1、Designation: E 697 96 (Reapproved 2006)Standard Practice forUse of Electron-Capture Detectors in Gas Chromatography1This standard is issued under the fixed designation E 697; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year
2、 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 the use of an electron-capturedetector (ECD) as the detection component of a gas chromato-gr
3、aphic system.1.2 This practice is intended to describe the operation andperformance of the ECD as a guide for its use in a completechromatographic system.1.3 For general gas chromatographic procedures, PracticeE 260 or Practice E 1510 should be followed except wherespecific changes are recommended i
4、n this practice for use of anECD. For a definition of gas chromatography and its variousterms, see Practice E 355. These standards also describe theperformance of the detector in terms which the analyst can useto predict overall system performance when the detector iscoupled to the column and other
5、chromatographic components.1.4 This standard does not purport to address all of thesafety concerns, 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 p
6、rior to use. For specific safetyinformation, see Section 3.2. Referenced Documents2.1 ASTM Standards:2E 260 Practice for Packed Column Gas ChromatographyE 355 Practice for Gas Chromatography Terms and Rela-tionshipsE 1510 Practice for Installing Fused Silica Open TubularCapillary Columns in Gas Chro
7、matographs2.2 CGA Standards:CGA P-1 Safe Handling of Compressed Gases in Contain-ers3CGA G-5.4 Standard for Hydrogen Piping Systems atConsumer Locations3CGA P-9 The Inert Gases: Argon, Nitrogen and Helium3CGA V-7 Standard Method of Determining Cylinder ValveOutlet Connections for Industrial Gas Mixt
8、ures3CGA P-12 Safe Handling of Cryogenic Liquids3HB-3 Handbook of Compressed Gases32.3 Federal Standard:Title 10, Code of Federal Regulations, Part 2043. Hazards3.1 Gas Handling SafetyThe safe handling of compressedgases and cryogenic liquids for use in chromatography is theresponsibility of every l
9、aboratory. The Compressed Gas Asso-ciation (CGA), a member group of specialty and bulk gassuppliers, publishes the following guidelines to assist thelaboratory chemist to establish a safe work environment.Applicable CGA publications include: CGA P-1, CGA G-5.4,CGA P-9, CGA V-7, CGA P-12, and HB-3.3.
10、2 The electron capture detector contains a radioactiveisotope that emits b-particles into the gas flowing through thedetector. The gas effluent of the detector must be vented to afume hood to prevent possible radioactive contamination in thelaboratory. Venting must conform to Title 10, Code of Feder
11、alRegulations, Part 20 and Appendix B.4. Principles of Electron Capture Detection4.1 The ECD is an ionizating detector comprising a sourceof thermal electrons inside a reaction/detection chamber filledwith an appropriate reagent gas. In packed column GC thecarrier gas generally fullfills the require
12、ments of the reagentgas. In capillary column GC the make-up gas acts as thereagent gas and also sweeps the detector volume in order topass column eluate efficiently through the detector. While thecarrier/reagent gas flows through the chamber the devicedetects those compounds entering the chamber tha
13、t are capableof reacting with the thermal electrons to form negative ions.These electron capturing reactions cause a decrease in theconcentration of free electrons in the chamber. The detectorresponse is therefore a measure of the concentration and thechange in concentration of electrons (1-17).51Th
14、is practice is under the jurisdiction of ASTM Committee E13 on MolecularSpectrography and is the direct responsibility of Subcommittee E13.19 on Chro-matography.Current edition approved March 1, 2006. Published March 2006. Originallyapproved in 1979. Last previous edition approved in 2001 as E 697 9
15、6 (2001).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.3Available from Compressed Gas Association (CGA), 17
16、25 Jefferson DavisHwy., Suite 1004, Arlington, VA 22202-4102.4Available from U.S. Government Printing Office Superintendent of Documents,732 N. Capitol St., NW, Mail Stop: SDE, Washington, DC 20401.5The boldface numbers in parentheses refer to a list of references at the end ofthis practice.1Copyrig
17、ht ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.4.2 A radioactive source inside the detector provides asource of b-rays, which in turn ionize the carrier gas to producea source of electrons (18). A constant or intermittent negativepotential,
18、 usually less than 100 V, is applied across the reactionchamber to collect these electrons at the anode. This flow of“secondary” electrons produces a background or “standing”current and is measured by a suitable electrometer-amplifierand recording system.4.3 As sample components pass through the det
