Abstract:
An IMS detector has a pin-hole or capillary inlet having a coating of an adsorbent material, such as polydimethylsiloxane, which is adsorbent to an analyte substance of interest. The analyte is adsorbed into the material until a heater is energized to heat the adsorbent material and release the adsorbed analyte substance for detection in a detector.

Description:
This application is related to three other concurrently filed copending patent applications, namely U.S. patent application Ser. No. 11/918,940, entitled “Detection Apparatus,” U.S. patent application Ser. No. 12/521,537, entitled “Detection Apparatus,” and U.S. patent application Ser. No. 12/521,549, entitled “Gas Preconcentrator for Detection Apparatus,” all assigned to the assignee of the present patent application, which three patent applications are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to detector apparatus of the kind having an inlet in the form of a pin-hole or capillary passage by which an analyte substance is admitted to the interior of the detector and to preconcentrators. 
     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. 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 known to use a preconcentrator at the inlet in order to produce a bolus of sample with increased levels of analyte. The preconcentrator contains an adsorbent material to which the analyte substance in gas supplied to the preconcentrator binds during an adsorption phase. The preconcentrator is subsequently heated to cause the analyte substance to be desorbed as a bolus of gas with an increased concentration of analyte. Other forms of detector also make use of preconcentrators. 
     It is desirable to provide alternative detector apparatus and preconcentrators. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided detector apparatus of the above-specified kind, characterized in that the surface of the pin-hole or capillary passage over which the analyte substance flows consists at least in part of an adsorbent material, and that the detector apparatus is arranged and configured to cause the adsorbent material to desorb adsorbed substance on demand. 
     The adsorbent material may be a coating on the surface of the pin-hole or capillary passage. The adsorbent material may include polydimethylsiloxane. The apparatus preferably includes a heater by which the adsorbent material is caused to desorb an adsorbed substance. The apparatus may be an IMS apparatus, with the pin-hole or capillary passage opening to a reaction region that is arranged to ionize admitted analyte molecules, and a reaction region that is arranged to supply the ionized molecules to a drift region for detection. The apparatus may include a plurality of inlets, each of which has a pin-hole or capillary passage with a surface consisting at least in part of an adsorbent material. 
     According to another aspect of the present invention there is provided a preconcentrator for a detector apparatus, with the preconcentrator being arranged to adsorb analyte substance and to release the adsorbed substance on demand, characterized in that the preconcentrator has an adsorbent surface providing a pin-hole or capillary passage, and that the preconcentrator is arranged to provide an inlet to the detector apparatus. 
     The adsorbent surface is preferably made of polydimethylsiloxane. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       An IMS detector apparatus including a preconcentrator inlet that is constructed and operated according to the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  shows an IMS apparatus schematically; and 
         FIG. 2  shows a modification of the IMS apparatus of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference first to  FIG. 1 , the 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  FIG. 1 ) and an ionization or reaction region  3  towards its opposite left-hand end (as shown in  FIG. 1 ). 
     An inlet  4  at the left-hand end of the housing  1  (as shown in  FIG. 1 ) opens into the interior of the reaction region  3  so that molecules of interest can pass from outside into the reaction region  3 . The inlet  4  will be described in detail 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  FIG. 1 ) 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  FIG. 1 ) 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  FIG. 1 ) 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  FIG. 1 ). 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  FIG. 1 ). A pressure pulser  8 , which may be an electromagnetic transducer similar to a loudspeaker, may be connected to the interior of 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, to draw the analyte substance into the housing  1  via the inlet  4 . As so far described, the apparatus is relatively conventional. 
     The inlet  4  differs from conventional inlets in that it is provided by a capillary tube  40  with an axial passage or bore  41  having a coating of an adsorbent material  42  that will adsorb the analyte substance of interest. Typically, the diameter of the bore  41  is approximately 0.5 mm (it is not shown to scale in the drawings), and the material used for the coating of the adsorbent material  42  may be polydimethylsiloxane. The coating of adsorbent material  42 , therefore, forms the surface over which all of the sample inlet gas flows as it enters into the reaction region  3 . It is not essential, however, that the adsorbent surface be provided by a coating thereupon, since it could alternatively be provided by a coating of adsorbent material on a tube or sleeve located within the inlet  4 . Alternatively, the inlet  4  (or the tube or sleeve) could be made entirely of the adsorbent material  42  instead of being coated with it. The inlet  4  also includes a heater  43  that is connected to be operated by the processor unit  15  and by which the temperature of the adsorbent material  42  can be raised on demand as necessary. 
     In operation, the detector apparatus initially functions in an adsorption phase in which no heat is applied to the inlet  4 , so that most of the analyte substance of interest is adsorbed by the adsorbent material  42 . After a set time, the apparatus starts a desorption phase during which the processor unit  15  energizes the heater  43  to increase the temperature of the adsorbent material  42  and thereby cause the adsorbed analyte substance to be driven off as a bolus or concentrated burst into the reaction region  3 . This momentary high concentration of the analyte substance enables greater numbers of analyte ions to be produced, and produces spectra with an increased signal-to-noise ratio. 
     Locating the preconcentrator adsorbent material  42  in the inlet  4  itself ensures intimate contact of the inlet gas with the adsorbent material  42 . This leads to an efficient adsorption. It also enables the bulk of the adsorbent material to be minimized, leading to rapid thermal cycling and reduced energy consumption, which can be important in battery-powered devices. 
     If a single inlet  4  does not allow for sufficient flow of analyte substance into the detector it would be possible to have more than one inlet in the manner shown in  FIG. 2 . In this arrangement two inlets  104  and  204  are mounted side-by-side to form parallel entry paths into the reaction region. In other respects, the apparatus shown in  FIG. 2  is that same as that shown in  FIG. 1  (and thus elements in  FIG. 2  that are identical to the elements of  FIG. 1  are not provided with reference numerals). 
     The invention is particularly useful in IMS apparatus, but may also have application in different forms of detector. 
     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.