Abstract:
A detecting method using an IMS apparatus with a preconcentrator outside its inlet aperture. Analyte vapor is adsorbed during a first phase when substantially no gas is admitted to the reaction region. The preconcentrator is then energized to desorb the analyte molecules and create a volume of desorbed molecules outside the IMS housing. Next, a pressure pulser is energized momentarily to drop pressure in the housing and draw in a small sip of the analyte molecules from the desorbed volume through the aperture. This is repeated until the concentration of analyte molecules in the desorbed volume is too low for accurate analysis, following which the apparatus enters another adsorption phase.

Description:
IDENTIFICATION OF RELATED PATENT APPLICATIONS 
     This patent application is a division of U.S. patent application Ser. No. 12/521,542, filed on Jun. 26, 2009, entitled “Detection Apparatus Accompanying Preconcentrator Pulsed Analyte Via An Aperture,” now U.S. Pat. No. 8,734,722, granted on May 27, 2014, which is a U.S. National Stage patent application under 35 U.S.C. Section 371 of PCT International Patent Application No. PCT/GB2007/004711, filed on Dec. 10, 2007, which in turn claims priority of Great Britain Patent Application No. 0625478.3, filed on Dec. 20, 2006, all of which are assigned to the assignee of the present patent application and all of which are hereby incorporated herein by reference in their entirety. 
     This application is related to three other patents, namely U.S. Pat. No. 8,668,870, granted on Mar. 11, 2014, entitled “Ion Mobility Spectrometer Which Controls Carrier Gas Flow to Improve Detection,” U.S. Pat. No. 8,158,933, granted on Apr. 17, 2012, entitled “Detector Apparatus and Preconcentrators,” and U.S. Pat. No. 8,022,360, granted on Sep. 20, 2011, entitled “Gas Preconcentrator for Detection Apparatus,” all assigned to the assignee of the present patent application, which three patents are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to detection apparatus of the kind for detecting or analyzing an analyte sample gas or vapor, the apparatus having an aperture through which analyte sample gas or vapor is admitted. 
     Ion mobility spectrometers or IMS apparatus are often used to detect substances such as explosives, drugs, blister and nerve agents, or the like. An IMS apparatus typically includes a detector cell to which a sample of air containing a suspected substance or analyte is continuously supplied as a gas or vapor. The cell operates at or near atmospheric pressure and contains electrodes energized to produce a voltage gradient along the cell. Molecules in the sample of air are ionized, such as by means of a radioactive source, UV source, or by corona discharge, and are admitted into the drift region of the cell by an electrostatic gate at one end. The ionized molecules drift to the opposite end of the cell at a speed dependent on the mobility of the ions. By measuring the time of flight along the cell, it is possible to identify the ions. In conventional IMS apparatus clean dry gas flows continuously through the reaction or ionization region. This arrangement allows for continuous sampling and short recovery times. Where the sample analyte is only present in small concentrations in the sample gas, there can be a relatively low signal-to-noise ratio and this can make reliable detection very difficult. 
     It is accordingly desirable to provide alternative detection apparatus. 
     The subject matter discussed in this background of the invention section should not be assumed to be prior art merely as a result of its mention in the background of the invention section. Similarly, a problem mentioned in the background of the invention section or associated with the subject matter of the background of the invention section should not be assumed to have been previously recognized in the prior art. The subject matter in the background of the invention section merely represents different approaches, which in and of themselves may also be inventions. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a detection apparatus of the above-specified kind, characterized in that the detection apparatus is arranged to establish a volume of analyte outside the detection apparatus and to supply analyte from the volume into the detection apparatus via an aperture in bursts smaller than that of the volume separated by periods when no analyte gas or vapor is admitted. 
     The detection apparatus preferably includes a preconcentrator by which the volume of analyte outside the apparatus is established. The preconcentrator may include polydimethylysiloxane. The detection apparatus preferably includes a pressure pulser connected with the interior of the detection apparatus by which the bursts of analyte are supplied into the detection apparatus. The detection apparatus may be an IMS, with the aperture opening into a reaction region and the reaction region opening into a drift region. 
     According to another aspect of the present invention there is provided a method of detecting an analyte sample gas or vapor, characterized in that the method includes the steps of establishing a volume of analyte, admitting bursts of analyte molecules less than the established volume to a reaction region, with the bursts being separated by periods of time during which substantially no analyte molecules are admitted, and detecting the presence of the admitted analyte molecules. 
