1、Designation: E 1698 95 (Reapproved 2005)Standard Practice forTesting Electrolytic Conductivity Detectors (ELCD) Used inGas Chromatography1This standard is issued under the fixed designation E 1698; the number immediately following the designation indicates the year oforiginal adoption or, in the cas
2、e of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers testing the performance of anelectrolytic conductivity detector (ELCD
3、) used as the detectioncomponent of a gas chromatographic system.1.2 This practice is directly applicable to electrolytic con-ductivity detectors that perform a chemical reaction on a givensample over a nickel catalyst surface under oxidizing orreducing conditions and employ a scrubber, if needed, t
4、oremove interferences, deionized solvent to dissolve the reac-tion products, and a conductivity cell to measure the electro-lytic conductivity of ionized reaction products.1.3 This practice covers the performance of the detectoritself, independently of the chromatographic column, in termsthat the an
5、alyst can use to predict overall system performancewhen the detector is coupled to the column and other chro-matographic system components.1.4 For general gas chromatographic procedures, PracticeE 260 should be followed except where specific changes arerecommended herein for the use of an electrolyt
6、ic conductivitydetector. For definitions of gas chromatography and its variousterms see Practice E 355.1.5 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.6 This standard does not purport to address all of thesafety concerns,
7、if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E 260 Practice for Packed Column Gas Chromato
8、graphyE 355 Practice for Gas Chromatography Terms and Rela-tionships3. Significance and Use3.1 Although it is possible to observe and measure each ofthe several characteristics of the ELCD under different andunique conditions, in particular its different modes of selectiv-ity, it is the intent of th
9、is practice that a complete set of detectorspecifications should be obtained at the same operating condi-tions, including geometry, gas and solvent flow rates, andtemperatures. It should be noted that to specify a detectorscapability completely, its performance should be measured atseveral sets of c
10、onditions within the useful range of thedetector. The terms and tests described in this practice aresufficiently general so that they may be used at whateverconditions may be chosen for other reasons.3.2 Linearity and speed of response of the recorder usedshould be such that it does not distort or o
11、therwise interferewith the performance of the detector. Effective recorder re-sponse should be sufficiently fast so that it can be neglected insensitivity of measurements. If additional amplifiers are usedbetween the detector and the final readout device, theircharacteristics should also first be es
12、tablished.4. Principles of Electrolytic Conductivity Detectors4.1 The principle components of the ELCD are representedin Fig. 1 and include: a control module, a reactor assembly,and, a cell assembly.4.1.1 The control module typically will house the detectorelectronics that monitor or control, or bot
13、h, the solvent flow,reaction temperatures, and the conductivity detector cell. It canbe functionally independent of the gas chromatography or, insome varieties, designed into the functional framework of thegas chromatograph. However, the reactor and cell assembliesare designed for specific models of
14、 gas chromatographs so it isimportant the proper components be assembled on the appro-priate chromatographic equipment.4.2 Fig. 2 is a block diagram representation of the GC/ELCD system. The electrolytic conductivity detector detectscompounds by pyrolyzing those compounds in a heated nickelcatalyst
15、(housed in the reactor), removing interfering reactionproducts with a scrubber (if needed), dissolving the reaction1This practice is under the jurisdiction of ASTM Committee E13 on MolecularSpectroscopy and is the direct responsibility of Subcommittee E13.19 on Chroma-tography.Current edition approv
16、ed Sept. 1, 2005. Published September 2005. Originallyapproved in 1995. Last previous edition approved in 2000 as E 1698 95 (2000).2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume informa
17、tion, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.products in a suitable solvent, and measuring the change inelectrical conductivity using a conductivity detector ce
18、ll. Othersuitable non-catalystic reaction tubes can be used for moreselective response characteristics. Using the conditions setforth in this practice, halogen (Cl, Br, I, F) compounds,nitrogen compounds, and sulfur compounds can be measuredselectively, even in the presence of each other.4.3 The ele
