Patent Publication Number: US-6656738-B1

Title: Internal heater for preconcentrator

Description:
GOVERNMENT RIGHTS 
     The United States Government has certain rights to the claimed invention pursuant to contract number F41624-98-C1000. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus for collecting chemical samples, and more specifically, to an apparatus for accumulating a concentration of chemicals over a period of time. 
     To prevent injury resulting from exposure to toxic chemicals, the presence of toxic chemicals must be detected while their concentrations are below toxic levels. Accordingly, to detect highly toxic chemicals, devices capable of detecting low concentrations within a short period of time are needed. 
     One prior art device for detecting low concentrations of chemicals includes a pump such as a diaphragm pump and a preconcentrator tube. The pump pumps air through a preconcentrator tube where the chemicals accumulate. The preconcentrator tube may comprise a low thermal mass tube that houses a sorbent material. The terms “preconcentrator tube” and “sorbent material” are well known in the art and correspond to a tube for accumulating chemicals and a material for sorbing (and, therefore, accumulating the chemicals), respectively. 
     A heating element wrapped around the preconcentrator tube is used to heat the sorbent material and thereby desorb the chemicals. A single pump is used to pump air through the preconcentrator tube and to a detector. Chemicals in the air accumulate in the sorbent material contained within the preconcentrator tube. The heater on the outside of the preconcentrator tube is activated, and chemicals adsorbed onto the sorbent material are released. The chemicals released from the sorbent material are entrained in the air being pumped to the detector. 
     This prior art configuration is simple and low cost. Additionally, this configuration consumes little power in comparison to other prior art designs. However, one drawback of this prior art configuration is that the detector is unable to measure chemicals contained in the air in real-time since a period of time is required to accumulate chemicals in the sorbent material. During the period of time while the chemicals are accumulating within the preconcentrator tube, the user is blind to the presence of toxic chemicals in the air. This period of time may last several minutes. During this time, the user will be exposed to the chemicals, which may be present in toxic levels. Only when heat is applied to the sorbent material are the chemicals released and detected. 
     Another disadvantage of this prior art design is that the desorbed chemical must be passed through the entire length of the sorbent material prior to reaching the detector. However, the chemical may react with the sorbent material as it is passed through it. Consequently, a sample of chemical traversing the sorbent material may not accurately reflect the concentration of chemical entering the sorbent material. Additionally, unless the preconcentrator tube is heated for a sufficiently long enough time, all of the chemicals accumulated in the sorbent material will not be released. Again, the sample of chemical released from the sorbent material that reaches the detector may not accurately reflect the concentration of chemical entering the sorbent material. Additionally, the device may exhibit a memory effect in which chemicals remaining in the sorbent material may be released when the preconcentrator tube is heated a subsequent time. Artificially higher levels of chemical may be produced at the detector during this subsequent heating. 
     Another prior art configuration employs two pumps, a first pump and a second pump, a three-port three-way valve, a preconcentrator tube, and a detector. With the three-port three-way valve in the first position, two separate paths are created. A first path extends from the first inlet to the preconcentrator tube and from the preconcentrator tube to the first pump. A second path extends from the second inlet to the detector and from the detector to the second pump. In a second position, the three-port three-way valve creates a flow path from the second inlet to the preconcentrator tube, from the preconcentrator tube to the detector, and from the detector to the second pump. 
     With the three-port three-way valve in the first position, air is drawn in the first inlet, pumped through the three-port three-way valve and through the preconcentrator tube, and pumped out an exhaust connected to the first pump. In this manner, chemicals are collected in sorbent material contained inside the preconcentrator tube. Simultaneously, chemicals are drawn from the second inlet through the valve and to the detector. Thus, real-time detection is provided for chemicals present at concentrations high enough to be sensed by the detector. 
     The first pump is subsequently turned off, the three-port three-way valve is switched to the second position, and a heater surrounding the preconcentrator tube is activated. With the heater activated, the chemicals collected in the sorbent material will be released and drawn into the detector by the second pump. 
     When the three-port three-way valve is in the second position, the direction that the air is pumped through the preconcentrator tube is reversed. Accordingly, all of the chemicals collected in the sorbent material do not have to travel through the sorbent material to reach the detector, thus, lowering the likelihood of a chemical reaction between the chemicals and the sorbent material. Desorption is also more efficient. The sorbent material does not need to be heated as long since the chemical does not have to pass through all the sorbent material. Despite these advantages, this prior art configuration has serious disadvantages. In particular, the three-port three-way valve is large in volume and requires large amounts of energy such that its use in portable chemical sensor systems is impractical. 
     A further prior art configuration substitutes the three-port three-way valve employed in the second prior art configuration with three single-port three-way valves that are magnetically latched and consume less power than non-magnetically latched valves. Overall power consumption can be reduced by switching to magnetically latched valves. Although the size of three single-port three-way valves is slightly larger than the size of a single three-port three-way valve, the number of batteries required for the three single-port three-way valves is less. Nevertheless, this configuration requires too much space and energy for many field applications. 
