ELECTROLYTIC CONDUCTIVITY DETECTOR

Abstract: A gas chromatograph system is provided with an im­ proved electrolytic conductivity cell for detecting a selected material species in a fluid stream from conventional gas chromatograph having both gas and liquid phases. Planar electrodes (80, 82) are spaced by an insulator (84) and define borehole diameters (88, 90, 92) which enhance the generated signal-to-noise ratio by providing s short fluid transit time within the conductivity detecting volume com­ pared with the effective time of the chromatographic event of interest. A pneumatic damper may be further included to reduce generated system signal noise.

Description: This invention relates to a chromatographic system having a conductivity cell for deriving an output signal functionally related to a throughput fluid. A divisional application has been filed in EP 88116330.7. FR-E-93 213 discloses a chromatographic system comprising a solid detection electrode and a reference electrode. The two electrodes are separated by an insulating disk. The electrodes and the insulating disk extend across the entire cross-sectional area of the chromatographic column; they consist of porous material. US-A-4 295 856 discloses an electrolytic conductivity detector for the selective and quantitative detection of N-nitrosamines in samples containing other nitrogen compounds. The detector requires a fluid reservoir to measure conductivity. In addition, fluid and gas must be separated. US-A-4 183 791 discloses an amperometric sensor device for electrochemical determination of polyhydroxy and polyamine compounds in liquid chromatography and high pressure liquid chromatography. There is no suggestion that the prior art sensor can measure conductivity in mixed phase media. US-A-4 032 296 discloses an electrolytic conductivity detector system that is particularly useful for gas chromatograhpy. The system comprises a conductivity cell where liquid phase is separated from gas phase and separated liquid phase utilized for conductivity measurement. The conductivity cell includes an inner electrode tube and an outer electrode block. Chromatography generally involves a separation process for redistributing molecules in a mixture from a thin material phase to a second bulk material phase. The two phases in contact may be liquid-liquid, liquid-solid, vapor-solid or vapor-liquid. Where the bulk phase is a gas stream, the term gas chromatography is used to describe the process. The gas stream is then analyzed to obtain information specific to the transfer molecules. Typical detection systems include thermal conductivity, flame ionization and argon detectors. More recent detectors include electrolytic conductivity detectors providing an electrical signal which is functionally related to the presence of a selected element. Electrolytic conductivity detectors have long been available for investigating the properties of electrolytes in solutions. Such devices in the prior art typically include electrode surfaces with a continuous phase liquid electrolyte therebetween. Conventional use of electrolytic conductivity detectors in a gas chromatograph system generally follows this prior art approach and separates any gas phase from the liquid phase prior to the detector cell which provides an output related to the electrolyte. An early typical electrolytic conductivity detector, the Coulson detector, is depicted in the Journal of Gas Chromatography , April, 1965, at pages 134-137. As therein depicted the electrolytic conductivity detector cell provides a liquid collecting and gas venting section for separating liquid from the gas flow and thereafter introducing the liquid to an electrode volume where the liquid electrolyte is used to form a continuous conductive path between electrode structures in the sidewalls. Yet another electrolytic conductivity detector is depicted in US-A-3 934 193. The Hall detector also separates the flow into a gas flow and a liquid flow and the liquid flow is captured between two concentric electrode surfaces for conductivity measurement. Thus, in both detectors, a slow moving volume of liquid is formed having a relatively long dwell time with respect to chromatographic events. The Coulson conductivity cell is typically a complex glass device which is difficult to interface with many chromatographs. Further, gases can accumulate in the detector section between electrodes. These gas bubbles can be difficult to displace and the liquid phase tends to merely divert about the gas phase bubble rather than to sweep away the bubble. The Hall conductivity detector is a substantially more rugged detector and easier to interface than the Coulson detector but the separating chamber is subject to deterioration. The relatively large electrode area per fluid volume ratio produces good sensitivity to the conductivity of the electrolytic fluid but can require frequent cleaning to maintain the volume cleanliness. Thus, it would be desirable to provide an electrolytic conductivity detector which is not subject to blockage by gas accumulation and which is substantially self cleaning, while retaining the large signal-to-noise ratio which is desired for sensitive results. It will be appreciated that the signal-to-noise ratio can be affected by short term variations in the fluid flow. Such short term variations are typically induced by the pump which