Abstract:
A detector for gas chromatography using two or more ionization sources within a single body to separately provide ionization energy to a column gas eluent to provide electrical discharge to two or more collecting electrodes provides improved selectivity and may be so used. Use is made of combined bias/collecting electrodes or of sets of separated bias and collecting electrodes. The use of multiple ionization sources permits generation of multiple detector outputs from within a common body and of a common constituent flow. The ionization sources and any applicable discharge gas and dopant may be selected based on desired selectivity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 13/675,727 filed Nov. 13, 2012 for “A photo ionization detector for gas chromatography having two separately ionizing sources and methods of use” priority to which is hereby claimed. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     An improvement to ionization detectors for gas chromatography and methods of use is disclosed. More specifically, a detector for gas chromatography with at least two separate ionization sources within a single body to separately provide ionization energy to, via a discharge gas, permit ionization of a column gas eluent to provide electrical discharge to two or more collecting electrodes, which may be combined bias/collecting electrodes or may be sets of separated bias and collecting electrodes, and methods of use provides improved selectivity, is disclosed. Additionally, the single body may include a structure to partially obstruct the flow path between, or partially isolate, the discharge sources to increase sensitivity. 
     2. Description of the Related Art 
     Gas chromatograph systems used to detect the presence of specific compounds include the well-known use of ionization detectors. A sample is vaporized and introduced to the head of a gas chromatographic column, where it is transported through the column by the flow an inert, gaseous mobile phase, and is separated according to physical properties of each compound in the sample. The eluent from this gas chromatography column is then flowed into the enclosed elongated chamber of an ionization detector. Helium, or another noble gas, or combinations thereof, is also introduced to the detector as a discharge gas and is ionized in a discharge chamber section of that enclosed elongated chamber, wherein energy is absorbed and the atoms transition to an exited state. The ion combine with another atom to form a diatomic metastable molecule, which then emits emitting a molecule emission—one or more photons. In an adjacent reaction chamber section of the enclosed elongated chamber, the separated constituents or compounds, in accordance with their retention time in the column, become ionized from photons emitted from the ionized discharge gas, each according to its composition. As each ionized compound encounters a collecting electrode, an electrical current is generated, which is useful in identification of each compound in the sample according to its ionization potential (IP) and retention time. Various benefits of this non-destructive nature of this detection have been identified. 
     Variations of the means of ionization of the discharge gas exist, including those which use an electric discharge, whether a direct current discharge (a pulsed discharge) or an alternating current discharge, others which use a dielectric barrier discharge, and others which use a lamp. Each ionization source provides its own benefits. The pulsed discharge detector (PDD), for example, has the advantages of stability and long lifetime. These detectors have been found useful in various areas, including trace gas analysis, semiconductor manufacturing, and environmental applications. 
     These detectors have used a single collecting electrode, which may be part of a system including at least one bias electrode, or may use a single combination bias/collecting electrode, where the collecting electrode is itself biased. Unfortunately, use of a single collecting electrode limits the information which may be obtained from within the detector cell. Moreover, these systems are non-selective, as they respond to all compounds except the carrier gas. While several methods for improving selectivity have been published, these have not been found sufficiently successful for adoption. Among these attempts has been the use of operating two separate gas detectors, each with a different discharge gas type, in parallel, using a division of the eluent from the gas chromatography column into each detector. Splitting the eluent, however, can create difficulties in ensuring an equal division of compounds between the two associated detectors. Moreover, the two detectors may produce differing results, such as by the result of contamination or deterioration of the detector components, which cannot be addressed by reference to a standard. 
     Thus, there is a need in the art for a photo ionization detector for gas chromatography with increased selectivity which features within a common body two separate ionization discharge locations which separately ionize an undivided eluent, and for methods of use thereof. 
     SUMMARY OF THE INVENTION 
     The present photo ionization detector uses two or more separately ionizing discharge sources and two or more combined bias/collecting electrodes or may be sets of separated bias and collecting electrodes. In operation, the use of two or more ionization sources permits generation of two or more detector outputs. The ionization sources and any applicable discharge gas and dopant may be selected based on desired selectivity. 
     The present photo ionization detector provides a multiple ionization source ionization detector body which includes an elongate chamber, a first ionization source, a column gas inlet, a second ionization source, an outlet, a first combined bias/collecting electrode or a first set of separated bias and collecting electrodes, and a second combined bias/collecting electrode or a second set of separated bias and collecting electrodes. The extent of the elongate chamber is defined by a first end and a second end. The first ionization source is positioned within the elongate chamber adjacent or at the first end, while the second ionization source is positioned within the enclosed elongated chamber adjacent or at the second end. The elongate chamber may include an obstruction or may be configured to better isolate the discharge sources. The column gas inlet, through a side of the detector body, which may be positioned in the elongate chamber on the opposite side of the first ionization source from the discharge gas inlet, provides an entrance into the elongate chamber between the first ionization source and the outlet, while the outlet is positioned to provide an exit from the elongate chamber between the point of entry of the column inlet and the second ionization source. An end of the first collecting electrode is exposed in the elongate chamber between the column gas inlet and the outlet. An end of the second collecting electrode is exposed in the chamber between the end of the first collecting electrode and the second ionization source. In an embodiment using pulsed discharge ionization as the first ionization source, the photo ionization detector also includes a first discharge gas inlet through the wall of the detector body and into the enclosed elongated chamber, near the chamber first end, so that the first ionization source is positioned between the first discharge gas inlet and the chamber second end. The end of the second collecting electrode may be positioned between the end of the first collecting electrode and a second gas discharge gas inlet, if the second ionization source is pulsed discharge. Finally, the outlet may be positioned between the end of the second collecting electrode and the chamber. 