     Accordingly, there is a need in the art for a chemical detection apparatus that may be miniaturized, is lightweight, and has relatively low power consumption. 
     SUMMARY OF THE INVENTION 
     An apparatus for collecting and detecting chemicals contained in a fluid comprises an enclosure which provides a fluid pathway for transmitting fluid therethrough. The enclosure has two fluid flow ports for allowing fluid to enter and exit the enclosure. A sorbent material and a heating element are contained within the enclosure. The apparatus further comprises at least one chemical sensor, a first pump, and a second pump. The first pump pumps fluid through the enclosure, thereby causing the chemicals to collect on the sorbent material. The second pump pumps the chemicals to the chemical sensor. The sorbent material may have a cavity therein with the heating element located within the cavity. Alternatively, the heating element may be interposed between the enclosure and the sorbent material. This heating element may comprises a resistive film formed on an interior surface of the enclosure. 
     A method for collecting and detecting chemicals in a fluid includes providing a sorbent material within an enclosure. The sorbent material has exterior walls and the enclosure has interior walls. The fluid is introduced into the enclosure thereby collecting the chemicals on the sorbent material. Heat is radiated from a source of heat located interior to the interior walls of the enclosure, thereby desorbing the chemicals from the sorbent material. The chemicals desorbed from the sorbent material are transferred out of the enclosure and to at least one chemical sensor. The method may comprise radiating heat from a source of heat located interior to the exterior walls of the sorbent material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a preferred embodiment of the present invention. 
     FIG. 2A is a fragmented cross-sectional view of a preconcentrator tube depicting a heating element passing through a sorbent material contained within the preconcentrator tube. 
     FIG. 2B is a perspective view of the preconcentrator tube and heating element shown in FIG.  2 A. 
     FIG. 3A is a fragmented cross-sectional view of a preconcentrator tube depicting a heating element comprising a heating element interposed between the preconcentrator tube and a sorbent material contained within the preconcentrator tube. 
     FIG. 3B is a perspective view of the preconcentrator tube and heating element shown in FIG.  3 A. 
     FIG. 4A is a fragmented cross-sectional view of a preconcentrator tube depicting a heating element surrounding the preconcentrator tube. 
     FIG. 4B is a perspective view of the preconcentrator tube and heating element shown in FIG.  4 A. 
     FIG. 5A is a schematic representation of the embodiment shown in FIG. 1 collecting chemicals in the sorbent material. 
     FIG. 5B is a schematic representation of the embodiment shown in FIG. 1 desorbing the chemicals from the sorbent material. 
     FIG. 6 is a plot, on axes of Time (in seconds) and Frequency Shift (in hertz), which depicts a sensor output during three cycles of collection and desorption. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An apparatus  10  for detecting chemicals, such as molecules, in accordance with a preferred embodiment of the present invention is shown in FIG.  1 . The apparatus  10  has a primary intake  12 , which corresponds to a first opening  14  leading into a primary intake passageway  16 . FIG. 1 shows a filter  17  inserted in the primary intake passageway  16 , which removes unwanted debris. 
     The primary intake passageway  16  extends to a preconcentrator element  18  comprising a tube having a front end portion  20  and a rear end portion  22 . The primary intake passageway  16  is connected to a side  24  of the preconcentrator tube  18  in the front end portion  20 . Although the sorbent (i.e. preconcentrator element)  18  in the embodiment shown in FIG. 1 is in the form of a tube, the present invention is not restricted to the use of a tube as a sorbent element. Chemicals may be accumulated in a chamber having a shape that is not tubular. 
     As used herein, the term “preconcentrator element,” is defined as a structure, such as a tube, for accumulating chemicals. In the preferred embodiment, such accumulation is achieved by a preconcentrator tube  18  containing a sorbent material  28 . The sorbent material  28  preferably comprises Tenax® TA 30/60 mesh, which is a porous polymer based on 2,6 diphenyl-ρ-phenylene oxide developed by AKZO Research Laboratories and which can be obtained from Alltech Associates. Alternatively, the sorbent material  28  may be selected from the group consisting of Tenax® GA, Carbosieve, or granulated charcoal. 
     Although a tube having sorbent material  28  is employed in the preferred embodiment, any sorbent element  18  may be used to accumulate a concentration of chemicals. For example, the sorbent element  18  may comprise a preconcentrator tube having a coated surface to which the chemicals adhere. Alternatively, the preconcentrator tube  18  may be cooled to promote adsorption on a surface therein. In any case, the sorbent element  18  permits fluid to flow therethrough, while collecting chemicals contained within the flowing fluid. 
