1、Designation: E1698 95 (Reapproved 2017)Standard Practice forTesting Electrolytic Conductivity Detectors (ELCD) Used inGas Chromatography1This standard is issued under the fixed designation E1698; the number immediately following the designation indicates the year oforiginal adoption or, in the case
2、of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers testing the performance of anelectrolytic conductivity detector (ELCD) u
3、sed 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, tore
4、move 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 analy
5、st 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, PracticeE260 should be followed except where specific changes arerecommended herein for the use of an electrolytic c
6、onductivitydetector. For definitions of gas chromatography and its variousterms see Practice E355.1.5 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, if an
7、y, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety, health, and environmental practices and deter-mine the applicability of regulatory limitations prior to use.1.7 This international standard was developed in accor-dance with internation
8、ally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2E260 Practi
9、ce for Packed Column Gas ChromatographyE355 Practice for Gas Chromatography Terms and Relation-ships3. 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 ofselec
10、tivity, it is the intent of this practice that a complete set ofdetector specifications should be obtained at the same operat-ing conditions, including geometry, gas and solvent flow rates,and temperatures. It should be noted that to specify a detectorscapability completely, its performance should b
11、e measured atseveral sets of conditions 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
12、 that it does not distort or otherwise 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, theircharacter
13、istics should also first be established.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 t
14、hat monitor or control, or both, 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 assemblies1This
15、 practice is under the jurisdiction of ASTM Committee E13 on MolecularSpectroscopy and Separation Science and is the direct responsibility of Subcom-mittee E13.19 on Separation Science.Current edition approved Oct. 1, 2017. Published October 2017. Originallyapproved in 1995. Last previous edition ap
16、proved in 2010 as E1698 95 (2010).DOI: 10.1520/E1698-95R17.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.Co
17、pyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of Interna
18、tional Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.1are designed for specific models of gas chromatographs so it isimportant the proper components be assembled on the appro-priate chromatographic equipment.4.2 Fig. 2 is a b
19、lock diagram representation of the GC/ELCD system. The electrolytic conductivity detector detectscompounds by pyrolyzing those compounds in a heated nickelcatalyst (housed in the reactor), removing interfering reactionproducts with a scrubber (if needed), dissolving the reactionproducts in a suitabl
20、e solvent, and measuring the change inelectrical conductivity using a conductivity detector cell. 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, a
21、nd sulfur compounds can be measuredselectively, even in the presence of each other.4.3 The electrolytic 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 whi
22、le sulfur compoundsare detected under oxidizing conditions. The effluent from thegas chromatographic column is combined with either hydrogen(reducing conditions) or air (oxidizing conditions) beforeentering the heated (800 to 1000 C) nickel reaction tube. Thecompound is converted to small inorganic
23、reaction productsdepending upon the reaction conditions as shown in Table 1.4.4 Table 2 shows the 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
24、gasesreach the detector cell. In the nitrogen-specific mode, halogenand sulfur products are removed 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 p
25、roducts pass to the conductivity cellwhere they are combined with the solvent. The followingsolvents are typically used for normal operation in eachindicated mode. Other solvents may be used to providechanges in selectivity and sensitivity (see 6.7):FIG. 1 ELCDPrincipal ComponentsFIG. 2 GC/ELCD Syst
26、em OverviewE1698 95 (2017)2Model SolventHalogen 1-PropanolSulfur 100 % MethanolNitrogen 10 %t-Butyl Alcohol/90 % Water4.6 The increase in electrical conductivity of the solvent asa result of the introduction of the reaction products is measuredby the sensing electrodes in the conductivity cell. The
27、solventpasses through the cell after being deionized through an ionexchange resin bed located between the conductivity cell andsolvent reservoir. In most instruments the solvent is recycledby taking the solvent from the cell back into the solventreservoir.5. Detector Construction5.1 There is some va
28、riation in the method of construction ofthis detector. In general, the geometry and construction of theconductivity cell is the single distinguishing component be-tween detector designs. It is not considered pertinent to reviewall aspects of the different detector designs available but ratherto cons
29、ider 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 low dead volume interfaceof the column to the reactor. The carrier gas, the reaction gas,and the make-up gas (if needed) are intro
30、duced 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 nickel pyrolysis tube interfaces tothe detector base and is heated by a heating element called thereactor which surrounds the tube.
