Abstract:
A sensor having electrodes connectable to an AC or DC voltage for powering an electrical discharge such as a corona, glow, arc, or the like. Additional electrodes connectable to analysis voltage may be proximate to the discharge providing electrodes. The discharge may ionize a sample fluid of varying chemical composition, flowing through a channel where the electrodes are situated. The discharge may be part of a group of sensors sensing the fluid flowing from a particle filter, gas chromatograph (GC) separator, thermal conductivity detector (TCD), optical sensors, photo ionization detector (PID), and to additional micro discharge devices (MDDs) and a mass spectrometer and/or a processor for analysis and processing to obtain results and information about the sample fluid composition.

Description:
[0001]    The present application claims priority under 35 U.S.C. § 119(e) (1) to co-pending U.S. Provisional Patent Application No. 60/440,108, filed Jan. 15, 2003, and entitled “PHASED-III SENSOR”, wherein such document is incorporated herein by reference. The present application also claims priority under 35 U.S.C. § 119(e) (1) to co-pending U.S. Provisional Patent Application No. 60/500,821, filed Sep. 4, 2003, and entitled “PHASED V, VI SENSOR SYSTEM”, wherein such document is incorporated herein by reference. The present application claims priority as a continuation-in-part to co-pending U.S. Nonprovisional Application No. 10/672,483, filed Sep. 26, 2003, and entitled “PHASED MICRO ANALYZER V, VI”, which claims the benefit of U.S. Provisional Application No. 60/414,211, filed Sep. 27, 2002, wherein the co-pending U.S. Nonprovisional Application No. 10/672,483 is incorporated herein by reference. The present application claims priority as a continuation-in-part to co-pending U.S. Nonprovisional Application No. 10/671,930, filed Sep. 26, 2003, and entitled “PHASED MICRO ANALYZER III, IIIA”. 
     
    
     
       BACKGROUND  
         [0002]    The present invention pertains to detection of fluids. Particularly, the invention pertains to ionization structures, and more particularly to the application of the structures as sensors for the identification and quantification of fluid components. The term “fluid” may be used as a generic term that includes gases and liquids as species. For instance, air, gas, water and oil are fluids.  
           [0003]    Aspects of structures and processes related to fluid analyzers may be disclosed in U.S. Pat. No. 6,393,894 B1, issued May 28, 2002, to Ulrich Bonne et al., and entitled “Gas Sensor with Phased Heaters for Increased Sensitivity,” which is incorporated herein by reference; U.S. Pat. No. 6,308,553 B1, issued Oct. 30, 2001, to Ulrich Bonne et al., and entitled “Self-Normalizing Flow Sensor and Method for the Same,” which is incorporated herein by reference.  
           [0004]    Presently available gas composition analyzers may be selective and sensitive but lack the capability to identify the component(s) of a sample gas mixture with unknown components, besides being generally bulky and costly. The state-of-the-art combination analyzers GC-GC and GC-MS (gas chromatograph—mass spectrometer) approach the desirable combination of selectivity, sensitivity and smartness, yet are bulky, costly, slow and unsuitable for battery-powered applications. In GC-AED (gas chromatograph—atomic emission detector), the AED alone uses more than 100 watts, uses water to cool its microwave discharges and is costly.  
           [0005]    Micro gas chromatography (μGC) detectors should be fast responding (&lt;1 ms), sensitive but not selective to specific compounds, of simple construction and low-cost, compact, and low-power (˜ mW). Presently available or conceived μGC detectors are either not very sensitive, such as thermal conductivity sensors (≧10 to 100 ppm of analyte); too selective to specific compounds such as fluorescence and electron-capture detectors; not low-cost such as the typical price tags in year 2003 of about $600, $3000 and upwards for many GC detectors; prone to drift due to soiled optics as micro-discharge devices (MDDs) monitored via spectral analysis; or not low-power such as the AEDs, as mentioned above.  