     Alternative embodiments of the photo ionization detector may further incorporate a third ionization source between the first ionization source and the second ionization source, associated with a third discharge gas, which may be a different discharge gas from that used in connection with the first ionization source or the second ionization source. This third ionization source may be positioned in an elongate second chamber in communication with the chamber. 
     Further alternative embodiments of the photo ionization detector may further include a chamber having a bend or change in direction, such as a right angle, therein, wherein the first ionization source and first collecting electrode are partially isolated from the second ionization source and second collecting electrode, where the bent piping may merely comprise a reduced cross sectional area or may provide a curve or bend which reduces the flow rate through the chamber. 
     In operation, the photo ionization detector may be utilized in a method of identifying separated constituents in a carrier gas. The separated constituents in a carrier gas are flowed from a column gas inlet into the elongate chamber, where they are ionized by photon emission from the first discharge gas, which has been ionized by the first ionization source, and by simultaneous photon emission from the second discharge gas, which has been ionized by the second ionization source. The separated constituents also may be simultaneously ionized by photon emission from a third discharge gas, which has been ionized by the third ionization source. By interaction with the ionized constituents, over time, a first time-dependent current is generated in the first collecting electrode and a second time-dependent current is generated in the second collecting electrode. The separated constituents are then flowed out of the detector body through the outlet. A visual representation of the first and second time dependent currents may then be displayed as a function of time, either directly or relation to one another. 
     A photo ionization detector using multiple separate ionization source, such as two or three discharge sources, and with two or more collecting electrodes provides improved selectivity. Beneficially, in use, the response ratio between any two collecting electrodes provides confirmation of the compound identities indicated by the retention time. Moreover, the photo ionization detector using multiple discharge sources and multiple collecting electrodes retains the universal and uniform response characteristics of a conventional photo ionization detector without the need for a second detector or for division of the effluent from the gas chromatography column. 
     Using helium doped with a first dopant selected from the group of argon, krypton, or xenon as the second discharge gas rather than pure helium provides a simultaneous, but different, discharge emission profile. Pure helium provides higher photon energy, ionizing compounds in the eluted sample. Use of a dopant results in a reduction in the photon energy, thus ionizing only selected components. Doped helium is used rather than a pure non-helium gas in order to retain the benefits of the helium: namely, its transparency for Ar, Kr, and Xe resonance radiation and its efficient cooling of the electrodes. Moreover, the photo ionization detector using a second discharge source with such doped discharge gas provides the simultaneous functionality of a specific photoionization detector for selective determination of aliphatics, aromatics, amines, and other species. For example, use of Argon provides selectivity for organic compounds, use of Kypton provides selectively for unsaturated compounds, and use of Xenon provides selectivity for polynuclear aromatics. 
     Similarly, using helium doped with a second dopant selected from the group of argon, krypton, or xenon as the third discharge gas rather than pure helium or helium with the first dopant, provides a simultaneous, but different, discharge emission profile. 
     Bias electrodes, biased by a voltage source may also be included in the detector body, but increase the number of components required. 
     Finally, determination of the ratio of the Ionization Potential for each compound within the sample, made possible from the dual discharge locations and multiple collecting electrodes, aids in identification of the compound. 
     Additional aspects, advantages, and embodiments of the photo ionization detector will become apparent to those skilled in the art from the following description of the various embodiments and related drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the described features, advantages, and objects of the photo ionization detector, as well as others which will become apparent, are attained and can be understood in detail; more particular description of the photo ionization detector briefly summarized above may be had by referring to the embodiments thereof that are illustrated in the drawings, which drawings form a part of this specification. It is to be noted, however, that the appended drawings illustrate only typical embodiments of the photo ionization detector and are therefore not to be considered limiting of its scope as the photo ionization detector may admit to other equally effective embodiments. 
       In the drawings: 
         FIG. 1  is an illustration of a detector body of the present photo ionization detector using pulsed discharge as the first and second ionization sources. 
         FIG. 2  is an illustration of a detector body of the present photo ionization detector using pulsed discharge as the first ionization source and a lamp as the second ionization source. 
         FIG. 3  is an illustration of the operation of the present photo ionization detector, showing current intensities as a function of retention time of the first collected current (E 1 ) at the first collecting electrode  138  and the second collected current (E 2 ) at the second collecting electrode  140  during operation of the present photo ionization detector with helium as a first discharge gas at the first discharge inlet  110  with krypton as a second discharge gas at the second discharge inlet  116  with operation of and the first ionization source  112  and the second ionization source  118 , showing the results at the first collecting electrode  138  and at the second collecting electrode  140 . 