19、ctrolytic conductivity detector pyrolyzes com-pounds as they elute from the chromatographic column througha hot nickel reaction tube. Halogen and nitrogen compoundsare detected under reducing conditions while sulfur compoundsare detected under oxidizing conditions. The effluent from thegas chromatog
20、raphic column is combined with either hydrogen(reducing conditions) or air (oxidizing conditions) beforeentering the heated (800 to 1000C) nickel reaction tube. Thecompound is converted to small inorganic reaction productsdepending upon the reaction conditions as shown in Table 1.4.4 Table 2 shows t
21、he chemistry and modes of selectiveresponse for the detector. Depending upon the mode ofoperation, various interfering reaction products are removed byemploying a selective gas scrubber before the product gasesreach the detector cell. In the nitrogen-specific mode, halogenand sulfur products are rem
22、oved by reaction with a causticscrubber. In the sulfur-specific mode, halogen products areremoved by a silver thread (or wire) scrubber. No scrubber isrequired for halogen mode operation.4.5 The reaction products pass to the conductivity cellwhere they are combined with the solvent. The followingsol
23、vents are typically used for normal operation in eachindicated mode. Other solvents may be used to providechanges in selectivity and sensitivity (see 6.7):Model SolventHalogen 1-PropanolSulfur 100 % MethanolNitrogen 10 %t-Butyl Alcohol/90 % Water4.6 The increase in electrical conductivity of the sol
24、vent asa result of the introduction of the reaction products is measuredby the sensing electrodes in the conductivity cell. The solventpasses through the cell after being deionized through an ionexchange resin bed located between the conductivity cell andsolvent reservoir. In most instruments the so
25、lvent is recycledby taking the solvent from the cell back into the solventreservoir.5. Detector Construction5.1 There is some variation in the method of construction ofthis detector. In general, the geometry and construction of theconductivity cell is the single distinguishing component be-tween det
26、ector designs. It is not considered pertinent to reviewall aspects of the different detector designs available but ratherto consider one generalized design as an example and recog-nize that variants may exist.5.2 Detector BaseThe base extends into the gas chroma-tography oven and permits an inert lo
27、w dead volume interfaceof the column to the reactor. The carrier gas, the reaction gas,and the make-up gas (if needed) are introduced at the detectorbase. The base is heated and controlled by the gas chromato-graph or allowed to track the gas chromatograph oven tem-perature.5.3 Reaction TubeThe nick
28、el pyrolysis tube interfaces tothe detector base and is heated by a heating element called thereactor which surrounds the tube. The normal operating tem-perature is 800 to 1000C for most applications.5.4 ScrubberA coiled tube, used in either the nitrogen orsulfur mode, containing a specific scrubbin
29、g material is placedbetween the exit of the pyrolysis tube and the entrance of theconductivity cell in order to remove certain reaction productswhich may interfere in the specific mode of operation. Re-placement of the scrubber is mandated by response to anyhalogen compound.5.5 Conductivity CellThe
30、conductivity cell consists of aplastic block containing two metal electrodes that measure theelectrolytic conductivity of the solvent. It is connected to thereactor exit by means of an inert (usually TFE-fluorocarbon)transfer tube. It provides the conductivity signal for the specificcompound. Gaseou
31、s products from the reaction tube enter intothe front of the cell and contact the solvent which is introducedthrough the side of the cell. Together, these entities passthrough the electrode area and then out through the back of thecell.5.6 SolventThe solvent is selected to provide the desiredsensiti
32、vity and selectivity for each mode of operation. Thesolvent must be deionized, having a low conductivity, neutralpH, and must be able to dissolve the appropriate reactionproducts. The increase in conductivity of the solvent due to thepresence of the reaction products results in a peak responsecorres
33、ponding to the original analyte. The solvent level in thereservoir should be maintained weekly and the solvent com-pletely replaced every three months using high-purity solventsfor best results.5.7 Solvent Delivery SystemThe system consists of apump and an ion exchange resin system which works to bo
34、thdeionize and neutralize the pH of the solvent.Aby-pass systemis used to allow the pump to run at a normal speed while stilldelivering the low solvent flow rates (30 to 100 L/min)required by the detector. For operation in the nitrogen modeFIG. 1 ELCDPrincipal ComponentsE 1698 95 (2005)2special solv