         SUMMARY  
         [0006]    The present detector system combines the sensitivity of photo-ionization detectors (PIDs) (down to 10 ppb), the non-specificity of TCDs, PIDs and plasma micro discharge devices (MDDs), low-power consumption (mW), short response time (&lt;1 ms), simple and low-cost design enabling MEMS co-planar fabrication and integration with μGC components without the need for complex design or micro-processing of photo-detectors, but offer ruggedness and reliability, thereby enabling operation at elevated temperatures and with relative immunity to soiling of optical elements. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0007]    [0007]FIGS. 1 a  and  1   b  illustrate a hollow-cathode discharge plasma and photo ionization detector in a rectangular flow channel;  
         [0008]    [0008]FIG. 2 is an illustration of a hollow-cathode “electrodeless” AC discharge plasma and photo ionization detector;  
         [0009]    [0009]FIG. 3 shows an electrode layout of an ionization detector having a co-planar design;  
         [0010]    [0010]FIG. 4 shows an electrode layout of an ionization detector having a another co-planar design;  
         [0011]    [0011]FIG. 5 shows an illustration of an ionization detector having prong- or interdigitated, finger-like co-planar electrodes;  
         [0012]    [0012]FIGS. 6 a ,  6   b ,  6   c  and  6   d  illustrate a capacitive discharge or AC device; and  
         [0013]    [0013]FIGS. 7, 8 and  9  show spectral emission intensity outputs of micro discharge versus wavelength for macro discharges of pure N 2  from a balloon, exhaled breath from a balloon, and automobile exhaust from a balloon, respectively; and  
         [0014]    [0014]FIG. 10 reveals a micro gas analyzer system, which may be connected upstream of the gas ionization sensor. 
     
    
     DESCRIPTION  
       [0015]    The essence of the invention is to harness both the charge carriers generated in a glow-discharge plasma and those generated by additional photo-electric effects of the discharge&#39;s UV spectral emission, for the detection of changes in the gas composition of a sample gas stream as occurs, e.g., at the exit of a GC or μGC.  
         [0016]    The present detector may have the following advantages over previously proposed or offered gas composition sensing devices. It may have a more advantageous combination of desired detector attributes (low-power, hi-speed, rugged and reliable, compactness, integratable with MEMS devices such as PHASED, simple and low-cost design, no need for drift-prone optical components), than other devices or approaches considered, previously. Further, it may be co-planar, simpler and lower-cost design than designs based on tubular discharge devices. It may consume less energy than conventional GC discharge detectors such as AEDs and be more temperature change tolerant than other devices involving photo-detectors, TCDs or flame ionization detectors (FIDs). No detector gas storage tank such as H 2  is needed for this detector as otherwise needed H 2  for FIDs. Also, no 10 to 12 eV UV window is needed, since the source and ionization test gas are either close together, or may be kept separate without a window by the assurance of laminar stratification, especially if a small flow of UV-transmissive discharge gas is provided.  
         [0017]    The present MDD-ionization combination may enable high power density of the discharge (10 5 -10 6  W/cm 3  according to the University of Illinois), short purge time constant and thus also shorter response to new passing gas peaks and greater sensitivity to their composition (rather then re-discharge old plasma gas) than macro-discharge devices. The present device may be more manufacturable than tubular structure discharge devices (such as those for ozone generators). Also, there may be advantageous use of ionization electrode materials with a self-generated oxide coating.  
         [0018]    [0018]FIGS. 1 a  and  1   b  illustrate a hollow-cathode discharge plasma and photo ionization detector in a rectangular flow channel, e.g., for a gas chromatograph (GC). The Figures represent a plasma-photo ionization detector  10  for an analyzer system such as a GC. It is a micro discharge device (MDD)  10  that has discharge electrodes  11  and  12  for providing a plasma glow discharge  25 . Source  21  may provide a higher voltage, e.g., 100 to 800 VAC, than the operating voltage, which may be needed for the ignition or start of the discharge  25 , voltage to the electrodes  11  and  12  and the current of which, after the start of the discharge  25 , the may be then lowered and limited, respectively, by a load resistor  18  to achieve an operation of the discharge device  10  in the low 0.01 to 1 mW range of dissipated power (with about 100 VAC). The operation may involve a capacitive discharge occurring at a frequency from about 20 kHz to 20 MHz between electrodes  11  and  12 . Electrode  12  may consist of the low conductivity silicon chip substrate material  15 . The ionization current may be measured via a relatively low-voltage DC circuit having a voltage source  22  of less than 60 VDC. Such DC circuit may minimize interference between the-two circuits of discharge and current measurement.  
         [0019]    The device of FIG. 2 uses a design similar to that of FIGS. 1 a  and  1   b , except for depositing the electrode  12  into the hollowed out area  27  on the insulating film  28  (e.g., SiO 2 , Si 3 N 4 , MgO) and covering the electrode with such insulating dielectric as well. One may note that in both devices  10  and  20 , electrodes  11 ,  12 ,  13 ,  14 , and thus the leads to them, may be located on two wafers  15  and  16 . Electrodes  11  and  12  may be on the bottom wafer  15 . Electrodes  13  and  14  may be attached to the top wafer. In both devices  10  and  20 , the top wafer  16  may form the gas flow channel  17 , for the sample gas flow  29 , of the separation column, and the bottom wafer  15 , in the situation of PHASED sensors, may support the column heaters.  