     A bi-directional pump is defined herein as a pump that is capable of pumping fluid in two directions. For example, a bi-directional pump  30  shown in FIG. 1 as having an intake  32  and a vent  34  can pump air from the intake to the vent or from the vent to the intake. Preferably, the bi-directional pump  30  comprises a rotary vane pump. Other types of bi-directional pumps  30  that can be used in accordance with the present invention include blowers and positive displacement pumps. Also, the bi-directional pump  30  preferably is capable of pumping air at a flow rate in the range between 25 and 2000 standard cubic centimeters/minute (sccm). Most preferably, the bi-directional pump  30  is capable of pumping air at a flow rate in the range between 200 and 1000 sccm. 
     As shown in FIG. 1, the rearend portion  22  of the preconcentrator tube  18  is connected to an intake  32  on the a bi-directional pump  30  by a passageway  36 . A first flow meter  38  and a first scrubber  40  are disposed in the passageway  36  connecting the preconcentrator tube  18  to the bi-directional pump  30 . A flow meter measures the flow of air passing through it or other properties related to flow, e.g., rate of flow. A scrubber is defined herein in accordance with its conventional usage to be a filter for purifying a fluid. 
     Preferably, the first flow meter  38  comprises a bi-directional flow meter, i.e., a flow meter capable of measuring flow passing over a single pathway in either of two directions. In particular, the first flow meter  38  preferably can measure the flow of air from the preconcentrator tube  18  to the bi-directional pump  30  and can measure the flow of air from the pump to the preconcentrator tube. 
     As shown in FIG. 1, an exhaust line  42  extending from the bi-directional pump  30  has an outlet opening  44 , which preferably vents to the ambient atmosphere. 
     A second scrubber  46  is disposed in the exhaust line  42 . Preferably, both the first scrubber  40  and the second scrubber  46  are comprises of the same material as the sorbent material  28  contained within the preconcentrator tube  18 . For example, the first scrubber  40  as well as the second scrubber  46  may contain Tenax® TA. 
     A detector  50  comprises a housing  52  having an entrance orifice  54  and an exit orifice  56 . A passageway  48  connects the entrance orifice  54  of the detector housing  52  to a second opening  60  in the primary intake passageway  16 . The detector housing  52  preferably contains a sensor array  58  comprising a plurality of sensors capable of sensing the chemicals to be detected. Alternatively, the housing  52  may contain a single sensor. 
     The sensor array  58  of the preferred embodiment preferably comprises a plurality of surface acoustic wave (SAW) sensors. A chemical sensor array employing an array of SAW devices is disclosed in the application of William D. Bowers, et al. entitled “Chemical Sensor Array”, Ser. No. 09/151,747, filed on Sep. 11, 1998, now U.S. Pat. No. 6,321,588 which is hereby incorporated herein by reference. Alternatively, the detector  50  may comprise other chemical detectors including, e.g., a gas chromatograph, an ion mobility spectrometer, or a mass spectrometer. 
     A separate passageway  62  connects the exit orifice  56  of the housing  52  of the detector  50  to an intake  66  of a detector pump  64  having a vent  68 . The detector pump  64  preferably is capable of pumping air at a flow rate in the range between 50 and 500 standard cubic centimeters/minute (sccm) and vents to ambient air. A second flow meter  70  is disposed in the passageway  62  between the detector  50  to the detector pump  64 . 
     Each of the passageways, the primary intake passageway  16 , the passageway  36  connecting the preconcentrator tube  18  to the bi-directional pump  30 , the exhaust passageway  42  extending from the bi-directional pump, the passageway  48  connecting the primary intake passageway to the detector  50 , and the passageway  62  connecting the detector to the detector pump  64  may, in accordance with the present invention, be formed by tubes. Alternatively, the passageways  16 ,  36 ,  42 ,  48 ,  62 , may have shapes other than tubular. For example, the passageways  16 ,  36 ,  42 ,  48 ,  62 , may be formed as integrated flow circuits that are part of a manifold. 
     FIG. 1 depicts a heater  72  and a temperature sensor  74  in thermal contact with the preconcentrator tube  18  and the sorbent material  28 . An electronic controller  76  is electrically connected to a power supply  78 , which is electrically connected to the heater  72 , the bi-directional pump  30 , and the detector pump  64 . The first flow meter  38 , the second flow meter  70 , and temperature sensor  74  are also electrically connected to the controller  76 . 
     As shown in FIG. 1, a detector unit  80  is comprised of the detector  50 , the second flow meter  70 , the detector pump  64 , the electronic controller  76  and the power supply  78 . The preconcentrator tube  18 , the first flow meter  38 , and the bi-directional pump  30  are electrically connected to the detector unit  80 . A detector unit  80  and a sample acquisition device for obtaining a gaseous sample from a surface, which may be employed in conjunction with the preferred embodiment of the present invention, is disclosed in the application of William D. Bowers, Ser. No. 09/151,743, filed on Sep. 11, 1998, now U.S. Pat. No. 6,269,703 entitled “Pulsed Air Sampler,” which is hereby incorporated by reference. 
     As best seen in FIGS. 2A and 2B, the heater  72  has a heating element  82  that extends into the preconcentrator tube  18  from the end  26  of the tube. The heating element  82  extends through a central, elongated, longitudinal cavity in the sorbent material  28 . 