31、The normal operating tem-perature is 800 to 1000 C for most applications.5.4 ScrubberA coiled tube, used in either the nitrogen orsulfur mode, containing a specific scrubbing material is placedbetween the exit of the pyrolysis tube and the entrance of theconductivity cell in order to remove certain
32、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 conductivity cell consists of aplastic block containing two metal electrodes that measure theelectrolytic conductivity of the s
33、olvent. It is connected to thereactor exit by means of an inert (usually TFE-fluorocarbon)transfer tube. It provides the conductivity signal for the specificcompound. Gaseous products from the reaction tube enter intothe front of the cell and contact the solvent which is introducedthrough the side o
34、f 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 desiredsensitivity and selectivity for each mode of operation. Thesolvent must be deionized, having a low conductivity, neutralpH, and must b
35、e able to dissolve the appropriate reactionproducts. The increase in conductivity of the solvent due to thepresence of the reaction products results in a peak responsecorresponding to the original analyte. The solvent level in thereservoir should be maintained weekly and the solvent com-pletely repl
36、aced 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 bothdeionize and neutralize the pH of the solvent.Aby-pass systemis used to allow the pump to run at a normal speed while stillde
37、livering the low solvent flow rates (30 to 100 L/min)required by the detector. For operation in the nitrogen modespecial solvent delivery systems may be required to ensure thepH of the water-based solvent remains neutral. Refer to specificinstructions provided by the manufacturer of the respectivede
38、tector you are employing on your gas chromatograph. It isimportant to note that each mode will require specific resinswhich will require periodic replacement and attention given toexpiration dates for their useful life-time. Resins should bemixed thoroughly before adding or replacing as the anion/ca
39、tion mixture used by most manufacturers will separate unlessa prepacked resin cartridge is used.5.8 ModuleAll operational functions, 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 V
40、ent ValveWhen opened, the vent valve provides away of preventing unwanted column effluents from enteringthe reaction tube. These effluents may include substances suchas the sample injection solvent and column bleed which cancause fouling or poisoning of the nickel reaction tubescatalytic surface. Th
41、e valve is otherwise kept closed to allowthe compounds of interest to pass into the reaction tube so thatthey may be detected. 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 measu
42、red daily to ensure reproducible results and retentiontimes.6. Equipment Preparation6.1 The detector will be evaluated as part 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 beult
43、ra-high purity (99.999 %) grade. Helium or hydrogen can beused as the GC column carrier gas. Nitrogen is extremelydetrimental 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 bem
44、ade here to guide the selection of optimum conditions, exceptTABLE 1 Pyrolysis Reaction Products Formed Under Oxidizingor Reducing ConditionsOxidizing Element ReducingCO2CCH4H2OH2NO/N2NN3HX, HOX X HXO2OH2OSO2/SO3SH2SE1698 95 (2017)3to state that experience has shown that gases of the highestavailabl
45、e 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 Hardw
46、areHigh-purity gases require ultra-cleanregulators, valves, and tubing. Use of clean regulators, employ-ing stainless steel valves, diaphragms, and tubing have beenfound to result in far fewer detector problems and difficulties.6.4 ColumnsAll columns, whether packed or capillary,should be fully cond
47、itioned 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 ofglass liners and columns) should be avoided since they havebeen shown to
48、poison the reactor tube.6.5 Reactor TemperatureThe target reactor temperature is800 to 900 C. However, 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 900 C for most halogen-mode applicat
49、ions,6.5.2 850 to 925 C for most nitrogen-mode applications,6.5.3 950 to 1000 C for polychlorinated biphenyls (PCBs),and6.5.4 900 to 950 C 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 mode of operationare listed as follows. Other solvents may be substituted in orderto enhance selectivity or sensitivity. However, there is usuallya sacrifice in se
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