Abstract:
IMS apparatus has an inlet with a preconcentrator opening into a reaction region where analyte molecules are ionized and passed via a shutter to a drift region for collection and analysis. A pump and filter arrangement supplies a flushing flow of clean gas to the housing in opposition to ion flow. A pressure pulser connects with the housing and is momentarily switched to cause a short drop in pressure, in the housing to draw in a bolus of analyte sample from the preconcentrator. Just prior to admitting a bolus of sample, the pump is turned off so that the flushing flow drops substantially to zero, thereby prolonging the time the analyte molecules spend in the reaction region.

Description:
IDENTIFICATION OF RELATED PATENT APPLICATIONS 
       [0001]    This application is related to three other concurrently filed copending patent applications, namely U.S. patent application Ser. No. ______, entitled “Detection Apparatus,” U.S. patent application Ser. No. ______, entitled “Detector Apparatus and Preconcentrators,” and U.S. patent application Ser. No. ______, 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  
       [0002]    This invention relates to detection apparatus of the kind including a reaction region and an analysis region where ion species produced in the reaction region are detected, and an arrangement for supplying a flow of clean gas through the reaction region. 
         [0003]    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. 
         [0004]    It is accordingly desirable to provide alternative detection apparatus. 
       SUMMARY OF THE INVENTION 
       [0005]    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 includes an arrangement for momentarily admitting an analyte gas or vapor to the reaction region, that the supply arrangement is arranged so as to reduce the flow of clean gas through the reaction region substantially to zero just prior to admitting the analyte gas or vapor to the reaction region such that the residence time of the analyte gas or vapor in the reaction region is increased, and that the supply arrangement is arranged subsequently to increase the flow of clean gas through the reaction region. 
         [0006]    The arrangement for momentarily admitting the analyte gas or vapor preferably includes a pressure pulser arranged to reduce pressure in the detection apparatus momentarily. The detection apparatus may have an inlet arrangement including a preconcentrator. The arrangement for flowing clean gas through the reaction region may be arranged and configured to flow the clean gas along substantially the length of the detection apparatus. Alternatively, the arrangement for flowing clean gas through the reaction region may include a first gas flow circuit connected between an end of the detection apparatus remote from its inlet and an end of the reaction region remote from the inlet and the detection apparatus may include a secondary circuit extending from the first circuit to an end of the reaction region adjacent the inlet, with the secondary circuit being closed when a sample is to be admitted. The detection apparatus may be an ion mobility spectrometer. 
         [0007]    According to another aspect of the present invention, there is provided a method of detecting substances including the steps of admitting a sample of the substance into a reaction chamber, flowing a gas through the reaction chamber, producing ions from the sample, passing ions from the reaction chamber to a collector for detection, and periodically reducing the flow of gas through the reaction chamber thereby to prolong the time during which the sample is present in the reaction chamber. 
         [0008]    The ions are preferably passed from the reaction chamber to the collector via a drift region having a voltage gradient along its length. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0009]    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 drawings, in which: 
           [0010]      FIG. 1  shows the detection apparatus schematically; and 
           [0011]      FIG. 2  shows alternative detection apparatus schematically. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0012]    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 ). 
         [0013]    An inlet conduit  4  opens at one end  5  to air or another source of gas or vapor to be sampled and analyzed. Air or gas is drawn through the conduit  4  by means of a pump  6  connected at the opposite end of the inlet conduit  4 . At some point along the conduit  4  a capillary passage  7  communicates between the conduit  4  and the interior of the reaction region  3  so that molecules of interest can pass from the conduit  4  into the reaction region  3 . There are various other conventional arrangements by which substances can be admitted to the apparatus, such as utilizing a pin-hole, a membrane, or other similar apparatus. A pressure pulser  8 , which may be an electromagnetic transducer similar to a loudspeaker, 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 reduce pressure in the housing  1  and hence draw sample vapor or gas into the reaction region  3  as a bolus. A preconcentrator  9  may be included in the inlet conduit  4  or in the capillary passage  7  into the apparatus itself. 
         [0014]    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 . 
         [0015]    As in a conventional IMS apparatus, 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 ). 
         [0016]    The apparatus differs from conventional IMS apparatus. The apparatus of the present invention is arranged so that initially the gas flow system  20  supplies clean dry gas to the housing  1  before a sample gas or vapor is admitted. Just prior to triggering the pressure pulser  8  to introduce a bolus of the sample gas or vapor, the gas flow to the housing  1 , and in particular to the reaction region  3 , is reduced to zero or near zero by turning off the pump  21 . The pressure pulser  8  is then triggered momentarily to inject a sample of analyte gas or vapor into the reaction region  3 . Alternatively, the pressure pulser  8  could be dispensed with and sample gas or vapor just allowed to diffuse into the reaction region  3 . Ions are produced continuously by the corona discharge point  10  from what is a substantially stationary sample cloud, which has a considerably increased residence time compared with conventional apparatus having a continuous gas flow. This enables the processor unit  15  to produce continuous ion mobility spectra. The ionization process does not significantly deplete the sample gas or vapor so a much longer average of ion mobility spectra can be acquired. This increases the signal-to-noise ratio. Just before the next analysis is required, the pump  21  is restarted to drive clean dry air through the apparatus and flush out the previous sample in the reaction region  3 . 
         [0017]    It is not essential to stop gas flow through the entire housing  1 ; rather, it is only necessary to stop or substantially reduce gas flow through the reaction region  3  in order to increase the residence time during which the sample gas or vapor is subject to ionization. Some IMS apparatus have separate gas flow paths in the drift region and the reaction region. An IMS apparatus of this kind that is adapted to the present invention is shown in  FIG. 2 , where equivalent items to those in  FIG. 1  are given the same reference numerals with the addition of  100 . It can be seen that the inlet pipe  124  that is connected with a first filter  122  is located towards the right-most, downstream end of the reaction region  103  close to the electrostatic shutter  111 . A spur pipe  126  forms a part of a secondary circuit and connects between the outlet of the pump  121  and a second filter  123 . The spur pipe  126  extends to the inlet of a third molecular sieve filter  127 . The outlet of the third filter  127  connects to a secondary outlet pipe  128 , which opens into the housing  101  via a valve  129 , with the opening of the secondary outlet pipe  128  into the housing  101  being located toward the left-hand end of the reaction region  103  (as shown in  FIG. 2 ). The valve  129  is controlled electrically by the processor unit  115  via a cable  130 . In this arrangement, the pump  121  operates continuously so that clean air flows in at the collector end of the drift region  102  and flows out close to the electrostatic shutter  111  at the downstream end of the reaction region  103 . When the processor  115  opens the valve  129 , gas will also flow via the spur pipe  126 , the third filter  127 , and the secondary outlet pipe  128  into the reaction region  103 . This gas will flow to the right and will pass out of the region  103  via the outlet pipe  124 . When a sample is to be admitted, the processor  115  closes the valve  129  to prevent gas entering the reaction region  103  via the pipe  128 . Some gas will still flow through the reaction region  103  from the drift region  102 , since this part of the gas flow is still operating, but this will be through a smaller portion of the reaction region  103  so the residence time for which the sample is exposed to the ionization effect will still be increased. 
         [0018]    The present invention is particularly suited to detection arrangements where the sample is administered to the apparatus in the form of a bolus, such as by means of a preconcentrator inlet system. The invention is not necessarily confined to IMS apparatus, but may also be applicable to other detection apparatus. 
         [0019]    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.