     The heating element  82  shown in FIGS. 2A and 2B comprises a film  85  that is formed on the exterior surface of a ceramic tube  84 . The film  85  covers at least that portion of the ceramic tube  84  that extends into the longitudinal cavity of the sorbent material  28 . The film  85  has a primary surface  88  which is juxtaposed with and in substantial contact with the sorbent material  28 . The portion of the ceramic tube  84  not covered with the film  85  is a no heat zone that does not generate heat. Preferably, this film  85  comprises a material, which when deposited, forms a resistive film. More specifically, this film  85  may comprise material selected from the group consisting of indium tin oxide (ITO) films and printed resistive ink films used in the semiconductor industry. This film  85  may also comprise a vapor deposited film, a thick film, or a thin film. The ceramic tube  84  may comprise aluminum oxide. Two heater wires  86 , which are connected to the power supply  78 , extend from the heater  72 . While the resistive film  85  formed on the ceramic tube  84  is shown in FIGS. 2A and 2B, other types of heaters  72  may be employed. 
     As shown in FIG. 2A, the sorbent material  28  is secured inside  90  the preconcentrator tube  18  with retainer screens  92   a ,  92   b . One of the retainer screens  92   a  is mounted on the ceramic tube  84  while the other retainer screen  92   b  is attached to an inside wall  94  of the preconcentrator tube  18 . In the preferred embodiment, the sorbent material  28  has a size such that it contacts the inside wall  94  of the preconcentrator tube  18  as shown in FIG.  2 A. 
     FIGS. 3A and 3B depict the heating element  82  interposed between the preconcentrator tube  18  and the sorbent material  28 . This heating element  82  is situated so as to surround the sorbent material  28 . The heating element  82  shown in FIGS. 3A and 3B comprises a film formed between the preconcentrator tube  18  and the sorbent material  28 . Preferably this film may comprise a material, which when deposited, forms a resistive film. More specifically, this film may comprise material selected from the group consisting of indium tin oxide (ITO) films and printed resistive ink films used in the semiconductor industry. This film may also comprise a vapor deposited film, a thick film, or a thin film. 
     The heating element  82  shown in FIGS. 3A and 3B has a first surface  96  which is in contact with and is juxtaposed with the sorbent material  28 . A second surface  98 , opposite to the first surface  96 , is juxtaposed with and contacts the inside wall of the tube  18 . 
     The preconcentrator tube  18  depicted in FIGS. 3A and 3B preferably comprises a ceramic. More preferably, the preconcentrator tube  18  comprises a ceramic having low thermal mass (or low mass). Most preferably, the preconcentrator tube  18  also has a low specific heat so as to enable rapid heating and cooling of the preconcentrator tube. 
     The sorbent material  28  is secured inside  90  the preconcentrator tube  18  with a retainer screens  92   a ,  92   b . In FIG. 3A, each of the retainer screens  92   a ,  92   b  is affixed to the inside wall  94  of the preconcentrator tube  18 . 
     FIGS. 4A and 4B depict the preconcentrator tube  18  with the heating element  82  wrapped around it. This heating element  82  is situated so as to surround the preconcentrator tube  18 . 
     The heater  72  shown in FIGS. 4A and 4B comprises a thin foil heater. Two heater wires  86 , which are connected to the power supply  78 , extend from the heating element  82 . Although a thin foil heater is shown in FIGS. 4A and 4B, other heaters  72  that heat the sorbent material  28  from outside the preconcentrator tube  18  may be employed. For example, a current could be passed through a wire wrapped around the preconcentrator tube  18 . A wire wound heater comprising heater wire with insulation wound around the wire could be used. 
     The heating element  82  for the foil heater shown in FIGS. 4A and 4B has a first primary surface  102  which is not in contact with but faces the sorbent material  28 . A second primary surface  104 , opposite to the first surface  102 , is neither in contact with nor facing the sorbent material  28 . The heating element  82  also has end surfaces  106 , perpendicular to the first and second primary surfaces  102 ,  104 , which are not in contact nor facing the sorbent material  28 . 
     The preconcentrator tube  18  depicted in FIGS. 4A and 4B preferably comprises a metal, and more preferable low thermal mass metal. Low thermal mass can be achieved with low mass; accordingly, a preconcentrator tube  18  having thin walls may be advantageously employed. 
     FIG. 4A also shows each of the retainer screens  92   a ,  92   b  affixed to the inside wall  94  of the preconcentrator tube  18  and the sorbent material  28  extending to the inside walls of the preconcentrator tube. 
     As illustrated in FIGS. 2A-2B,  3 A- 3 B, and  4 A- 4 B, the heating element  82  can be either within the sorbent material  28  or outside the sorbent material. However, a heating element  82  within the sorbent material  28  provides more even heating therein. Additionally, heating from within the sorbent material  28  is more efficient and faster. When the heating element  82  is contained within the sorbent material  28 , most of the surface area of the heating element will face and be in contact with the sorbent material. Accordingly, less heat is lost by being radiated or conducted away from the sorbent material  28 . 
