Abstract:
A sampling system for detecting an analyte, comprising a diaphragm pump, a buffer chamber, a sensor head and intake port. The pump includes a chamber for receiving fluids via first and second ports. A buffer chamber is located at the second port for holding a quantity of air, and a sensor head is adapted to identify the presence of a desired analyte and produce a signal in response to the quantity identified. Preferably the diaphragm pump has a volume of gas per stroke capacity slightly larger than the volume of the sensor head such that the pump chamber has a greater volume than the buffer chamber and the buffer chamber has about the same volume as the sensor head. A sensor intake port intakes a sample potentially containing the analyte for contact with the sensor head upon operation of the diaphragm pump and out of the sensor head by jet-action caused by rapid movement of the diaphragm pump. A filter may be used for filtering air drawn through the pump chamber. The diaphragm pump may be either a single chamber pump or a multiple chamber diaphragm mesopump. In the purging mode, the system can also contain a pump for filling the system with clean air. When a mesopump is used, a single pumping channel can provide both AC and DC operation.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a sampling pump for a chemical sensing system. More particularly the invention relates to a sensing system using a sniffing mode, alternately inhaling and exhaling in each pumping cycle to expose the sensor or other chemically responsive surface to doses of reference gas and analyte.  
         BACKGROUND OF THE INVENTION  
         [0002]    A large number of chemical and biological sensors are based on changes in the properties of a chemically sensitive material, such as changes in conductivity, surface charge or luminescence, that occur upon adsorption of analyte molecules. Analytes are, of course, the gas to be chemically analyzed. These changes can be monitored through physical methods and are related to the concentration of the analyte in the environment. In order to reduce the time associated with the diffusion of the analyte to the sensor site and to increase the amount of analyte seen by the sensor, sampling methods are used which force the air from the environment into direct contact with the sensitive polymer or other sensor material.  
           [0003]    A typical present day configuration includes a material having physical properties that change when its surface is exposed to a gas containing certain chemical or biological species. These properties may be optical, electrical or mechanical, for example. A gas sampling system is used to bring fresh gas samples into contact with the surface of the material. Then, a read-out and signal processing system of electronics is used to convert the physical change to a useful output.  
           [0004]    While these sensors have been demonstrated to work over a short time basis, they have been found to be adversely affected by long term drift that limits their practical use. The baseline drift frequently exceeds the minimum detectable signal by orders of magnitude, so that a sensitive technique is rendered essentially useless. It would be of great advantage to the art to greatly reduce the effects of baseline drift in gas sensing systems.  
           [0005]    It would be another great advance in the art if a system would be developed to enable or significantly enhance the use of a wide variety of chemical and biological sensing techniques now not useful due to an inability to distinguish between the response to an analyte and the effect of baseline drift.  
           [0006]    Other advantages will appear hereinafter.  
         SUMMARY OF THE INVENTION  
         [0007]    It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, the present invention provides a sampling system for a chemical sensor that works in a sniffing mode, that is, it produces an alternating flow pattern that alternately exposes the sensor head to a dose of reference gas and then to a dose of analyte gas during each sampling cycle.  
           [0008]    The flow pattern during the exhaling phase is sufficiently powerful to insure a fresh sample at each intake phase of the sniffing cycle. The principle, used by all breathing animals, has not been previously suggested or used for active gas sampling.  
           [0009]    The sampling system also functions in a purge mode to restore the baseline output of the sensor. The present invention permits the use of signal processing techniques that suppress background and sensor baseline drift, and thus significantly improve the sensitivity and usefulness of chemical sensors.  
           [0010]    The diaphragm pump used in the present invention operates in two different modes to accomplish the goals of the invention. In a DC mode, the pump produces a gas flow in one direction through a filter or other cleaning device. This cleaned air is further used as a reference gas for the second mode, known as the AC operation regime.  
           [0011]    The second or AC operation mode performs the sniffing function by causing the direction of flow to alternate during each cycle. Gas flow in this regime is analogous to the electric current in an AC electrical circuit.  