moves the solvent liquid, used to dissolve portions of the gas phase, through the system and through the electrolytic conductivity detector. The signal-to-noise ratio could desirably be improved by reducing or eliminating the induced flow fluctations. The disadvantages of the prior art are overcome by the present invention, however, and improved apparatus is provided for electrically detecting gas chromatograph products and for enhancing the overall sensitivity of a gas chromatograph with an electrolytic conductivity detection system. An objective of the present invention is to directly measure the conductivity of a mixed liquid-gas fluid stream. Another objective of the present invention is to provide a conductivity detecting path which is not subject to gas bubble blockage. Yet another objective is to obtain a conductivity measurement region which is substantially self cleaning. One other objective is to reduce noise signals due to pump action on the solvent. SUMMARY OF THE INVENTION: The invention achieves these objections by means of a chromatographic apparatus, as claimed in claim 1 and a conductivity cell as claimed in claim 5. These objectives and other features and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a simplified block diagram of a gas chromatograph with an electrolytic conductivity detection system. FIGURE 2 is a cross section of one embodiment of a detector cell including the present invention. FIGURE 3 is a cross section more particularly depicting the electrode system. FIGURE 4 depicts one embodiment of a pneumatic damper. DETAILED DESCRIPTION Referring first to FIGURE 1, there is depicted in block diagram form a gas chromatography system having an electrolytic conductivity detector cell for signal derivation. Gas chromatograph 10 generates a sample gas flow from a selected material within gas chromatograph 10. The output sample gas is combined with reaction gas 12 for input to reactor 14. The output from reactor 14 generally contains inorganic compounds that will support electrolytic conductivity, such as HCl, SO₃, NH₃, or CO₂. Thus, the reaction gas 12 may be an oxidant or a reductant with respect to the effluent gas from chromatograph 10 to obtain appropriate compounds for the conductivity detector. The gaseous output from reactor 14 is provided to detector cell 16 and combined with a suitable solvent from solvent reservoir 18, as hereinafter described. Detector cell 16 provides a solvent return 25 to solvent reservoir 18 and provides an output signal to conductivity amplifier 24 to obtain an output from device 26. Solvent may be added to the system from reservoir 18 by the action of pump 20. The solvent may also flow through ion exchange resin bed 22 for solvent purification and removal of extraneous electrically conductive materials prior to reuse. Ion exchange bed 22 is conveniently designed, according to one embodiment of the present invention, to include a pneumatic reservoir for damping pressure and flow changes resulting from the action of pump 20. The response of detector cell 16 is a function of the product from reactor 14 and the specific solvent from reservoir 18 which are mixed and passed through detector cell 16 to return line 25 for reuse. Further, the output of reactor 14 may be determined by the reaction temperature which is effected by temperature controller 28. The overall gaseous input is further determined by vent timer 30 actuating solenoid vent valve 32 for venting detrimental system gases during a measurement cycle. Scrubbed reaction products are passed by tube 65 to the electrolytic conductivity detector cell shown in FIGURE 2. Thus, reaction products are introduced through tube 65 and a selected solvent is introduced through entry tube 66 to meet in upper cell assembly 76. The solvent in tube 66 may be either an aqueous or an organic solvent selected for the materials sought to be detected in the system. As herein used, the term fluid refers to the mixed stream of gas and liquid formed in upper cell assembly 76. The liquid-gas fluid thus formed passes through electrode assembly 77, generating an output signal, as hereinafter more particularly discussed. The liquid-gas fluid exhausts through tube 68 for return 25 to solvent reservoir 18 (see FIGURE 1). Fittings 70, 72 and 74 are conveniently provided to sealingly connect tubes 65, 66 and 68, respectively, to upper cell assembly 76 and lower cell assembly 78. Electrode assembly 77 (FIGURE 2) is more particularly shown in FIGURE 3. Flow bore path 86 is provided for the liquid-gas fluid for fluid movement through upper electrode 80, insulator 84 and lower electrode 82. Upper electrode 80 and lower electrode 82 are preferably planar electrodes arranged in a spaced facing relationship with planar insulator 84 therebetween. The fluid in bore path 86 forms a conductive path from upper electrode 80 to lower electrode 82 for transmitting an electrical signal therebetween. As used herein, the terms upper electrode and lower electrode designate the upstream and downstream positions, respectively, in the flow path. Electrodes 80 and 82 and insulator 84 are sealingly clamped between upper cell assembly 76 and lower cell assembly 78 to confine fluid flow through flow bore path 86. It will be appreciated that the flow through bore path 86 is a liquid-gas mixture, contrary to the teachings of the prior art. It is