     The volume of analyte is preferably established by adsorption and desorption. The analyte is preferably adsorbed while there is substantially no flow into the reaction region, with the analyte being subsequently desorbed and pressure momentarily reduced to draw desorbed analyte molecules into the reaction region. The pressure may be repeatedly reduced to draw a plurality of sips of analyte molecules from the desorbed molecules progressively to deplete the analyte molecules in the volume. Analyte sample gas or vapor may be adsorbed again by preconcentration, and pressure reduction may be stopped when the concentration of analyte in the volume has been depleted to an extent where analysis becomes less reliable, with desorption and pressure reduction being restarted again after sufficient time for detectable amounts of analyte to be released. 
    
    
     
       DESCRIPTION OF THE DRAWING 
       An IMS apparatus that is constructed and operated according to the present invention will now be described, by way of example, with reference to the accompanying FIGURE which shows IMS apparatus schematically. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the FIGURE, the detection apparatus takes the form of an ion mobility spectrometer (“IMS”) having a generally tubular housing  1  with an analysis or drift region  2  towards its right-hand end (as shown in the FIGURE) and an ionization or reaction region  3  towards its opposite left-hand end (as shown in the FIGURE). 
     An inlet conduit  4  opens at one end  5  to air or another source of gas or vapor to be sampled and analyzed. At its other end, the inlet conduit  4  connects with a pump  17 , by which the sample is drawn through the inlet conduit  4 , and an aperture  6  provided by a capillary passage or pin-hole, which communicates between the inlet conduit  4  and the interior of the reaction region  3  so that molecules of interest can pass from the inlet conduit  4  into the reaction region  3 . The aperture  6  could be provided by a membrane or other similar apparatus. The inlet conduit  4  includes a preconcentrator  7  or other similar apparatus for establishing a volume of analyte gas or vapor, the purpose of which will be described later. 
     The reaction region  3  contains apparatus to ionize molecules of the analyte substance, such as a corona discharge point  10 , at a high potential. The reaction region  3  and the drift region  2  are both at atmospheric pressure or just slightly below atmospheric pressure. The reaction region  3  and the drift region  2  may be separated from one another by an optional, conventional electrostatic shutter  11  such as a Bradbury Nielson gate by which the flow of ions into the drift region  2  may be controlled. The drift region  2  has a series of pairs of electrodes  12  on opposite sides thereof which are longitudinally spaced from one another along the length of the drift region  2 . A voltage supply  13  applies a voltage to each electrode pair  12 , which voltage increases from the left to the right along the length of the drift region  2  (as shown in the FIGURE) so that ions passed by the electrostatic shutter  11  are subject to a voltage gradient, which draws them along the length of the drift region  2 . A collector plate  14  mounted at the far, right-hand end of the drift region  2  (as shown in the FIGURE) collects ions after passage along the drift region  2 . The charge produced by each ion when it impacts the collector plate  14  is supplied as an electrical signal to a processor unit  15 . The processor unit  15  analyzes the signals to produce spectra representative of the mobility of the different ions detected and supplies these to a display or other utilization apparatus  16 . 
     A gas flow system  20  provides a flow of clean dry air along the inside of the housing  1  against the flow of the ions. The gas flow system includes a pump  21  with molecular sieve inlet and outlet filters  22  and  23  respectively located at its inlet and outlet. The inlet filter  22  connects with an inlet pipe  24 , which opens into the housing  1  towards the inlet end of the reaction region  3  (shown on the left end in the FIGURE). The outlet filter  23  connects with an outlet pipe  25 , which opens into the housing  1  towards the downstream end of the drift region  2  (shown on the right end in the FIGURE). The pump  21  operates to draw gas from the reaction region  3  so that it flows through the first filter  22 , the pump  21 , and the second filter  23  before flowing back into the housing  1  at the right-most end of the drift region  2  (as shown in the FIGURE). 
     The apparatus also includes a pressure pulser  8 , which may be an electromagnetic transducer similar to a loudspeaker, which is connected to the housing  1  in the manner described in U.S. Pat. No. 6,073,498, to Taylor et al., which is hereby incorporated herein by reference. The pressure pulser  8  is operated intermittently, momentarily to draw small volumes of sample vapor or gas into the reaction region  3  to produce a pressure pulse pattern of the kind illustrated. 