         FIG. 4  is an illustration of the ratios of various peaks according to  FIG. 3   c.    
         FIG. 5  is an illustration of a detector body of the first alternative embodiment for the present photo ionization detector using pulsed discharge as the first, second, and third ionization sources. 
         FIG. 6  is an illustration of a detector body of the second alternative embodiment for the present photo ionization detector using a non-linear chamber. 
         FIG. 7  is an illustration of the operation of the present photo ionization detector illustrated in  FIG. 3 , showing the current intensity as a function of retention time of the first collected current (E 1 ) at the first collecting electrode  138  during operation of the present photo ionization detector with helium as a first discharge gas at the first discharge inlet  110  with krypton as a second discharge gas at the second discharge inlet  116  with operation of and the first ionization source  112  and the second ionization source  118 , showing the results at the first collecting electrode  138 . 
         FIG. 8  is an illustration of the operation of the present photo ionization detector illustrated in  FIG. 3 , showing the current intensity as a function of retention time of the second collected current (E 2 ) at the second collecting electrode  140  during operation of the present photo ionization detector with helium as a first discharge gas at the first discharge inlet  110  with krypton as a second discharge gas at the second discharge inlet  116  with operation of and the first ionization source  112  and the second ionization source  118 , showing the results at the second collecting electrode  140 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present photo ionization detector provides a detector body for multiple discharge photo ionization detector with two or more collecting electrodes and a method of use for gas chromatography. 
     Referring to  FIG. 1 , an illustration of a detector body of the present photo ionization detector using pulsed discharge as the first and second ionization sources, the ionization detector body  102  includes an enclosed elongated chamber  104  having a chamber first end  106  and a chamber second end  108 , a first ionization source  112 , a second ionization source  118 , a column gas inlet piping  114 , a first collecting electrode  138  having a first collecting electrode end  120  which may be constructed to function as a combined bias/collecting electrode, and a second collecting electrode  140  having a second collecting electrode end  122  which may be constructed to function as a combined bias/collecting electrode, and an outlet piping  136  from the enclosed elongated chamber  104 . Where a set of separated bias and collecting electrodes is utilized, the biasing electrode is positioned near the collecting electrode. The detector body may further include a first discharge gas inlet  110  and a second discharge gas inlet  116 . The first discharge gas inlet  110  may be positioned into or at the enclosed elongated chamber  104  proximate, i.e at or near, the chamber first end  106 . The second discharge gas inlet  116  may be positioned into or at the enclosed elongated chamber  104  proximate, i.e at or near, the chamber second end  108 . The first discharge gas inlet  110  may therefore provide a pathway into the enclosed elongated chamber  104  proximate the chamber first end  106  for a first discharge gas while the second discharge gas inlet  116  provides a pathway into the opposing end of enclosed elongated chamber for a second discharge gas proximate the chamber second end  108 . The first ionization source  112  is positioned in the enclosed elongated chamber  104  adjacent the chamber first end  106  and, in operation, downstream of the first discharge gas inlet  110  while the second ionization source  118  is positioned in the enclosed elongated chamber  104  adjacent the chamber second end  108 , which, in operation, will also be downstream of the second discharge gas inlet  116 . As can be appreciated, this positions, in operation, the first ionization source  112  downstream of the first discharge gas inlet  110  and the second ionization source  118  downstream of the second discharge gas inlet  116 . The column gas inlet piping  114  is positioned between the first ionization source  112  and the second ionization source  118 . This further positions, in operation, the column gas inlet piping  114  downstream of the first discharge gas inlet  110 . In operation, the column gas inlet piping  114  provides a pathway into the enclosed elongated chamber  104 , which should be positioned closer to first ionization source  112  than the second ionization source downstream of the first ionization source  118 . The first collecting electrode end  120  is exposed to the enclosed elongated chamber  104  and, in operation, downstream, with respect to the first discharge gas, of the column gas inlet piping  114  and the first discharge gas inlet  110 , thus between the column gas inlet piping  114  and the outlet piping  136 . The second collecting electrode end  122  is exposed to the enclosed elongated chamber  104  and, in operation, positioned downstream, with respect to the first discharge gas, of the column gas inlet piping  114 , the first discharge gas inlet  110 , and the first collecting electrode end  120 , so as to be between the first collecting electrode  138  and the second discharge gas inlet  116 , when present. The first ionization source  112  may be between the first collecting electrode end  120  and the second ionization source  118 . The outlet piping  136  is positioned in or at the enclosed elongated chamber  104 , which should be between the second ionization source  118 , such as the second pair of spaced-apart electrode tips  126 , and the second collecting electrode end  122 , and therefore the column gas inlet piping  114 . The column gas inlet piping  114  and the outlet piping  136  are hollow and/or cylindrical tubing. 
     Further, a first biasing-or-ground electrode  152  is positioned in the column gas inlet piping  114  so its end is at least adjacent, i.e., in or at the edge, of the enclosed elongated chamber  104 . Similarly, a second biasing-or-ground electrode  154  is positioned in the outlet piping  136  so its end is at least adjacent, i.e., in or at the edge of the enclosed elongated chamber  104 . 