believed that the planar arrangement of electrodes and insulator with a bore therethrough provides a quite small conductive volume relative to conductive volumes formed in the prior art whereby fluids entering the volume have a short residence time in the effective conductive volume, with a transit time of about 1 millisecond, compared to the chromatographic event information of interest, typically greater than one second. Momentary fluctations in the signal from the passage of the mixed liquid and gas phases can be accommodated by conventional electronic filtering and signal processing. By way of example, a typical measurement volume is 1 x 10⁻⁴ cubic centimeters and the volumetric flow rates through that volume may typically be 10 to 100 cubic centimeters per minute for the gas phase and 0.1 to 1.0 milliliters per minute for the liquid phase. Thus, the volume is rapidly swept, producing an output signal and maintaining a continuous sweep of the gas phase from the measurement volume to enhance the signal-to-noise ratio. In one conventional signal processing technique, a bipolar square wave voltage is applied to electrode 80 or electrode 82 and the resulting transmitted signal through the fluid forming the conductive path is detected on the opposed electrode. The signal is processed to filter the undesired capacitive component of the resulting signal from the desired resistive component, the resulting signal is amplified and is provided as an output from electrode assembly 77. The performance of the electrolytic conductivity detector cell depicted in FIGURE 2 was evaluated using an O.I. Corporation Model 610 Total Organic Halogen Analyzer. This unit contains apparatus for sparging volatile halides from water, collecting the liberated halides on an absorbent trap, and thereafter desorbing the absorbed halides. In one instance, the desorbed halides were provided to a separating-type electrolytic conductivity detection cell, such as depicted in US-A-3 934 193 and the signal-to-noise ratio was compared with the performance of an electrolytic conductivity detection cell as depicted in FIGURE 2. The performance of any gas chromatographic electrolytic conductivity detector depends upon five primary factors: (1) efficiency of formation of the monitored species from the chromatographed compound, (2) efficiency of transport of the monitored species to the detector cell, (3) cell design, (4) stability of the purity and flow rate of the electrolytic conductivity solvent, and (5) conductivity measurement electronics. The formation and transport of the monitored species (HCl) was optimized in the experimental systems and the measurement electronics were standardized to permit an evaluation of the signal-to-noise ratio based solely on cell design. The results of this comparison are presented in table A as follows: Under the evaluation conditions, the signal-to-noise ratio for the standard prior art system was in the range from 10,000-20,000. It is apparent from Table A that the system herein described obtains a minimum signal-to-noise ratio equal to the ratio obtained from a prior art electrolytic conductivity cell, and generally obtains a superior performance as the electrode design is optimized. Referring again to FIGURE 3, and Table A, the signal-to-noise ratio has been determined as a function of the upper electrode bore diameter 88, insulator bore diameter 90 and lower electrode bore diameter 92. The test results indicate that improved results are obtained with upper electrode bore diameter 88 and insulator bore diameter 90 the same dimension and with lower electrode bore diameter 92 reduced. The results also indicate that a pneumatic damper in the solvent delivery system 18, 20, 22, (FIGURE 1) can further improve the results. Referring now to FIGURE 4, there is depicted one embodiment of a pneumatic damper incorporated with ion exchange bed 22 (FIGURE 1). Solvent entry fitting 100 generally directs solvent flow within resin cartridge 102 through ion exchange resin reservoir 106 to outlet fitting 104. Pneumatic damper reservoir 108 provides a pressurized chamber for accommodating pressure and flow fluctations during the action of pump 20 (FIGURE 1). Pneumatic damper reservoir 108 reduces the short-term flow pulsations normally associated with small gear pumps and reciprocating pumps. Pneumatic damper reservoir 108 is conveniently contained within resin cartridge 102, but might be provided as a separate pressurizer-type unit. The application of pneumatic damper reservoir 108 with the embodiment of the electrolytic conductivity detector according to the present invention produces the enhanced results depicted in Table A. However, the use of a pneumatic damper reservoir may have application to any pumped solvent delivery system used with electrolytic conductivity detection cells already existing in the prior art for use in gas chromatography. It is therefore apparent that the present invention is one well adapted to attain all of the objects and advantages herein above set forth together with other advantages which will become obvious and inherent from the description of the apparatus itself. It will be understood that certain combinations and subcombinations are of utility and may be obtained without reference to other features and subcombinations. This is contemplated by and is within the scope of the present invention.