     For example, the heating element  82  shown in FIGS. 2A-2B is more efficient than the foil heater shown in FIGS. 4A-4B because more of the surface area on the heating element is usable for transferring heat to the sorbent material  28 . A substantial portion of the primary surface  88  of the heating element  82  depicted in FIGS. 2A-2B is in contact with or facing the sorbent material  28 . Heat, thus, can be transferred to the sorbent material  28  by conduction or radiation through the primary surface  88 . 
     In contrast, although the first primary surface  102  of the foil heater depicted in FIGS. 4A-4B faces the sorbent material  28 , the second primary surface  104  does not; and the second primary surface is the same size as the first primary surface. Thus, at least half of the surface area of the foil heater does not participate in the direct transfer of heat to the sorbent material  28 . Only the first primary surface  102  of the foil heater directs thermal energy to the sorbent material  28 . The end surfaces  106  of the foil heater depicted in FIGS. 2A-2B do not directly transfer heat to the sorbent material  28  since they neither faces nor contact the sorbent material. 
     Accordingly, the heating element  82  shown in FIGS. 2A-2B is more efficient than the heating element in FIGS. 4A-4B. For example, less heat is radiated away from the sorbent material  28 . Additionally, the heating element  82  in FIGS. 2A-2B is directly in contact with the sorbent material  28  while the heater foil shown in FIGS. 4A-4B is not. 
     The heating element  82  in FIGS. 3A-3B is also more efficient than the heating element in FIGS. 4A-4B. The heating element  82  in FIGS. 3A-3B has a first surface  96  that is in direct contact with and faces the sorbent material  28  whereas the thin foil heater in FIGS. 4A-4B is not in direct contact with the sorbent element. 
     FIGS. 5A and 5B depicts the operation of the embodiment shown in FIG.  1 . FIG. 5A illustrates a first mode of operation, the collection mode, wherein chemicals are collected in the sorbent material  28 . FIG. 5B depicts a second mode of operation, the desorption mode, wherein chemicals collected on the sorbent material are desorbed from it. 
     In the collection mode, chemicals such as molecules are collected on the sorbent material  28  by activating the bi-directional pump  30  such that air is pulled into the primary intake  12  and through the preconcentrator tube  18 . This air is exhausted out the vent  34  of the bi-directional pump  30 . To cause the air to flow from the intake  12  through the preconcentrator tube  18 , the bi-directional pump  30  operates in a first direction, herein designated as the forward direction. 
     FIG. 5A shows arrows  108 ,  110 , which represent the flow of air into the primary intake  12  and through the primary intake passageway  16 . The air drawn into the primary intake  12  passes through the first filter  17 , which removes unwanted debris from the air, thereby preventing this unwanted debris from accumulating on the sorbent material  28 . 
     The air travels from the primary intake passageway  16  into the preconcentrator tube  18  as indicated by another arrow  112 . The air entering the preconcentrator tube  18  flows through the sorbent material  28  which traps chemicals  114  therein. 
     The air exits the rear  22  of the preconcentrator tube  18  and moves into the passageway  36  connecting the preconcentrator tube and the bi-directional pump  30 . The air passes through the first flow meter  38  as illustrated by two arrows  116   a ,  116   b . The first flow meter  38  measures the flow or rate of flow through the preconcentrator tube  18  and through the sorbent material  28 . 
     Electrical connection to the controller  76  enables the first flow meter  38  to provide feedback to regulate the rate at which the bi-directional pump  30  passes air through the preconcentrator tube  18 . More specifically, the first flow meter  38  outputs a signal indicative of the flow or flow rate through the preconcentrator tube  18  enabling the controller  76  to adjust the pump rate of the bi-directional pump  30  accordingly. The controller  76  will set the power from the power supply  78  that is directed to the bi-directional pump  30 , thus, controlling the rate that the bi-directional pumps the air through the sorbent material  28 . 
     The bi-directional pump may also have a tachometer that measures and outputs pump speed (e.g., revolutions/minute). This tachometer can, therefore, provide feedback for controlling the flow rates as well as provide general diagnostics information. 
     The number density of chemicals  114  collected on the sorbent material  28  is a function of the chemical nature of the sorbent, the concentration of the chemicals in the air, and the total volume of air pumped through the sorbent material. The total volume of air pumped through the sorbent material depends on the flow rate of the air through the sorbent material  28  and the length of time the air passes through the sorbent material. Higher flow rates through the sorbent material  28  mean shorter the times required for collecting a sufficient number density of chemicals for detection. Accordingly, high flow rates through the sorbent material are preferred. These flow rates may range between 25 and 2000 standard cubic centimeters/minute (sccm), and more preferably, between 200 and 1000 sccm. Most preferably, the maximum flow rate provided by the bi-directional pump  30  is employed during the collection mode. 