     Described with reference to the position of the components, the dual ionization source ionization detector body  102  first includes an enclosed elongated chamber  104  having a chamber first end  106  and a chamber second end  108 . A first discharge gas inlet  110  provides an inlet into the enclosed elongated chamber  104  near, which necessarily includes at, adjacent to and abutting, the chamber first end  106 . A second discharge gas inlet  116  provides an inlet into the enclosed elongated chamber  104  near the chamber second end  108 . A first ionization source  112  is positioned in or about the enclosed elongated chamber  104  adjacent the chamber first end  106  and/or between the first discharge gas inlet  110  and the chamber second end  108  so as to provide ionization energy into the enclosed elongated chamber. A column gas inlet piping  114  provides an inlet from a gas chromatography column into the enclosed elongated chamber  104  between the first ionization source  112  and the chamber second end  108  through the wall of the side of the detector body  102 . A first collecting electrode end  120  is exposed to the enclosed elongated chamber  104  between the column gas inlet piping  114  and the chamber second end  108 . A second collecting electrode end  122  is exposed to the enclosed elongated chamber  104  between the first collecting electrode end  120  and second ionization source  118  or the chamber second end  108  and/or adjacent the chamber second end  108 . An outlet piping  136 , to provide venting or outflow from the enclosed elongated chamber  104  is positioned in the elongated chamber  104  between the second collecting electrode end  122  and chamber second end  108 , or more narrowly, the second discharge gas inlet  116 . Finally, a second ionization source  118  is positioned in the enclosed elongated chamber  104  between the outlet piping  136  and the chamber second end  108 . The dual ionization source ionization detector body  102  may be further defined by the first discharge gas inlet  110  being positioned in the enclosed elongated chamber  104  near the chamber first end  106 , and the chamber second end  108  being positioned in the enclosed elongated chamber  104  near the chamber second end  108 . 
     The chamber  104  may have a chamber first end  106  and a chamber second end  108 . The first discharge gas inlet  110  may be positioned proximate, that is on, at, abutting, is close proximity, near, or adjacent to, the chamber first end  106 . The second discharge gas inlet  116  may be positioned proximate, that is on, at, near, or adjacent to, the chamber second end  108 . 
     The flow rates associated with the first discharge gas inlet  110  and the second discharge gas inlet  116  need not be equivalent, but provide better results when non-equivalent. The outlet piping  136  has a sufficient flow rate for outflow, whether by size or pressure differential, to permit outflow at a rate equivalent to the inflow at both the first discharge gas inlet  110  and the second discharge gas inlet  116 . 
     The first ionization source  112  and the second ionization source  118  use ultraviolet light (photons) to ionize gas atoms. The discharge of ultraviolet light may be may be obtained by various technology known in the art. These include electrical discharge, particularly generation of an electrical spark between two electrodes as well as ultraviolet lamps, also known as glow-discharge lamps, wherein typically a lamp is filled with a low-pressure inert gas, which when energized, such as by electrodes within the lamp or by an external radiation source produces ultraviolet spectral radiation. In the case of electrical discharge, the wavelength of the emitted photos is dependent on the discharge gas flowing about the discharge. In the case of the glow-discharge lamp, the wavelength is determined by the gas in the lamp. For example, when excited krypton will emit 123.9 nm and 116.9 nm radiation, or the equivalent of 10 eV and 10.6 eV. In the case of electrical discharge, the wavelength of the emitted photos is dependent on the discharge gas flowing about the discharge. In the case of the glow-discharge lamp, the wavelength is determined by the gas in the lamp. For example, when excited krypton will emit 123.9 nm and 116.9 nm radiation, or the equivalent of 10 eV and 10.6 eV. The first ionization source  112  and the second ionization source  118  need not be of an identical type of ionization source and, depending on first discharge gas and second discharge gas selected, need not be of equivalent energies. 
     The first discharge gas may be helium while the second discharge gas is selected, in connection with the second ionization source  118 , to transmit lower photonic energy. This may be accomplished by doping pure helium with a second noble gas, such as Argon, Kypton, Neon and Xenon, in connection with electrical pulsed discharge or by using pure helium with a ultraviolet radiating lamp. Utilizing a different type of ionization source for the second ionization source  118  permits operation of the detector body with a single discharge gas source, particularly helium, and therefore eliminating the need for a second discharge gas source and the associated equipment. 
     As depicted in  FIG. 1 , the first ionization source  112  may be a pulsed discharge system, and may include a first pair of spaced-apart discharge electrode tips  124 , positioned sufficient for spark formation therebetween within the enclosed elongated chamber  104  for pulsed discharge. Likewise, the second ionization source  118  may also be a pulsed discharge system, which may include a second pair of spaced-apart discharge electrode tips  126 , positioned sufficient for spark formation therebetween within the enclosed elongated chamber  104  for pulsed discharge, in which case the discharge gas would be helium with a dopant. 
     The enclosed elongated chamber  104  may be divided to include a first discharge chamber section  128 , a reaction chamber section  130  and a second discharge chamber section  132 , and may be have an enclosed elongated chamber longitudinal axis  134 . The first discharge chamber section  128  is located between the chamber first end  106  and the column gas inlet piping  114 . The first discharge chamber section  128  may be further identified according to its first discharge chamber section cross sectional area perpendicular to the enclosed elongated chamber longitudinal axis  134 , which is generally constant throughout the first discharge chamber section. 
     Similarly, the second discharge chamber section  132  is located between the chamber second end  108  and the outlet piping  136 . The second discharge chamber section  132  may also be further identified according to its second discharge chamber section cross sectional area perpendicular to the enclosed elongated chamber longitudinal axis  134 , which is generally constant throughout the second discharge chamber section. The second discharge chamber section cross sectional area should be equivalent to the first discharge chamber section cross sectional area. 