         [0020]    Devices  30  and  40  in FIGS. 3 and 4 may use a co-planar electrode structure, where the collection electrodes  31 ,  32 , and discharge electrodes  33 ,  34  of detector  30  may be deposited on the same wafer or substrate, e.g., wafer or substrate  15 . Likewise, collection electrodes  41 ,  42  and discharge electrodes  43 ,  44  may be deposited on the same wafer or substrate  15  of detector  40 . The dimension and location differences between detectors  30  and  40  involve minimizing high e-field regions between the two circuits (discharge and ionization), and insuring that the ion+electron collection electrodes  31 ,  32  field is lower than the field between the discharge electrodes  33 ,  34 . One of the collection electrodes  31 ,  32  of device  30  may be smaller than the other. Collection electrodes  41  and  42  of device  40  may be about the same size. The device  30  discharge electrodes  33  and  34  may have a greater distance between them for larger discharges than the electrodes  43  and  44  of device  40 .  
         [0021]    The photo and plasma ionization device  50  may maintain a high-power density (W/cm 3 ) and brightness of micro-discharges, i.e., higher than that of macro-discharge devices, while increasing the total ion+electron+photon output relative to the outputs of the above noted devices  10 ,  20 ,  30  and  40 . Discharge electrodes  53  and  54  may have an inter-meshed prong- or finger-like (i.e., interdigitated) design for greater electrode-to-electrode area for potentially providing a larger and more intense discharge  25 . One may place the collection electrodes  51  and  52  and discharge electrodes  53  and  54  at a position further downstream of the channel or column to optimize collection in a high sample gas flow  29  (about 100 to 200 cm/s). Co-planar electrodes  51 ,  52 ,  53  and  54  may be situated on the heater wafer  15 .  
         [0022]    [0022]FIGS. 6 a ,  6   b ,  6   c  and  6   d  describe a capacitive glow discharge device  60  which is a macro-discharge assembly. It may be a set-up that is based on an ozone generator. FIG. 6 a  shows a side view of a channel  61  with a gas flow  29  going through it. Channel  61  may be a 1 inch by 1 inch channel composed of SiO 2 . Situated in channel  61  is a side view of discharge device  60  which is shown in more detail in a FIG. 6 b  side view. Device  60  may have a substrate layer  62  composed of Al 2 O 3  with a thickness of about 0.5 mm (19.7 mils). On a portion of the back side of substrate  62  may be a layer of Cu. On the front side of layer  62  may be a layer  64  which is a thin electrode having dimensions of 0.75 mm by 32 mm. On a significant portion of electrode  64  may be an electrode cover film  65 . Electrode cover film  65  may be composed of, for example, MgO, SiO 2  or Si 3 N 4 . On film  65  may be a discharge region  66 . Region  66  may sustain about a 20 kHz 6.8 kV discharge. FIG. 6 c  is an axial view of device  60  with discharge region  66  at its front and situated in channel  61 . FIG. 6 d  is a top view of device  60  situated in channel  61 .  
         [0023]    The expected spectral output of an MDD  10 ,  20 ,  30 ,  40  or  50  may cover N 2 , O 2  and OH and other analyte plasma reaction products in normal air as a carrier gas. One may note the spectra of these fluids in FIGS. 7, 8 and  9 , respectively, with relative intensity versus wavelength, obtained with the macro-discharge assembly  60  shown in FIGS. 6 a ,  6   b ,  6   c  and  6   d . The ionization sensor of the sample fluid  29  may be part of a set of spectral and other sensors, all geared to maximize reliability of detection and quantification of the analytes of interest in the fluid, especially when discharge current, discharge-induced photo-ionization and spectral emission outputs of MDDs can be detected simultaneously and/or in relation to each other as sample gas composition changes.  
         [0024]    [0024]FIG. 7 involves a macro discharge in pure N 2  from a balloon. FIG. 8 involves a macro discharge in exhaled breath from a balloon. FIG. 9 involves a macro discharge in automobile exhaust from a balloon. Data for FIGS. 7-9 were recorded by Caviton, Inc. One may note the various wavelengths of NO emission at 247.2 or 258.8±1.4 nm, and reference N 2  at 336.9 or 357.5±2 nm. Other bands of OH, C 2  and CH may be known from flame spectras. Still others may be known from absorption measurements of NH 3 , CO, SO 2 , and the like. When GC peaks of CO, CO 2 , CH 4 , CnHm, etc., elute, more ions+electrons and different spectral emission bands are likely to be generated, all contributing to a simultaneous change (generally an increase) in the measurable ionization current. There may be measurable changes in discharge current as a composition of the gas in the discharge changes with time, in accordance with concentration peaks eluting from a gas chromatography analyzer.  