     Arrows  116   b ,  116   c  on two sides of the first scrubber  40  illustrate the passage of the air through a first scrubber prior to entering the bi-directional pump  30 . The first scrubber  40  filters out any chemicals that would otherwise reach the bi-directional pump  30 . As discussed above, the first scrubber  40  preferably comprises sorbent material that will trap the type of chemicals to be detected at the detector  50 . Without the first scrubber  40 , these chemicals would reach and contaminate the bi-directional pump  30 . As a result, some of these chemicals might be ejected by the bi-directional pump  30  back into the passageway  36  between the preconcentrator tube  18  and the bi-directional pump whenever bi-directional pump pumps in the reverse direction. These chemicals would then flow through the preconcentrator tube  18  and into the detector  50 , creating an artificially high reading by the detector. In the preferred embodiment, however, the air in passageway  36  between the preconcentrator tube  18  and the bi-directional pump  30  is pumped through the first scrubber  40  when in the collection mode. Chemicals are trapped in the sorbent material in the first scrubber  40  and do not reach the bi-directional pump  30 . 
     After passing through the first scrubber  40 , the air moves into the bi-directional pump  30 . The bi-directional pump  30  provides the pumping power to transfer the air from the primary intake  12  and through the preconcetrator tube  18 . The bi-directional pump  30  is shown in FIG. 5A as a rotary vane pump. An arrow  118  indicates the motion of the rotor and vanes  120  in the bi-directional pump  30  when the apparatus  10  for detecting chemicals operates in the collection mode. 
     The air drawn into the bi-directional pump  30  is expelled out the vent  34  of the bi-directional pump and into the exhaust line  42 . The air passes through the second scrubber  46  as illustrated by two arrows  122   a ,  122   b  on two sides of the second scrubber  46 . Another arrow  124  represents the air being discharged from the opening  44  in exhaust line  42 . 
     During the collection mode, while the bi-directional pump  30  pumps air through the preconcentrator tube  18 , the detector pump  64  samples air drawn in the primary intake  12  to detect the presence of the chemicals in real-time. In particular, a portion of the air entering the primary intake  12  and passing through the primary intake passageway  16  is drawn by the detector pump  64  into the passageway  48  between the primary intake passageway and the detector  50 . Arrows  126  indicate the flow of air from the primary intake passageway  16 , through the passageway  48  between the primary intake passageway and the detector  50  and to the detector. The air enters the detector  50  through the entrance orifice  54  and passes over the sensor array  58 . The sensor array  58  detects the presence the chemicals to be detected and outputs a signal, which is communicated to the user of the apparatus  10 . 
     The air passing over the sensor array  58  exits the detector  50  through the exit orifice  56  and enters the passageway  62  between the detector  50  and the detector pump  64  as indicated by another arrow  128 . The air proceeds through the second flow meter  70  to the detector pump  64 . The air is drawn into the intake  66  of the detector pump  64  and expelled out the vent  68  of the detector pump. The detector pump  64  provides the pumping power to draw a portion of the air from the primary intake passageway  16  to the detector  50  while the second flow meter  70  measures the flow or rate of flow through the detector. Electrical connection of the second flow meter  70  to the controller  76  enables the second flow meter to provide feedback to regulate the rate at which the detector pump  64  passes air through the detector  50 . More specifically, the second flow meter  70  outputs a signal indicative of the flow or flow rate through the detector  50 , thereby enabling the controller  76  to set the power from the power supply  78  that is directed to the detector pump  64 . 
     After a period of time during which chemicals  114  are collected in the preconcentrator tube  18 , the apparatus  10  for detecting chemicals is shifted from the collection mode to the desorption mode. In the desorption mode, chemicals  114  collected on the sorbent material  28  are desorbed by activating the heater  72  while air is pumped from the exhaust line  42  of the bi-directional  30  and through the preconcentrator tube  18 . 
     To shift from the collection mode to the desorption mode, the bi-directional pump  30  is switched from pumping in the first direction (i.e. the forward direction) to pumping instead in a second direction, herein designated the reverse direction. Accordingly, the flow of the air through the bi-directional pump  30  and through the pre-concentrator tube  18  is reversed, as air is pumped from the opening  44  in the exhaust line  42  of the bi-directional pump to the preconcentrator tube. 
     FIG. 5B depicts the operation of the apparatus for detecting chemicals in the desorption mode. Air is drawn into the opening  44  in the exhaust line  42 . This air is pumped through the exhaust line  42  to the bi-directional pump  30 . Before reaching the bi-directional pump  30 , the air passes through the second scrubber  46  as illustrated by two arrows  130   a ,  130   b.    
     The second scrubber  46  removes unwanted chemicals from the air entering the exhaust line  42 , thereby preventing these unwanted chemicals from entering the bi-directional pump  30 . As discussed above, the second scrubber  46  preferably comprises sorbent material that will trap the type of chemicals to be detected at the detector  50 . Chemicals are trapped in the sorbent material in the second scrubber  46  and never reach the bi-directional pump  30 . Accordingly, these unwanted chemicals will not flow through the preconcentrator tube  18  and into the detector  50 , affecting the output of the detector. 