     While the reaction chamber section  130  intermediate the first discharge chamber section  128  and the second discharge chamber section  132  may have a reaction chamber section cross sectional area perpendicular to the enclosed elongated chamber longitudinal axis  134  equivalent to the first discharge chamber section cross sectional area or the second discharge chamber section cross sectional area, the reaction chamber section cross sectional area should be greater than the first discharge chamber section cross sectional area or the second discharge chamber section cross sectional area. 
     The first collecting electrode end  120  may be the end of a first collecting electrode  138 , which may be a combined voltage-biased bias/collecting electrode in electrical connection with a first voltage bias source and for transmittal of the current collected at the first collecting electrode  138  or which may be a first set of separated bias and collecting electrodes where only the first bias electrode is so connected. Similarly, the second collecting electrode end  122  may be the end of a second collecting electrode  140 , which is a combined voltage-biased bias/collecting electrode in electrical connection with a second voltage bias source and for transmittal of the current collected at the second collecting electrode  140  or which may be a second set of separated bias and collecting electrodes where only the second bias electrode is so connected. 
     Referring to  FIG. 2 , an illustration of a detector body of the present photo ionization detector using pulsed discharge as the first ionization source and a lamp as the second ionization source, the general structure of the detector body  102  is maintained with changes pertinent to selection of a glow-discharge lamp  204  as the second ionization source  118 . As the glow-discharge lamp, positioned at the chamber second end  108 , contains the second discharge gas, no second discharge gas inlet  116  is needed. 
     Bias electrodes, biased by a voltage source, may also be included in the detector body, but increase the number of components required. 
     By virtue of this structure, two chromatograms may be generated in operation, differing as a result of the differing ionization energy transmitted from the two discharge gases and the responses generated from different constituent compounds in the sample. A current of higher intensity will be generated in the first collecting electrode  138  at its first collecting electrode end  120 . This higher intensity current is due to the interaction of the sample compounds and the ionized pure helium used in the first discharge chamber section  128 , which produced higher ionization energy. Conversely, a current of lower intensity will be generated in the second collecting electrode  140  at its second collecting electrode end  122 . This lower intensity current is due to the interaction of the sample compounds and the ionized noble gas combination used in the second discharge chamber section  132 , which produced lower ionization energy. Notably, while this structure provides a higher intensity for each electrode, some constituent compounds in the sample exhibit different responses, such as substantially increased peaks, to the two different ionization levels. 
     Thus, in operation using dual pulsed discharge ionization sources, helium is flowed through the first discharge gas inlet  110  into the first discharge chamber section  128  at the chamber first end  106  of the enclosed elongated chamber  104  while a combination of helium and a second noble gas is flowed through the second discharge gas inlet  116  into the second discharge chamber section  132  at the chamber second end  108  of the enclosed elongated chamber  104  in an opposing direction of the helium and therefor towards it. The helium is ionized in the first discharge chamber section  128  as it passes the first ionization source  112  while the combination of helium and a second noble gas, a noble gas combination) is ionized in the second discharge chamber section as it passes the second ionization source  118 . A carrier gas with sample compounds or constituents (also known as an analyte) is introduced into the enclosed elongated chamber  104  through the column gas inlet piping  114 , associated with a gas chromatography column which has already separated the various compounds or constituents of the sample according to various properties of each, and which thereby flows counter to the flow of the two discharge gases. The sample compounds are ionized by the photons emitted by the ionized helium and by the ionized noble gas combination. As the column gas approaches the first collecting electrode end  120  its compounds are ionized by the photons according to the ionization potential of each. As each ionized compound passes the first collecting electrode end  120  an electric current is generated. The retention time and intensity of each current discharge is indicative of the compound. As the column gas approaches the second collecting electrode end  122  its compounds are now ionized by the ionized noble gas combination, having lower ionization energy, according to the IP of each. As each ionized compound passes the second collecting electrode end  122  a second electric current is generated. The retention time and intensity of each current discharge in this second current, represented as a peak on a chromatogram, is also indicative of the compound. 
     The response from the first collecting electrode  138  and the response from the second collecting electrode  140  are then used for identification. A chromatogram for each collecting electrode may be generated by displaying, by printing or by depiction on a computer screen, the collected current intensity in relation to retention time. 
     Referring to  FIGS. 3 ,  7  and  8  and Table 1, two chromatograms generated by the operation of the present photo ionization detector are illustrated jointly in  FIG. 3  and separately in  FIG. 7  (solely E 1 ) and  FIG. 8  (solely E 2 ), showing current intensities as a function of retention time of the first collected current (E 1 ) at the first collecting electrode  138  and the second collected current (E 2 ) at the second collecting electrode  140  during operation of the present photo ionization detector with helium as a first discharge gas at the first discharge inlet  110  with krypton as a second discharge gas at the second discharge inlet  116  with operation of and the first ionization source  112  and the second ionization source  118 , showing the results at the first collecting electrode  138  and at the second collecting electrode  140 . The constituents associated with each peak and associated data are presented in Table 1. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Peak No. 
                 Constituent 
                 eV 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 1,1-dichloroethene 
                 9.79 
               
