1、Designation: E1698 95 (Reapproved 2010)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 and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E260 Practice for Packed Column Gas Chromatography
8、E355 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 this prac
9、tice 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 conditio
10、ns 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 otherwis
11、e 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 establish
12、ed.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 both, the
13、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 gas ch
14、romatographs 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 (housed
15、 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 Separation Science and is the direct responsibility of Subcom-mittee E13.19 on Separation Science.Cu
16、rrent edition approved Nov. 1, 2010. Published November 2010. Originallyapproved in 1995. Last previous edition approved in 2005 as E1698 95 (2005).DOI: 10.1520/E1698-95R10.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For A
17、nnual Book of ASTMStandards volume information, 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 con
18、ductivity 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, and sulfur compounds can be measuredselectively, even
19、 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 while sulfur compoundsare detected under oxidizing cond
20、itions. The effluent from thegas chromatographic 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 conditio
21、ns 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 gasesreach the detector cell. In the nitrogen-specifi
22、c 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 products pass to the conductivity cellwhere they are c
23、ombined 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):Model SolventHalogen 1-PropanolSulfur 100 % MethanolNitrogen 10 %t-Butyl Alcohol/90 % Water4.6 The incr
24、ease 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 solventpasses through the cell after being deionized through an ionexchange resin bed located between the conductivity cell andsol
25、vent 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 variation in the method of construction ofthis detector. In general, the geometry and construction of theconductivity cell is the si
26、ngle distinguishing component be-tween detector 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 ch
27、roma-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 introduced at the detectorbase. The base is heated and controlled by the gas chromato-graph or allowed to track the gas chromatograph o
28、ven 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. The normal operating tem-perature is 800 to 1000C for most applications.5.4 ScrubberA coiled tube, used in either the nitrogen ors
29、ulfur 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 reaction productswhich may interfere in the specific mode of operation. Re-placement of the scrubber is mandated by response to any
30、halogen compound.5.5 Conductivity CellThe 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 conductivi
31、ty 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 of the cell. Together, these entities passthrough the electrode area and then out through the back of thecell.5.6 SolventThe solvent
32、 is selected to provide the desiredsensitivity 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
33、 products results in a peak responsecorresponding 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 i
34、on 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 stilldelivering the low solvent flow rates (30 to 100 L/min)required by the detector. For operation in the nitrogen modeFIG. 1 ELCDPrincip
35、al ComponentsE1698 95 (2010)2special 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 respectivedetector you are employing on your gas chromatograph. It isimportant to note that each
36、 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/cation mixture used by most manufacturers will separate unlessa prepacked resin cartri
37、dge 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 Vent ValveWhen opened, the vent valve provides away of preventing unwanted column eff
38、luents 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. The valve is otherwise kept closed to allowthe compounds of interest to pass into the
39、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 measured daily to ensure reproducible results and retentiontimes.6. Equipment Preparation
40、6.1 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 beultra-high purity (99.999 %) grade. Helium or hydrogen can beused as the GC column carr
41、ier 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 bemade here to guide the selection of optimum conditions, exceptto state that experienc
42、e has shown that gases of the highestavailable 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 o
43、f air works best for the sulfurmode.6.3 HardwareHigh-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, whe
44、ther packed or capillary,should be fully 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 Overvi
45、ewTABLE 1 Pyrolysis Reaction Products Formed Under Oxidizingor Reducing ConditionsOxidizing Element ReducingCO2CCH4H2OH2NO/N2NN3HX, HOX X HXO2OH2OSO2/SO3SH2SE1698 95 (2010)3glass liners and columns) should be avoided since they havebeen shown to poison the reactor tube.6.5 Reactor TemperatureThe tar
46、get reactor temperature is800 to 900C. 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 900C for most halogen-mode applications,6.5.2 850 to 925C for most nitrogen-mode applicatio
47、ns,6.5.3 950 to 1000C for polychlorinated 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 forrecommendation
48、s.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 selectivity in order to gain sensitivity andvice-versa.Halogen Mode Sensitivity Selectivity1-Propano
49、l Normal Normalisopropyl Alcohol Normal 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 biweek
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