         [0025]    Some features of the invention include coplanar MEMS MDDs and ionization sensing electrodes, interdigitated MDDs to achieve both high power density (i.e., brightness and high UV and ion+electron output), short residence time of discharge gas due to short diffusion distances across the microdischarge (10 to 100 microns), which favors sensitivity to sample gas, and high total power and ionization signal and fast response. These may be all with co-planar ionization collection electrodes. There may be a positioning of the ionization collection electrodes shifted downstream, for optimal collection of ions in a fast gas flow (100 to 200 cm/s). An application of DC ionization collection voltage may be had for least interference between charge carrier generation and measurement circuits.  
         [0026]    To minimize the probability, P, of mis-identifying a gas mixture component, it is desirable to obtain as many independent measurements of an analyte as possible. Measurements with GC-MS (MS=mass spectrometer), GC-GC and GC-GC-MDD may be noted. The point is that sensing MDDs spectral emission together with ionization current features can help to reduce P. Such features could be AC and DC measurements, ion-drift (i.e., ionization current) phase-lag relative to the known generation of the charge carriers, and rectification effects enabled by the use of dissimilar electrodes, as practiced in flame rectification circuits.  
         [0027]    There may be an application of AC ionization collection voltage with a pair of equal electrodes and a phase-locked amplifier tied to ion generation frequency, to enable measurement of ionization amplitude and phase shift, which may relate to the size and polarity of the ion, as in ion drift spectrometry. On the other hand, there may be an application of AC ionization collection voltage with a pair of un-equal electrodes and a phase-locked amplifier tied to an ion generation frequency, to enable measurement of ionization amplitude, phase shift and rectification, which may be had to better quantify the size and polarity of the ion, and to further reduce P.  
         [0028]    There may be the use of a differential ionization (really a charge-carrier) collection circuit, where the steady-state input sample gas ionization may be compared with that of gas exiting from the GC, which features the separated gas constituent peaks. Also, one may sense MDD power and/or current and/or ionization, all vs. applied voltage and frequency, in addition to MDD spectral output and ionization current to reduce P.  
         [0029]    The present ionization gas detectors may include the following items. There may be the use of plasma hollow-cathode micro glow discharge device (MDD) for gas sensing via spectral emission of unknown gas mixture samples, to generate pairs of ions and electrons and additional pairs via photo-ionization, especially of gas mixture components (i.e., analytes) of low ionization potential. Also, there may be the use of co-planar electrodes (e.g., thick film-Pt on alumina) for MEMS MDDs but with added co-planar ionization sensing electrodes.  
         [0030]    [0030]FIG. 10 reveals certain details of micro gas apparatus  115 . Sample stream  125  may enter input port  134  from pipe or tube  119 . There may be a particle filter  143  for removing dirt such as soot from exhaust and other particles from the stream of fluid  125  that is to enter apparatus  115 . This removal is for the protection of the apparatus and the filtering should not reduce the apparatus&#39; ability to accurately analyze the composition of fluid  125 . Dirty fluid (with suspended solid or liquid non-volatile particles) might impair proper sensor function. A portion  29  of fluid  125  may flow through the first leg of a differential thermal-conductivity detector (TCD, or chemi-sensor (CRD), or photo-ionization sensor/detector (PID), or other device)  227  and a portion  147  of fluid  125  may flow through tube  149  to a pump  151 . By placing a “T” tube immediately adjacent to the inlet  29 , sampling with minimal time delay may be achieved because of the relatively higher flow  147  to help shorten the filter purge time. Pump  151  may cause fluid  147  to flow from the output of particle filter  143  through tube  149  and exit from pump  151 . Pump  153  may effect a flow of fluid  29  through the sensor via tube  157 . From detector  227 , fluid  29  may flow through ionizer  224 , flow sensor  225 , separator  226  and through detector  228  (which may be like detector  227 ) on to pump  153 . Separator  226  may be for separating individual gas constituents of sample fluid  29 , particularly if the fluid is a gas mixture. There may be additional or fewer pumps, and various tube or plumbing arrangements or configurations for system  115  in FIG. 10. Data from detectors  227  and  228 , flow sensor  225 , ionizer  224 , and separator  226  may be sent to controller  230  for processing, analysis and results about fluid  29 .  
         [0031]    Although the invention has been described with respect to at least one illustrative embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.