     The air in the exhaust line  42  proceeds to the bi-directional pump  30  where it drawn into the vent  34  on the bi-directional pump. The bi-directional pump  30  pushes this air out of the intake  32  of the bi-directional pump and into the passageway  36  connecting the bi-directional pump with the preconcentrator tube  18 . In this manner, the bi-directional pump  30  provides the pumping power to draw air from the opening  44  in the exhaust line  42  and force the air through the preconcetrator tube  18 . 
     The bi-directional pump  30  is shown in FIG. 5B as a rotary vane pump. An arrow  132  indicates the motion of the rotor and vanes  120  in the bi-directional pump  30  when the apparatus  10  for detecting chemicals operates in the desorption mode. In the case where the bi-directional pump  30  is a rotary vane pump, to switch the direction of the bi-directional pump from the forward direction to the reverse direction, the polarity of the voltage supplied by the power supply  78  to the bi-directional pump is reversed. Switching polarity causes the rotor and vanes  120  to spin in an opposite direction. 
     Arrows  134  in FIG. 5B represent the flow of air through the passageway  36  connecting the bi-directional pump  30  with the preconcentrator tube  18 . FIG. 5B depicts the air passing through the first scrubber  40 , which filters out any chemicals released by the bi-directional pump  30 . Such chemicals are trapped in the sorbent material in the first scrubber  40  and do not reach the preconcentrator tube  18  or the detector  50 . 
     The air, after passing through the first scrubber  40 , continues on through the first flow meter  38 , which measures the flow or rate of flow through the preconcentrator tube  18  and through the sorbent material  28 . Electrical connection to the controller  76  enables the first flow meter  38  to provide feedback to regulate the rate at which the bi-directional pump  30  passes air through the preconcentrator tube  18 . 
     The air travels from the passageway  36  connecting the bi-directional pump  30  to the preconcentrator tube  18  and on into the preconcentration tube where it flows through the sorbent material  28 . The heater  72  is activated when the apparatus  10  is in the desorption mode so as to provide energy to cause the chemicals  114  collected in the sorbent material  28  to be desorbed into the air flowing through the preconcentrator tube  18 . These chemicals  114 , once desorbed, are carried away by the air flowing through the preconcentrator tube  18  as illustrated by an arrow  136  at the front of the preconcentrator tube shown in FIG.  5 B. 
     Power is supplied to the heater  72  by the power supply  78 , which is regulated by feedback from the temperature sensor  74 . The temperature sensor  74 , in thermal contact with the sorbent material  28  through the preconcentrator tube  18 , sends a signal to the controller  76 , which then adjusts the amount of power from the power supply  78  that is delivered to the heater  72 . 
     Preferably, the heater  72  heats the sorbent material  18  to a temperature in the range between about 70° C. and 250° C. (200° C. for Tenax®) to desorb chemicals therefrom. More preferably, the temperature controller  76  is programmed to raise the temperature of the sorbent material  28  in stages, holding the temperature at a number of temperature setpoints prior to reaching the temperature at which the chemicals to be detected are desorbed from the sorbent material. 
     In the case where low concentrations of a chemical are to be detected, a small amount of the chemical may be mixed with a large number of background chemicals. These background chemicals can be present in concentrations that are several orders of magnitude higher than the chemicals of interest. If the preconcentrator tube  18  is heated in a single step to a temperature at which all of the chemicals to be detected are desorbed from the sorbent material, the background chemicals may be released with the chemicals of interest. The release of the background chemicals creates “chemical noise,” that is, the detector  50  must be able to distinguish the presence of the chemicals to be detected from the background chemicals. 
     However, the ability to distinguish the chemicals of interest from other chemicals can be improved by separating the times at which different chemicals are desorbed from the sorbent material  28 . The temperature of the sorbent material  28  can be raised in stages, being held for a period of time at one or more temperatures lower than the temperature at which the chemicals to be detected are desorbed from the sorbent material. The high concentrations of background chemicals will then reach the detector at a different time as the chemicals to be detected. 
     For example, if non-volatile toxic chemicals are to be detected, the preconcentrator tube  18  can be heated first to a temperature lower than the temperature at which the non-volatile toxic chemical is desorbed, then to the temperature at which the non-volatile toxic chemical is desorbed. The background chemicals with low volatility are thereby first driven off the sorbent material  28 . Accordingly, chemical noise at the detector  50  is reduced at the point in time when the non-volatile toxic chemical reaches the detector. 
     The chemicals  114  once desorbed from the sorbent material  28 , are carried away by the air flowing through the preconcentrator tube  18  into the primary intake passageway  16 . Pumping power to push the air out the preconcentrator tube  18  and into the primary intake passageway  16 , is provided by the bi-directional pump  30 . 