           [0012]    In the present invention, operation sequence first includes filling the whole sampling system with cleaned reference gas using the above referenced pumping cycle.  
           [0013]    An intake phase of the sniffing mode follows, where a fresh sample of analyte gas from the environment of interest is brought into the sensing head. Outside air is kept from getting into the pumping chamber, avoiding contamination of the pump.  
           [0014]    An exhaling cycle follows. The diaphragm action pushes the sample out of the sensing chamber and fills the sensing chamber again with the reference air from the buffer and the pump chambers.  
           [0015]    The cycle starts over again by inhaling another sample.  
           [0016]    In its simplest embodiment, the mesosniffer system consists only of a diaphragm pump working in the AC mode and a chemical sensor. The AC mode pump moves air back and forth across a sensor surface which is responsive to the desired analyte. The interaction between the analyte and the sensor is assumed to have an irreversible component, so that all the absorbed analyte is not desorbed, this causes a slowly increasing output which cannot be distinguished from baseline drift. AC signal processing eliminates the effect of the slowly changing baseline and provides an output proportional to the concentration of analyte.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    For a more complete understanding of the invention, reference is hereby made to the drawings, in which:  
         [0018]    [0018]FIG. 1 a  is a schematic illustration of a sensor/sampler system of the present invention which uses two separate pumps, while FIGS. 1 b  and  1   c  show the DC or purge pump operation, and FIGS. 1 d  through  1   g  show the sequential operation of the diaphragm pump used for AC operation;  
         [0019]    [0019]FIG. 2 is a graphical illustration of the output of the system shown in FIG. 1;  
         [0020]    [0020]FIGS. 3 a  and  3   b  are side elevational, schematic illustration of a single chamber diaphragm pump operating in the AC regime in both intake and output modes;  
         [0021]    [0021]FIG. 4 illustrates a multiple chamber mesopump structure;  
         [0022]    [0022]FIG. 5 illustrates the operation of a multiple chamber diaphragm mesopump operating in the DC regime;  
         [0023]    [0023]FIG. 6 illustrates the signal used to drive the mesopump of FIG. 5 operating in the DC regime; and  
         [0024]    [0024]FIG. 7 illustrates the operation of the mesopump of FIG. 4 in the AC regime.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    Turning now to the drawings, FIG. 1 a  illustrates a sampling system  10  generally, showing a pump  11  for AC pumping, a separate pump  25  for DC pumping and a valve  15  to enable the switching from DC pumping to AC pumping. Pumps  11  and  25  and valve  15  all may be elements of the pump generically known as a mesopump, such as that described in commonly owned U.S. Pat. No. 5,835,750. Also shown are a sensing device  23  connected to pump  11  through a buffer chamber  19  and a filter  21  that cleans the gas entering the sampling system during the purging phase of the operation cycle as disclosed below. FIGS. 1 b  to  1   f  are schematic representations of the different phases of the operation cycle of the sampling system. FIG. 1 b  shows the purging phase of the operation. In this phase, valve  15  is opened, allowing the gas to move from the purging port  27  to the sampling port  29 . The gas filling the system from purging port  27  is cleaned by filter  21 , serving as a reference gas for the following phases of the operation cycle. FIG. 1 c  shows the end of the purging phase. Valve  21  is closed, blocking the flow to and from the purging port  27 . During purging, pump  11  can contribute to the pumping action by suitable synchronization with pump  25  or can be kept inactive, in a newtrual position.  
         [0026]    [0026]FIGS. 1 b ,  1   d ,  1   e ,  1   f  and  1   g  show the operation of the diaphragm pump with arrows indicating the induced air flow outside the system. A volume of gas equal to the pump chamber  17  and proportionally larger than the volume of sensing chamber  23  is expelled out of the system in pump  11   b  by movement of diaphragm  13  in pump chamber  17 , downward in the drawing. However, enough cleaned air remains in the sampling system to refill pump chamber  17  in later cycles. During this phase, FIG. 1 d , of operation, the base-line level and drift of the chemical sensor located in sampling chamber  23  are established as shown as phase d of FIG. 2. It should be noted that these systems are independent of orientation and terms such as “downward” and “one end” or other directional terms are used for descriptive purposes only and are merely convenient terms for illustrating the operation of a particular system in a specific orientation.  