               
                 2 
                 methylene chloride 
                 11.32 
               
               
                 3 
                 1,1-dichloroethane 
                 11.06 
               
               
                 4 
                 benzene 
                 9.26 
               
               
                 5 
                 carbon tetrachloride 
                 11.47 
               
               
                 6 
                 1,2-dichloropropane 
                 10.87 
               
               
                 7 
                 trichloroethene 
                 9.47 
               
               
                 8 
                 2-chloroethyl vinyl ether 
               
               
                 9 
                 1,1,2-trichloroethane 
                 11 
               
               
                 10 
                 Dibromochloromethane 
                 10.59 
               
               
                 11 
                 Tetrachloroethene 
                 9.32 
               
               
                 12 
                 Chlorobenzene 
                 9.06 
               
               
                   
               
             
          
         
       
     
     Referring to  FIG. 4 , a ratio of the first collected current (E 1 ) to the second collected current (E 2 ) may be determined and used in identification, as depicted in Table 2. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Peak No. 
                   
                 eV 
                 E1 
                 E2 
                 E1/E2 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 1,1-dichloroethene 
                 9.79 
                 509 
                 311 
                 1.6367 
               
               
                 2 
                 methylene chloride 
                 11.32 
                 458 
                 108 
                 4.2407 
               
               
                 3 
                 1,1-dichloroethane 
                 11.06 
                 498 
                 193 
                 2.5803 
               
               
                   
                 benzene 
                 9.26 
                 overlap 
               
               
                   
                 carbon tetrachloride 
                 11.47 
                 overlap 
               
               
                 6 
                 1,2-dichloropropane 
                 10.87 
                 517 
                 167 
                 3.0958 
               
               
                 7 
                 trichloroethene 
                 9.47 
                 547 
                 408 
                 1.3407 
               
               
                   
                 2-chloroethyl vinyl ether 
                   
                 620 
                 380 
                 1.6316 
               
               
                 9 
                 1,1,2-trichloroethane 
                 11 
                 467 
                 182 
                 2.5659 
               
               
                 10 
                 Dibromochloromethane 
                 10.59 
                 277 
                 116 
                 2.3879 
               
               
                 11 
                 Tetrachloroethene 
                 9.32 
                 537 
                 341 
                 1.5748 
               
               
                 12 
                 Chlorobenzene 
                 9.06 
                 714 
                 523 
                 1.3652 
               
               
                   
               
             
          
         
       
     