35、ent delivery systems may be required to ensure thepH of the water-based solvent remains neutral. Refer to specificinstructions provided by the manufacturer of the respectivedetector you are employing on your gas chromatograph. It isimportant to note that each mode will require specific resinswhich w
36、ill require periodic replacement and attention given toexpiration dates for their useful life-time. Resins should bemixed thoroughly before adding or replacing as the anion/cation mixture used by most manufacturers will separate unlessa prepacked resin cartridge is used.5.8 ModuleAll operational fun
37、ctions, except for detectorbase temperature, are controlled from the module. On somesystems, vent time can be controlled from the gas chromato-graph as an external event.5.9 Vent ValveWhen opened, the vent valve provides away of preventing unwanted column effluents from enteringthe reaction tube. Th
38、ese effluents may include substances suchas the sample injection solvent and column bleed which cancause fouling or poisoning of the nickel reaction tubescatalytic surface. The valve is otherwise kept closed to allowthe compounds of interest to pass into the reaction tube so thatthey may be detected
39、. The valve interfaces with the detectorbase by means of a vent tube connected at the column exit inthe base. It is important that the gas flow from the vent (if used)be measured daily to ensure reproducible results and retentiontimes.6. Equipment Preparation6.1 The detector will be evaluated as par
40、t of a gas chro-matograph using injections of gases or liquid samples whichhave a range of component concentrations.6.2 GasesAll gases passing through the reactor should beultra-high purity (99.999 %) grade. Helium or hydrogen can beused as the GC column carrier gas. Nitrogen is extremelydetrimental
41、 to the performance of the detector in all modes, andtherefore cannot be used as a carrier of makeup gas nor can itbe tolerated as a low level contaminant. No attempt will bemade here to guide the selection of optimum conditions, exceptto state that experience has shown that gases of the highestavai
42、lable purity result in far fewer detector problems anddifficulties. Poor quality, hydrogen has been found to be thecause of noise, low response, wandering baseline, and peaktailing when operating in the halogen or nitrogen modes.Similarly, the highest grade of air works best for the sulfurmode.6.3 H
43、ardwareHigh-purity gases require ultra-clean regu-lators, valves, and tubing. Use of clean regulators, employingstainless steel valves, diaphragms, and tubing have been foundto result in far fewer detector problems and difficulties.6.4 ColumnsAll columns, whether packed or capillary,should be fully
44、conditioned according to suppliers specifica-tions prior to connecting to the detector. Certain liquid phasesthat are not compatible with the mode of operation should beavoided. Use of silanes (such as those used in deactivation ofFIG. 2 GC/ELCD System OverviewTABLE 1 Pyrolysis Reaction Products For
45、med Under Oxidizingor Reducing ConditionsOxidizing Element ReducingCO2CCH4H2OH2NO/N2NN3HX, HOX X HXO2OH2OSO2/SO3SH2SE 1698 95 (2005)3glass liners and columns) should be avoided since they havebeen shown to poison the reactor tube.6.5 Reactor TemperatureThe target reactor temperature is800 to 900C. H
46、owever, other reactor temperatures may befound to provide better results with certain compound types.Some typical reactor temperatures are given as follows:6.5.1 800 to 900C for most halogen-mode applications,6.5.2 850 to 925C for most nitrogen-mode applications,6.5.3 950 to 1000C for polychlorinate
47、d biphenyls (PCBs),and6.5.4 900 to 950C for sulfur compounds, such as sulfides.6.6 Reaction Gas Flow RateReaction gas flow rates fallwithin a range from 50 to 100 mL/min, depending upondetector design and application. Consult the manufacturer forrecommendations.6.7 SolventTypical solvents for each m
48、ode of operationare listed as follows. Other solvents may be substituted in orderto enhance selectivity or sensitivity. However, there is usuallya sacrifice in selectivity in order to gain sensitivity andvice-versa.Halogen Mode Sensitivity Selectivity1-Propanol Normal Normalisopropyl Alcohol Normal
49、NormalMethanol Highest LowestEthanol Higher Lower1-Butanol Lowest HighestSulfur ModeMethanol Lower HigherMethanol/20 % Water Normal NormalEthanol Lowest HighestNitrogen Mode10 % t-Butyl Alcohol/Water Higher Higher50 % 1-Propanol/50 % Water Normal Normal6.7.1 In solvent systems requiring water, use only deionizedwater with a resistivity of 18 MV or better. It should also benoted the binary solvent systems will change in their propor-tions due to normal evaporation. It is suggested that thosesolvents be checked biweekly and the reservoir topped off withfresh