         [0027]    An intake cycle follows as shown in FIG. 1 e , which is of course phase e of the cycle, wherein a fresh sample of analyte gas from the environment is brought into sampling chamber  23 , functioning in the inhaling phase of the sniffing mode. Diaphragm  13  moves up with valve  15  closed, so sample air is drawn into sensor head  23 . Buffer chamber  19  is used to prevent outside air from getting into pump chamber  17 , thus avoiding contamination of the pump  11  with particles and with the analyte gas. During this phase the sensor in sampling chamber  23  will produce an output indicating the analyte concentration.  
         [0028]    [0028]FIG. 1 f  illustrates the exhaling phase of the sniffing system, where rapid movement of diaphragm  13 , by electrostatic action for example, causes a jet-formation regime to expel everything from sensor head  23  to insure a fresh sample during the next intake phase. The action of diaphragm  13  pushes the sample out of the sensor head and fills the sampling chamber  23  again with the reference air from buffer chamber  19  and pump chamber  17 . During this phase, shown as FIG. 1 f , the reading from the sensor in sampling chamber  23  will correspond to the reference gas and is expected to be stable. Any variation in the output of the sensor during this phase will indicate base-line drift of the chemical sensor used in sensor head  23  as phase f of FIG. 2.  
         [0029]    The cycle then begins again by inhaling another sample as shown in FIG. 1 g  and phase g of FIG. 2. It should be noted that the analyte may accumulate in the sensing device after a certain number of cycles or until a saturation level is detected. At this point a purging cycle is initiated, as shown in FIG. 1 b , where valve  15  is opened to permit DC cycle operation purging using purge pump  25  and filter  21 .  
         [0030]    In the preferred embodiment of the present invention, it is desirable that the dimensions of the device and flow rates conform to certain constraints. In the jet-action expulsion phase shown in FIGS. 1 d  and  1   f , the flow pattern is highly irreversible. To achieve jet-action during the exhaling regime, the flow in the sampling chamber  23  must be fast enough to provide sufficient momentum to the air so that it is expelled into the open space away from the sensor head. Thus, this air will not be drawn back into the sensor during an inhaling phase which brings in fresh analyte. Because of this feature, the inhaled air comes from the near neighborhood of the sensor head and is therefore different and fresher than the exhaled air. This principle, though used by all breathing animals to avoid breathing in their own exhaled air, has not been used to date in active sampling systems.  
         [0031]    In a preferred embodiment, the sensing chamber  23  Is small enough and the inhale phase, FIG. 1 e , is of long enough duration that most of the analyte molecules diffuse to and absorb onto the sensing surface therein. The exhaling phase is fast enough and the sensing head opening is small enough to generate an exhaust jet with sufficient momentum to avoid being drawn in during the next inhale cycle. The air from the environment is retained in the sensor head  23  for a time long enough to diffuse to the walls but short enough to avoid diffusion into pump chamber  17 .  
         [0032]    In designing a prototype and preferred embodiment using the mesopumps described in the above referenced patents, certain volumes have been chosen as an example of good functional operation. The sampling chamber  23  would have a preferred volume of two microliters for each centimeter length of the chamber, such as 10 microliters for 5 centimeters of sampling chamber. Pump chamber  17  would have a volume of 3 microliters for a pump chamber with a 7.5 mm diameter, or 10 microliters for the 10 mm chamber. With these dimensions, mesopumps have functioned in the AC mode at a period of operation of 50 Hz.  