19、ector, theycombine with electrons. This causes a decrease in the standingcurrent or an increase in frequency of potential pulses depend-ing on the mode of ECD operation (see 5.3). The magnitude ofcurrent reduction or frequency increase is a measure of theconcentration and electron capture rate of th
20、e compound. TheECD is unique among ionizing detectors because it is this lossin electron concentration that is measured rather than anincrease in signal.4.4 The two major classifications of electron-capture reac-tions in the ECD are the dissociative and nondissociativemechanisms.4.4.1 In the dissoci
21、ative-capture mechanism, the samplemoleculeAB reacts with the electron and dissociates into a freeradical and a negative ion:AB + eA+B. This dissociativeelectron-capture reaction is favored at high detector tempera-tures. Thus, an increase in noncoulometric ECD response withincreasing detector tempe
22、rature is evidence of the dissociativeelectron-capture reaction for a compound. Naturally, detect-ability is increased at higher detector temperatures for thosecompounds which undergo dissociative mechanisms.4.4.2 In the nondissociative reaction, the sample moleculeAB reacts with the electron and fo
23、rms a molecular negativeion: AB + e AB. The cross section for electron absorptiondecreases with an increase in detector temperature in the caseof the nondissociative mechanism. Consequently, the nondis-sociative reaction is favored at lower detector temperatures andthe noncoulometric ECD response wi
24、ll decrease if the detectortemperature is increased.4.4.3 Beside the two main types of electron capture reac-tions, resonance electron absorption processes are also possiblein the ECD (for example, AB+e=AB). These resonancereactions are characterized when an electron absorbing com-pound exhibits a l
25、arge increase in absorption cross section overa narrow range of electron energies. This is an extremelytemperature sensitive reaction due to the reverse reactionwhich is a thermal electron deactivation reaction. For solutes inthis category a maximum detector temperature is reached atwhich higher tem
26、peratures diminish the response to the analyte(55).4.5 The ECD is very selective for those compounds thathave a high electron-capture rate and the principal use of thedetector is for the measurement of trace quantities of thesematerials, 109g or less. Often, compounds can be derivatizedby suitable r
27、eagents to provide detection of very low levels byECD (19, 20). For applications requiring less sensitivity, otherdetectors are recommended.4.6 A compound with a high electron-capture rate oftencontains an electrophoric group, that is, a highly polar moietythat provides an electron-deficient center
28、in the molecule. Thisgroup promotes the ability of the molecule to attach freeelectrons and also may stabilize the resultant negativemolecule-ion. Examples of a few electrophores are the halo-gens, sulfur, phosphorus, and nitro- and a-dicarbonyl groups(21-25).4.7 A compound could also have a high el
29、ectron-capturerate without containing an obvious electrophore in its structure,or its electron-capture rate could be much greater than that dueto the known electrophore that might be present. In these casescertain structural features, which by themselves are onlyweakly electrophoric, are combined so
30、 as to give the moleculeits electrophoric character. A few examples of these are thequinones, cyclooctatetracene, 3,17-diketosteroids, o-phthalatesand conjugated diketones (26-32).4.8 Enhanced response toward certain compounds has beenreported after the addition of either oxygen or nitrous oxide tot
31、he carrier gas. Oxygen doping can increase the responsetoward CO2, certain halogenated hydrocarbons, and polycyclicaromatic compounds (33). Small amounts of nitrous oxide canincrease the response toward methane, carbon dioxide, andhydrogen.4.9 While it is true that the ECD is an extremely sensitived
32、etector capable of picogram and even femtogram levels ofdetection, its response characteristics vary tremendously fromone chemical class to another. Furthermore, the responsecharacteristic for a specific solute of interest can also beenhanced or diminished depending on the detectors operatingtempera
33、ture (56) (see 4.4 and 5.5). The detectors responsecharacteristic to a solute is also dependent on the choice ofreagent gas and since the ECD is a concentration dependentdetector, it is also dependent on the total gas flow rate throughthe detector (see 5.5). These two parameters affect both theabsol
34、ute sensitivity and the linear range an ECD has to a givensolute. It is prudent of the operator of the ECD to understandthe influence that each of the aforementioned parameters hason the detection of a solute of interest and, to optimize theparameters prior to final testing.5. Detector Construction5