     Referring to Table 2 as depicted in  FIG. 4 , the response ratio (E 1 /E 2 ), which covers a wide concentration range, may be used to confirm the compound identifies indicated by the retention time. While the response ratio varies with the intensities of the first ionization source  112  and the second ionization source  118 , and with the geometry of the ionization detector body  102 , namely the distance between the ionization source  112 ,  118  and the end  120 ,  122  of the respective collecting electrode  138 ,  140 , these characteristics are generally fixed. Beneficially, the response ratio is independent of flow, temperature and concentration changes. The variance due to intensities of the first ionization source  112  and the second ionization source  118  may be eliminated by normalizing the values to a known standard, such as benzene. The scale factor necessary for normalizing may be obtained by dividing the ionization potential for benzene according to the second collecting electrode  140  by the ionization potential for benzene according to the first collecting electrode  138 . The response ratio for a compound, whether normalized or not, is basically dependent on the Ionization Potential of the compound and its structure, as illustrated in  FIG. 4 . 
     The constituents in a sample may thus be identified by providing a detector according to the structure provided above, flowing discharge gases and the sample therethrough and displaying or visually representing to the user the results on a computer screen, or printer, or other display options perceivable by the user. This may be accomplished by providing an enclosed elongated chamber, the chamber having a first end and a second end; providing a first discharge gas inlet into the enclosed elongated chamber proximate the chamber first end; providing a first ionization source, the first ionization source positioned in the enclosed elongated chamber downstream of the first discharge gas inlet; providing a column gas inlet into the enclosed elongated chamber downstream of the first ionization source; providing a second discharge gas inlet into the enclosed elongated chamber proximate the chamber second end; providing a second ionization source, the second ionization source positioned in the enclosed elongated chamber downstream of the second discharge gas inlet; providing an outlet from the enclosed elongated chamber positioned between the column gas inlet and the second ionization source; providing a first collecting electrode end in the enclosed elongated chamber and downstream of the column gas inlet and between the column gas inlet and the outlet; and providing a second collecting electrode end in the enclosed elongated chamber and between the first collecting electrode end and the second ionization source and, when the second ionization source  118  is pulsed discharge, downstream of the second discharge gas inlet. The relevant gases may be flowed through the detector and the electrical output obtained. This includes flowing helium through the first discharge gas inlet into the enclosed elongated chamber; ionizing the flowing helium, the first discharge gas, as it passes the first ionization source to generate photons; flowing a combination of helium and a second noble gas through the second discharge gas inlet into the enclosed elongated chamber; ionizing the flowing combination of helium and a second noble gas, the second discharge gas, as it passes the second ionization source to generate an ionized noble gas combination; flowing a carrier gas and sample compounds mixed therewith through the column gas inlet into the enclosed elongated chamber for exposure to the photons to generate high-ionized sample compounds and for exposure to the ionized noble gas combination to generate lower-ionized sample compounds; applying a first bias voltage to a first collecting electrode associated with the first collecting electrode end; applying a second bias voltage to a second collecting electrode associated with the second collecting electrode end; generating over time a first time-dependent current at the first collecting electrode end from interaction with the high-ionized sample compounds, and generating over time a second time-dependent current at the second collecting electrode end from interaction with the lower-ionized sample compounds. The method of use is completed by displaying or visually representing to the user the first time-dependent current as a function of time and the second time-dependent current as a function of time. 
     The method of use may be continued by determining a time-dependent current ratio of each peak in the first time-dependent current to each peak in the second time-dependent current, each time-dependent current ratio indicative of the ionization potential of the constituent associated with the each peak, and displaying or visually representing to the user a visual representation of the time-dependent current ratio. 
     Additionally, the structure may be used in connection with a method to identify separated constituents in a carrier gas. The method includes the steps of:
         a. flowing the separated constituents in a carrier gas through the column gas inlet into an enclosed elongated chamber having a chamber first end, a chamber second end, and an outlet,   b. first ionizing those constituents by a first ionization source, positioned between the column gas inlet and the chamber first end, such as by interaction with a first discharge gas ionized by the first ionization source,   c. applying a first bias voltage to a first collecting electrode having a first collecting electrode end positioned between the column gas inlet and the outlet,   d. generating over time a first time-dependent current at the end of the first collecting electrode from interaction with said constituents ionized by interaction with the first discharge gas ionized by the first ionization source,   e. secondly ionizing the constituents by a second ionization gas, positioned between the end of the first collecting electrode and the chamber second end, ionized by a second ionization source,   f. applying a second bias voltage to the second collecting electrode having a second collecting electrode end positioned between the end of the first collecting electrode and the outlet,   g. generating over time a second time-dependent current at the end of a second collecting electrode from interaction with said constituents ionized by interaction with the second discharge gas ionized by the second ionization source,   h. flowing the separated constituents in the carrier gas out of the enclosed elongated chamber at the outlet, where the outlet is positioned between the end of the second combined collecting electrode and the second ionization source, and   i. displaying or visually representing to the user, whether on a computer display or printout, a visual representation of the first time-dependent current as a function of time and said second time-dependent current as a function of time.
 
The method may further include:
   j. determining a time-dependent current ratio of each peak in the first time-dependent current to each peak in the second time-dependent current, and   k. displaying or visually representing to the user a visual representation of that time-dependent current ratio as a function of ionization potential, whether on a computer display or printout.
 