         [0033]    In this example, an AC mode flow at high frequency, of about 20 Hz to 50 Hz, can be maintained for about ten seconds, resulting in a 2 ml total sample volume for the 10 mm chamber, followed by purging at a higher pump speed, such as 100 ml/min, achieved by using the purging pump.  
         [0034]    Also, if the detected level of baseline drift is above a predetermined margin, heaters may be used for the purging phase to assist in desorption. A heater may be placed in buffer chamber  19 , for example, to accomplish this step.  
         [0035]    [0035]FIG. 2 illustrates the output of the system of FIG. 1. Specifically, V represents the voltage applied to the bottom electrode. More generally, it represents the position of the diaphragm, or an output derived from a flow sensor, or a diaphragm position sensor. Output ( 1 ) is the detector output for a reversible analyte-sensor interaction and Output ( 2 ) represent the detector output for an irreversible analyte-sensor interation. With AC pumping, the sensor output provides information on the reversibility of the chemical interaction between the analyte molecules and the material of the chemical sensor. For a reversible reaction, the analyte molecules adsorbed during inhaling will be desorbed during the exhaling phase and the sensor output will be in phase with the pumping motion, shown as Output ( 1 ). At the other extreme, Output ( 2 ) illustrates the case where the analyte molecules have reacted irreversibly with the sensor material, so a dosimeter-type response is obtained. The chemical sensor output will increase during inhaling and will remain constant during exhaling, with the base-line monotonically increasing. The output of the chemical sensor will have a Fourier component at the sniffing frequency, proportional to the concentration of analyte but out of phase with the pumping action. Phase-sensitive detection or other means of correlating the pumping action with the sensor output allows the separation of the montonically increasing signal from the output, producing an output proportional to the analyte concentration. Without the active sniffing action, it would be very hard to distinguish between the analyte response and the base-line drift of the sensor. Output variations due to temperature changes, for example, will be much slower than the pumping action and can be subtracted by AC signal processing.  
         [0036]    For irreversible interaction between the analyte and the sensor a simple AC system will provide the same benefits without the buffer chamberr or the purging pump as shown in FIGS. 3 a  and  3   b . FIGS. 3 a  and  3   b  illustrate the intake mode and output mode of a single chamber diaphragm pump operating in the AC regime. Molded pump body  31  includes a port  32  for input in FIG. 3 a  and output in FIG. 3 b , leading to a chamber  33  which is filled or emptied by movement of diaphragm  34  upon action of a driving signal from signal source  36  to create electrostatic forces on electrodes  37 . Back pressure vent  38  is included in pump body  31  to facilitate movement of diaphragm  34  and elimination of back pressure. Pump body  31  is, of course, connected via port  32  to aa sensing chamber  37 . he dimensions and frequency of operation are chosen so that most of the analyte is absorbed during the inhaling cycle. The resulting clean air provides the referernce for the exhale cycle.  
         [0037]    The multi-chamber mesopump described in our earier patent is a versitile pumping system that performs both the AC and DC pumping action. FIG. 4 illustrates one mesopump channel  41 , draws air into inlet  42  through filter  43 , through chambers  44   a ,  44   b ,  44   c  and  44   d  to produce flow in the direction of arrow  46 , for discharge via port  47 . In this mesopump, which of course can be in an array, four chambers  44   a - d  operate electrostatically to move fluid through the sequence in the manner of mesopump operation.  
         [0038]    [0038]FIG. 5 illustrates the operation of a mesopump in what has been called the DC operation regime for purposes. In FIG. 5, the flow is from left to right. In FIG. 5 a , filtered purging air enters at  51  into chamber  52 , then in FIG. 5 b  to pump  53  and chamber  54 , sequentially in FIG. 5 c  to chamber  56 , then in FIG. 5 d  to chamber  60 . In FIG. 5 e , the fluid is expelled at  62 , respectively. FIG. 6 illustrates the DC operation of FIG. 5 with applied signals for the electrodes of chambers  52 ,  544 , 56  and  60 .  