35、.1 Geometry of the Detector Cell:5.1.1 Three basic types of b-ray ionization-detector geom-etries can be considered applicable as electron-capture detectorcells: the parallel-plate design, the concentric-tube or coaxial-tube design, and recessed electrode or asymmetric type (34-37). The latter could
36、 be considered a variation of theconcentric-tube design. Both the plane-plate geometry andconcentric geometry are used almost exclusively for pulsedoperation. Although the asymmetric configuration is primarilyemployed in the d-c operation of electron-capture detectors, aunique version of the asymmet
37、ric design (referred to as adisplaced-coaxial-cylinder geometry) has been developed forpulse-modulated operation. The optimum mode of operation isusually different for each detector geometry and this must beconsidered, where necessary, in choosing certain operatingparameters.E 697 96 (2006)25.1.2 In
38、 general, more efficient operation is achieved if thedetector is polarized such that the gas flow is counter to theflow of electrons toward the anode. In this regard, the radio-active source should be placed at the cathode or as near to it aspossible.5.1.3 Other geometric factors that affect cell re
39、sponse andoperation are cell volume and electrode spacing, which may ormay not be altered concurrently depending upon the construc-tion of the detector. Of course, both these variables can besignificant at the extremes, and optimum values will alsodepend upon other parameters of operation. In the pu
40、lsedoperational mode, the electrons within the cell must be able toreach the anode or collector electrode during the 0.1 to 1.0-svoltage pulse. Generally, electrode distances of 0.5 to 1.0 cmare acceptable and can be used optimally by the proper choiceof operating conditions. Cell volume should be s
41、mall enough tomaintain effective electron capture without encountering othertypes of electron reactions and also small enough so as not tolose any resolution that may have been achieved by high-resolution chromatographic systems. Typical ECD cell vol-umes range from approximately 2 to 0.3 cm3. A det
42、ector cellwith a relatively low internal volume is particularly importantwhen the ECD is used with open tubular columns. In additionto the preceding electrical and chromatographic requirements,the electrode dimensions of the detector are also determined bythe range of the particular b-rays.5.2 Radio
43、active Source:5.2.1 Many b-ray-emitting isotopes can be used as theprimary ionization source. The two most suitable are3H(tritium) (38, 39). and63Ni (40).5.2.1.1 TritiumThis isotope is usually coated on 302stainless steel or Hastelloy C, which is a nickel-base alloy. Thetritium attached to the forme
44、r foil material is in the form of Ti3H2; however, there is uncertainty concerning the exact meansof tritium attachment to the scandium (Sc) substrate of theHastelloy C foil. The proposed methods of attachment includeSc3H3and3H2as the occluded gas.The nominal source activityfor tritium is 250 mCi in
45、titanium sources and 1000 mCi inscandium sources. Department of Energy regulations permit amaximum operating temperature of 225C for the Ti3H2sourceand 325C for the Sc3H3source. Naturally, detector tempera-tures that are less than the maximum values will lengthen thelifetimes of the tritiated source
46、s by reducing the tritiumemanation rates. The newer scandium sources are more effec-tive at minimizing the contamination problems associated withelectron-capture detectors because of their capability for op-eration at 325C. Furthermore, the tritiated-scandium sourcedisplays a factor-of-three detecta
47、bility increase for dissociativeelectron-capturing species, that is, halogenated molecules.Another advantage of scandium tritide sources is their avail-ability at much higher specific activities than nickel-63sources; therefore, Sc3H3sources are smaller and permit theconstruction of detector cells w
48、ith smaller internal volumes.The maximum energy of the b-rays emitted by tritium is 0.018MeV.5.2.1.2 Nickel-63 (63Ni)This radioactive isotope is usu-ally either electroplated directly on a gold foil in the detectorcell or is plated directly onto the interior of the cell block.Since the maximum energ
49、y of the b-rays from the63Ni is 0.067MeV and63Ni is a more effective radiation source than tritium,the normal63Ni activity is typically 10 to 15 mCi. Anadvantage of63Ni is its ability to be heated to 350C and theconcomitant decrease in detector contamination during chro-matographic operation. Another advantage of the high detectortemperatures available with63Ni is an enhanced sensitivity forcompounds that undergo dissociative electron capture.5.2.2 Although the energies and the practical sourcestrengths for these two radioactive isotopes are different, nosignificant d