In those additional steps, each time-dependent current ratio is indicative of the ionization potential or the constituent associated with each peak.
       

     As provided above, the first ionization source  112  may comprise a first pair of spaced-apart discharge electrode tips  124  positioned sufficient for spark formation therebetween for pulsed discharge or may comprise a lamp. Similarly, the second ionization source may comprise a second pair of spaced-apart discharge electrode tips  126  positioned sufficient for spark formation therebetween for pulsed discharge, or a lamp, for radiation of less photonic energy that the first ionization source. 
     Referring to  FIG. 5 , an illustration of a detector body of the first alternative detector body  502  for the present photo ionization detector using pulsed discharge as the first, second, and third ionization sources is provided. In addition to the components of the embodiment of  FIG. 1 , the first alternative embodiment includes an elongate, i.e. having a length greater than its width, second chamber  504  having a second chamber first end  506  and a second chamber second end  508  and in communication at a second chamber second end  508  with the elongate main chamber  104  between the first collecting electrode  112  and the second collecting electrode  140 . The angle of the communication between the elongate main chamber  104  and the second chamber  504  may be adjusted to control flow rates, but preferably is a right angle. A third ionization source  510  is positioned in the second chamber  504  adjacent the second chamber first end  506 . Preferably, the first alternative detector body  502  also includes a third discharge gas inlet  512  into the second chamber  504  proximate the second chamber first end  506  wherein the third ionization source comprises a third pair of spaced-apart discharge electrode tips  514  positioned sufficient for spark formation therebetween within the second chamber  504  for pulsed discharge. As at the other ends  106 ,  108 , a third biasing-or-ground electrode  516  is positioned in into the second chamber  504  intermediate the third ionization source  510  and the second chamber second end  508  so its end is at least adjacent, i.e., in or at the edge of, the second chamber  504 , and is electrical connection with a third voltage bias source. A third biasing and collecting electrode  518  is position within the second chamber  504  downstream of the third discharge gas inlet  512 , the third ionization source  510 , and the third pair of spaced-apart discharge electrode tips  514 . 
     As provided above, using helium doped with a second dopant selected from the group of argon, krypton, or xenon as the third discharge gas rather than pure helium or helium with the first dopant, provides a simultaneous, but different, discharge emission profile. The multiple discharge emission profiles, which may require a time shift to account for flow through the first alternative detector body  502 , provide a more accurate result without the need for additional detectors. 
     As previously noted, the detector body  502  may further include a first bias electrode adjacent the enclosed elongate main chamber  502  intermediate the first ionization source  112  and the first combined bias/collecting electrode  138 . Moreover, the detector body  502  may further include a second bias electrode adjacent the enclosed elongate main chamber  502  intermediate the second ionization source  118  and the second combined bias/collecting electrode  140 . 
     Referring to  FIG. 6 , an illustration of a detector body  602  of the second alternative embodiment for the present photo ionization detector using a non-linear chamber is provided. Unlike the preferred embodiment of  FIG. 1 , the enclosed elongate chamber  604  of the detector body  602  of the second alternative embodiment includes a bend or change in direction  638  intermediate the first combined bias/collecting electrode  152  and the second combined bias/collecting electrode  154 . The extent of the bend or change in direction  638  is preferably ninety (90) degrees, though the bend or change in direction  638  may be more or less than ninety (90) degrees. The bend or change in direction  638  may be accomplished by bending of the body  602  or may be accomplished by joining two segments  602   a ,  602   b  of a body  602 , each with its own longitudinal axis, at an angle greater than zero, such as by joining two segments  602   a ,  602   b  of a body  602  at a right angle. Thus, the enclosed elongated chamber  604  may have a first enclosed elongated chamber longitudinal axis  634  associated with the first body segment  602   a  and a second enclosed elongated chamber axis  636  associated with the second body segment  602   b , where the first enclosed elongated chamber longitudinal axis  634  is non-coaxial with the second enclosed elongated chamber axis  636 . The bend or change in direction  638  provides a localized area of pressure loss in the enclosed elongate chamber  604  due to the friction in enclosed elongate chamber  604  and the momentum exchange incident to the change in direction. The extent of the angle of the bend or change in direction  638 , and therefore the angle between the first enclosed elongated chamber longitudinal axis  634  and the second enclosed elongated chamber axis  636 , affects the extent of the pressure differential. The bend or change in direction  638  therefore results is increased separation between the first ionization source  112  and the second ionization source  118  within a relatively small body  602 , minimizing interference between the two sources  112 ,  118  and in increased selectivity. 
     As previously noted, the detector body  602  may further include a first bias electrode adjacent the enclosed elongate main chamber  602  intermediate the first ionization source  112  and the first combined bias/collecting electrode  138 . Moreover, the detector body  502  may further include a second bias electrode adjacent the enclosed elongate main chamber  502  intermediate the second ionization source  118  and the second combined bias/collecting electrode  140 . 
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof.