour that permeates through the valve seals, providing a lower rate of diffusion. Consequently, the size of the vapour absorbent assembly  115  (e.g., the length, surface area, etc. of the vapour-permeable passage  117  and/or the dead end vapour-permeable passages  317 ) may be reduced. 
     In  FIGS. 1 through 4 , the vapour absorbent  116  is illustrated as extending around the vapour-permeable passage  117  and/or the dead end vapour-permeable passages  317 . However, in implementations, the entire vapour generator  100  may be at least substantially enclosed in a vapour absorbent so that vapour does not substantially escape from the vapour generator  100  in the off state. 
     The on-demand vapour generator  100  of the present disclosure provides for efficient trapping of vapour. The vapour generator  100  is not confined to use in doping detectors but could be used in other applications. For example, the vapour generator  100  may be used to provide a periodic internal calibrant material in a detection system  10 . The detection system  10  may be an IMS detection system, gas chromatograph system, a mass spectrometer or other system. The vapour generator  100  may be used for calibration or testing of other detectors, filters, and so forth. 
     As will be appreciated in the context of the present disclosure, the vapour generator need not generate new vapour, it may generate pre-existing vapour obtained from a vapour source, e.g. a reservoir of vapour. As will also be appreciated in the context of the present disclosure, the term “absorption” need not imply chemical or molecular action, and may be taken to comprise at least one of adsorbing the vapour onto a surface, chemical absorption, take up of the vapour by chemical or molecular action, and at least temporary capture of the vapour in a porous material. As will also be appreciated, the volume flow rate along a flow passage may depend on the length and cross section of the flow passage, and the pressure difference applied to drive flow along the passage. Accordingly, a vapour permeable passage provides an example of a flow impeder in that the volume flow rate along the passage is impeded by the finite cross section and finite width of the passage. Flow may also be impeded by other examples of flow impeders such as any means of inhibiting flow, for example by slowing flow by means of adsorption, absorption, or by interposing a barrier in the flow. 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described. Although various configurations are discussed the apparatus, systems, subsystems, components, and so forth can be constructed in a variety of ways without departing from this disclosure. Rather, the specific features and acts are disclosed as example forms of implementing the claims.