Claims: A chromatographic apparatus having a conductivity cell for deriving an output signal functionally related to a throughput fluid, comprising: means (10) for generating a gas stream from a material sample and means (18) for generating a solvent stream effective to separate selected material species from said gas stream, characterized by: means (65, 66) for mixing said gas stream with said solvent stream to form a stream of said throughput fluid, and an electrode assembly (77) comprising a first (80) and second (82) planar electrode having disposed therebetween an insulator (84), the electrode assembly sealingly clamped between an upper (76) and lower (78) cell assembly and defining a hole (86) bored therethrough for receiving said throughput fluid stream and effective to generate an electrical output signal responsive to said selected material species in said solvent; the first (80) and second (82) planar electrodes and said insulator (84) disposed in a spaced parallel facing relationship and defining respective first (88) and second (92) boreholes and an insulator bore (90) therethrough for receiving said throughput fluid, the diameter of the first borehole (88) and the insulator bore diameter (90) are equal or the insulator bore diameter (90) is less than the first borehole diameter (88) and the second borehole diameter (92) is reduced with respect to the insulator bore diameter (90). Apparatus according to Claim 1, wherein said means (18) for generating a solvent stream further includes: a pneumatic damper (108) for reducing spurious flow induced signal noise. Apparatus according to Claim 2, wherein said means for generating a gas stream further includes: a reactor (14) having a removable cap, a reaction tube within said reactor, and compression fitting means for securing and supporting said reaction tube within said reactor. Apparatus according to Claim 3, wherein said compression fitting means further comprises: a first reaction tube nut receiving said reaction tube and threadedly engaging said removable cap, a first compression ferrule engaging a top portion of said reaction tube within said first nut, a first compression nut threadedly engaging said first nut and compressing said first ferrule to sealingly engage said reaction tube, a second compression ferrule engaging a bottom portion of said reaction tube adjacent a sealing surface within said reactor, and a second compression nut for sealing said second ferrule about said reaction tube and against said sealing surface. A conductivity cell for measuring the conductivity of a fluid stream derived from a chromatographic event and having a mixed fluid phase and gas phase characterized by: first (80) and second (82) planar electrodes and an insulator (84) in spaced parallel facing relationship, the first (80) and second (82) electrodes and insulator (84) sealingly clamped between an upper (76) and lower (78) cell assembly, said electrodes and insulator having a common central fluid path comprising coaxial first (88) and second (92) boreholes and insulator bore (90) for transmitting said fluid stream through said first and second boreholes respectively, said first and second boreholes located in said spaced electrodes and in coaxial alignment to define in cooperation with insulator bore (90) a measurement volume for said fluid stream to transit in a time substantially less than a time defining said chromatographic event, the diameter of the first borehole (88) and the bore diameter (90) are equal or the insulator bore diameter (90) is less than the first borehole diameter (88) and the second borehole diameter (92) is reduced with respect to the insulator bore diameter (90). Apparatus according to claim 1, including means (108) for dampening pressure and flow changes resulting from the generation of the solvent stream effective to separate selected material species from said gas stream; and wherein the diameter of the first borehole (88) and the insulator bore diameter (90) are equal to 1 mm or less. A conductivity cell according to claim 5, including means (108) for dampening pressure and flow changes resulting from the derivation of a fluid stream from the chromatographic event; and in that the diameter of the first borehole (88) and the insulator bore diameter (90) are equal to 1 mm or less.

Inventors: HALL, RANDALL C.
Applicants: O.I. CORPORATION
Application year: 1985
Main classification: G01N30
IPC codes: G01N31, G01N30, G01N27