         [0039]    Operation of the mesopump of FIG. 4 in the AC regime is shown in FIG. 7, where pump  71  is operated with one diaphragm  73  operating to intake and exhale fluid at port  74 , while diaphragms  75 ,  76  and  77 , acting as valves, are kept in a fixed position to close the inter-chamber conduits.  
         [0040]    During each sampling cycle, information will be generated at the sensor head. A first output occurs during exposure to the fresh sample of analyte gas, and the sensor can typically be read at a frequency much higher than the frequency of the sampling cycle. The sensor output will indicate the accumulation of analyte in the sensing layer during this phase of the sampling cycle. Also, the sensor output during exposure to reference (cleaned) air will occur without analyte being attached to the sensing layer and the output of the sensor during this exposure to clean air should be stable or decrease as analyte is desorbed. If, however, an increase in the level of the analyte during exposure to clean air samples would be detected, this would indicate a drift in the base-line and could be accounted for in the measurement process.  
         [0041]    Since these two conditions set out above are synchronized to the pumping action, it is possible to use the powerful AC signal processing technique of synchronous demodulation or phase-sensitive detection to significantly enhance the performance of the chemical sensor. In the AC gas sampling, a reference gas and a sample gas are presented to the sensor alternatively, allowing compensation for base-line drift. By integrating the output of the synchronous detector over long integration times, very low levels of signals can be detected. This technique is used to detect a modulated light sources in the presence of a large unmodulated background. Electrical chopping and synchronous detection are used to accurately measure small DC voltages that are smaller than the drifting offset voltages of the input amplifier.  
         [0042]    The advantages of AC signal processing are well known in th art of infrared detection, and in fact are an essential element of it. In IR detection a chopper is used, and the IR sensor is alternately exposed to the shutter and to the IR source. It is commonly used to detect infrared radiation through the use of a mechanical chopper. In this example, the infrared imager alternately sees the scene and the chopper blade that presents it with a reference temperature. By phase-sensitive detection methods, the temperature of the chopper blade is subtracted from the temperature of the scene, providing a thermal image in which full contrast corresponds to a few degrees of temperature difference in the scene. The meso-sniffer of this invention has been discovered to function as the equivalent of a chopper blade to enable phase sensitive detection for a chemicl sensor.  
         [0043]    It is well known that phase-sensitive detection provides two output signals instead of one: the in-phase and the out-of-phase components of teh signal. With the meso-sniffer, these two components provide information on the reversibiltiy of the chemical interaction between the analyte molecules and the material of the chemical sensor. We first consider the case of a chemically reversible reaction. The analyte molecules adsorbed during inhaling will be desorbed during the exhaling phase and the sensor output will be in phase with the pumping motion. See FIG. 2, detector output ( 1 ).  
         [0044]    At the other extreme, if the analyte molecules react irreversibly with the sensor material a dosimeter-type response is obtained, as in FIG. 2, detector output ( 2 ). The chemical sensor output will increase during inhaling and wil remain constant during exhaling, with the baseline monitonically increasing. The output of the chemical sensor will have a Fourier component at the sniffing frequency, proportional to the concentration of analyte but out of phase with the pumping action. Phase-sensitived detection subtracts the monotonically increasing signal from the output, producing an output proportional to the alanlyte concentration as shown in FIG. 2, detector output ( 3 ). Without the active sniffing action, it is very hard to distinguish the monotinic increase from baseline drift of the sensor. Output variations due to temperatur changes, for example, will be much slower than the sniffing action and are much reduced by AC signal processing.  
         [0045]    The use of phase-sensitive detection or equivalent signal processing methods significantly advances the state of the art of chemical sensing, since the present invention can be applied to many types of chemical sensors. This mode of operation is especially beneficial to the problem of land mine detection, where rapid detection of very low levels of explosives is needed.  
         [0046]    While particular embodiments of the present invention have been illustrated and described, it is not intended to limit the invention, except as defined by the following claims.