     The preconcentrator  7  includes a quantity of a material that will adsorb analyte vapor of interest and that can be arranged to desorb the vapor. A typical material that could be used is polydimethylsiloxane. Baffles could be arranged around the preconcentrator  7  to reduce sample losses due to diffusion. The preconcentrator  7  is located close to the aperture  6 . 
     In operation, during the adsorption phase, air to be sampled is flowed into the conduit  4  by means of the pump  17  so that there is a continuous flow in and out of the conduit  4 . During this adsorption phase, analyte vapor is adsorbed by the preconcentrator  7  and there is substantially no flow into the housing  1 . The apparatus then goes through a desorption phase during which the pump  17  is turned off so that the inlet flow to the conduit  4  is stopped to prevent the collected sample from being blown away. The preconcentrator  7  is then heated or otherwise actuated to release the adsorbed analyte vapor into the volume  9  between the preconcentrator  7  and the aperture  6 . At the same time, the pump  21  is turned off, or flow is substantially reduced, to prevent or reduce gas flow along the housing  1 . The pressure pulser  8  is then activated to cause pulsed reductions in pressure within the housing  1 . This has the effect of drawing in small bursts, puffs, or sips of the vapor in the volume  9  through the aperture  6  as jets into the reaction region  3 . The vapor in each such burst is ionized in the reaction region  3 , and ion mobility spectra are produced by the processor unit  15 . The volume of each burst caused by the pressure pulser  8  is substantially less than the static volume of vapor established by the preconcentrator  7  in the volume  9 . As such, the pressure pulser  8  can take many sips of the static vapor in the volume  9  before the concentration of the analyte in the volume  9  becomes depleted to an extent that the spectra produced become unreliable. When this happens, the detector reverts again to an adsorption phase. 
     By taking multiple sips from an established volume of analyte vapor, many spectra samples can be obtained, thereby enabling prolonged averaging and an increase in the signal-to-noise ratio. This is especially valuable where the analyte is only present in very low concentrations. The small sips of sample taken also avoid overloading the detector with excessive quantities of analyte substance where it is present in high quantities and enable the dynamic range of the detector apparatus to be extended. If the vapor concentration detected in the first burst is high, the detector apparatus could be arranged such that no further bursts are taken. Furthermore, the arrangement of the present invention avoids the problems that can occur when a high level of moisture is present, since this can prevent efficient ionization. By taking small bursts, the moisture is diluted to an extent such that it does not prevent detection of the compounds of interest. 
     Although it is preferable for gas flow within the housing  1  that are produced by the gas flow system  20  to be stopped when sample bursts are drawn into the reaction region  3 , it is not essential to do so, since the flow could be maintained to flush away the sample burst, providing that the sample stays within the reaction region  3  sufficiently long for analysis to take place. It should also be noted that it is not necessary to stop gas flow along the entire housing  1 , since the detection apparatus could have a separate gas flow path within the reaction region  3 . In such an arrangement, this separate gas flow could be stopped, and the remaining flow along the drift region  2  could be maintained. In another arrangement, the gas flow could be stopped when each burst is taken, and then restarted between bursts to flush away the analyte substance. Alternatively, the flow of gas through the reaction region  3  could be stopped for a period extending over several discrete bursts. 
     There are various alternative ways in which a volume of sample substance could be established. Microelectromechanical systems (“MEMS”) processes can be used to construct small chambers containing sorbent material onto which the vapor is adsorbed and from which it is desorbed. Instead of using heat to desorb the adsorbed substance, it would be possible to use radiation, pressure, or vibration to encourage desorption, either by itself or in conjunction with some other process. 
     The present invention can be used to enable small concentrations of analyte to be detected with improved signal-to-noise ratio. The present invention is particularly useful in IMS apparatus, but may also have application in different forms of detectors. 
     Although the foregoing description of the present invention has been shown and described with reference to particular embodiments and applications thereof, it has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the particular embodiments and applications disclosed. It will be apparent to those having ordinary skill in the art that a number of changes, modifications, variations, or alterations to the invention as described herein may be made, none of which depart from the spirit or scope of the present invention. The particular embodiments and applications were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such changes, modifications, variations, and alterations should therefore be seen as being within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled. 
     While the current application recites particular combinations of features in the claims appended hereto, various embodiments of the invention relate to any combination of any of the features described herein whether or not such combination is currently claimed, and any such combination of features may be claimed in this or future applications. Any of the features, elements, or components of any of the exemplary embodiments discussed above may be claimed alone or in combination with any of the features, elements, or components of any of the other embodiments discussed above.