     The detector pump  64  continues to draw air into the detector  50  while the apparatus  10  for detecting chemicals is in the desorption mode. Consequently, air being pumped through the preconcentrator tube  18  and into the primary intake passageway  16  is directed into the passageway  48  connecting the primary intake passageway to the detector  50 . Arrows  138  indicate the flow of air from the primary intake passageway  16  into the passageway between the primary intake passageway and the detector  50 . The air then enters the detector  50  and passes over the sensor array  58 . The sensor array  58  detects the presence the chemicals and outputs a signal, which is communicated to the user of the apparatus  10 , preferably after some processing. 
     The detector pump  64  provides the pumping power to draw the air from the primary intake passageway  16  to the detector  50 . The second flow meter  70  measures the flow or rate of flow through the detector  50  and provides feedback to the controller  76 . The controller  76  is used to regulate the rate at which the detector pump  64  passes air through the detector  50 . 
     Preferably, the bi-directional pump  30  pumps the air through the preconcentrator tube  18  at a rate that is about equal to the rate at which the detector pump  64  pumps air to the detector  50 . If the flow rate of air through the preconcentrator tube  18  exceeds the flow rate of air to the detector  50 , some air passing through the preconcentrator tube will not enter the passageway  48  connecting the primary intake passageway  16  to the detector  50 . Instead, some air will escape through the primary intake  12 . Accordingly, some of the chemicals to be detected will not reach the detector  50  for detection. In contrast, if the flow rate of air to the detector  50  exceeds the flow rate of air through the preconcentrator tube  18 , additional air drawn in through the primary intake  12  will reach the detector, diluting the concentration of chemicals  114  released from the preconcentrator tube. Thus, the flow rate of air passing through the preconcentrator tube  18 , which carries the chemicals  114  out of the sorbent material  28  should be about equal to the flow rate of air to the detector  50 . Toward this end, the first and second flow meters  38 ,  70  can be used to provide feedback to the controller  76 , to thereby match the rate of flow of air through the preconcentrator tube  18  with the rate of flow of air to the detector  50 . Alternatively, the pressure at the preconcentrator tube  18  could be matched with the pressure at the detector  50 . For the foregoing reasons, the measurements made by the detector  50  will be most accurate when the pressure at the preconcentrator tube  18  is approximately equal to the pressure at the detector  50 . 
     FIG. 6 illustrates the reproducability of measurements obtained using the apparatus  10  depicted in FIG. 1 with a SAW sensor as the detector  50 . A SAW sensor outputs an oscillating signal having a frequency that shifts when chemicals are adsorbed onto the sensor. This frequency shift, in hertz, is plotted over time, in seconds, to track the output of the SAW sensor during three cycles of collection and desorption. The intervals when the detector pump  64  pumps air across the detector  50  while chemicals collect in the preconcentrator tube  18  correspond to three periods of time T 1 , T 2 , T 3 . The SAW sensor output during these three periods T 1 , T 2 , T 3  is relatively small, i.e., in the range of between 40 to 100 Hz. In contrast, three peaks  140  results with the release of the chemicals  114  during desorption. The magnitude of the peaks is fairly reproduceable, 560, 580, and 580 Hz, respectively, indicating that chemicals are not randomly released from the preconcentrator tube  18 , the first and second scrubbers  40 ,  46  nor the bi-directional pump  30  during desorption. 
     Employing a bi-directional pump  30  and reversing its direction to effectuate detection of the chemicals  114  released from the preconcentrator tube  18  eliminates the requirement for a valve to switch the direction of flow through the preconcentrator tube. Without needing such a valve to alter the flow of air, the apparatus  10  for detecting chemicals becomes less complex, smaller in size, and less costly. Additionally, since no valve is required to direct the flow of air during the collection mode and the desorption mode, the apparatus  10  consumes less power. 
     Switching the direction of pumping of the bi-directional pump  30  reverses the flow through the preconcentrator tube  18 , and thus, the thermally desorbed chemicals do not need to proceed completely through the sorbent material  28 . The desorbed chemicals only need to travel out the side they were introduced. As a result, the chemicals  114  are less likely to chemically react with the sorbent material  28 . Additionally, desorption efficiency is increased, and the sorbent material  28  need not be heated as long. Furthermore, real-time measurement is possible as the detector  50  continuously monitors the air during both the collection and the desorption modes. 
     Although the preferred embodiment is described as detecting chemicals contained in air, separate embodiments of the invention may be used to detect chemicals in other fluids. Additionally, the apparatus  10  may be employed to detect the presence of chemicals other than molecules, such as sub-micron, neutrally charged chemicals or chemicals in the form of aerosols. 
     The present invention may be embodied in other specific forms without departing from the essential characteristics as described herein. The embodiments described above are to be considered in all respects as illustrative only and not restrictive in any manner. The scope of any invention is, therefore, indicated by the following claims rather than the foregoing description. Any and all changes which come within the meaning and range of equivalency of the claims are to be considered in their scope.