1、Designation: E1642 00 (Reapproved 2010)Standard Practice forGeneral Techniques of Gas Chromatography Infrared (GC/IR) Analysis1This standard is issued under the fixed designation E1642; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revisio
2、n, 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 techniques that are of general use inanalyzing qualitatively multicomponent sample
3、s by using acombination of gas chromatography (GC) and infrared (IR)spectrophotometric techniques. The mixture is separated intoits individual components by GC and then these individualcomponents are analyzed by IR spectroscopy. Types of GC-IRtechniques discussed include eluent trapping, flowcell, a
4、ndeluite deposition.1.2 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.3 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 standa
5、rd 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:2E131 Terminology Relating to Molecular SpectroscopyE168 Practices for General Techniques of Infrared Quanti-tative AnalysisE260
6、Practice for Packed Column Gas ChromatographyE334 Practice for General Techniques of Infrared Mi-croanalysisE355 Practice for Gas Chromatography Terms and Rela-tionshipsE932 Practice for Describing and Measuring Performanceof Dispersive Infrared SpectrometersE1252 Practice for General Techniques for
7、 Obtaining Infra-red Spectra for Qualitative AnalysisE1421 Practice for Describing and Measuring Performanceof Fourier Transform Mid-Infrared (FT-MIR) Spectrom-eters: Level Zero and Level One TestsE1510 Practice for Installing Fused Silica Open TubularCapillary Columns in Gas Chromatographs3. Termin
8、ology3.1 DefinitionsFor definitions of terms and symbols, referto Terminology E131 and Practice E355.4. Significance and Use4.1 This practice provides general guidelines for the properpractice of gas chromatography coupled with infrared spectro-photometric detection and analysis (GC/IR). This practi
9、ceassumes that the chromatography involved in the practice isadequate to separate the compounds of interest. It is not theintention of this practice to instruct the user how to perform gaschromatography properly.5. General GC/IR Techniques5.1 Three different types of GC/IR technique have beenused to
10、 analyze samples. These consist of analyte trapping,flowcell, or lightpipe, and direct eluite deposition and arepresented in the order that they were first used.5.2 The GC eluent must not be routed to a destructive GCdetector (such as a flame ionization detector) prior to reachingthe IR detector as
11、this will destroy or alter the individualcomponents. It is acceptable to split the eluent so that part ofthe stream is directed to such a detector or to pass the streamback to the detector after infrared analysis if such techniquesare feasible.5.3 Eluent Trapping TechniquesAnalyte trapping tech-niqu
12、es are the least elaborate means for obtaining GC/IR data.In these techniques, the sample eluting from the chromato-graph is collected in discrete aliquots to be analyzed. Inutilizing such techniques, it is essential that a GC detector beemployed to allow definition of component elution. If adestruc
13、tive detector is employed, then post-column splitting tothat detector is required. GC fractions can be trapped in thecondensed phase by passing the GC effluent through a solvent,1This practice is under the jurisdiction of ASTM Committee E13 on MolecularSpectroscopy and Separation Science and is the
14、direct responsibility of Subcom-mittee E13.03 on Infrared and Near Infrared Spectroscopy.Current edition approved March 1, 2010. Published April 2010. Originallyapproved in 1994. Last previous edition approved in 2010 as E1642 00 (2010).DOI: 10.1520/E1642-00R10.2For referenced ASTM standards, visit
15、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.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959
16、, United States.a powdered solid, or a cold trap for subsequent analysis (seePractice E1252) (1).3Vapor phase samples can be trapped in aheated low-volume gas cell at the exit of the GC, analyzed,then flushed with the continuing GC effluent until the nextaliquot of interest is in the gas cell when t
17、he flow is stoppedagain for analysis (2). Since the analyte of interest is staticwhen employing an analyte trapping technique, the spectrumcan be recorded using a long co-addition time to improve thesignal-to-noise (SNR) ratio. However, in analyte trapping,sample integrity can be compromised by slow
18、 decomposition.A spectrum should be obtained with a short co-addition timefirst, to create a reference spectrum to ensure the integrity ofthe spectrum obtained after long co-addition.5.4 Flowcell Detection of Vapor Phase ComponentsThemost common GC/IR technique is the flowcell or “light-pipe”techniq
19、ue. The GC eluent stream is monitored continuously inthe time frame of the chromatography (real-time) by the IRspectrometer with the use of a specially designed gas cellcalled a light-pipe. In this design, the light-pipe is coupleddirectly to the GC by a heated transfer line. Individualcomponents ar
20、e analyzed in the vapor phase as they emergefrom the transfer line. This technique typically yields lownanogram detection limits for most analytes (3-5). Instrumentsthat include the IR spectrometer, the gas chromatograph,heated transfer-line, and light-pipe are commercially available.5.4.1 The rapid
21、ity with which spectra of the individualcomponents must be recorded requires a Fourier-transforminfrared (FT-IR) spectrometer. Such instruments include acomputer that is capable of storing the large amount ofspectroscopic data generated for subsequent evaluation.5.4.2 The transfer line from the GC t
22、o the light-pipe must bemade of inert, non-porous material (normally fused silicatubing) and be heated to prevent condensation. The tempera-ture of the transfer line is normally held constant during acomplete analysis at a level chosen to avoid both condensationand degradation of the analytes. Typic
23、al working temperaturesare about 100 to 300C (normally 10C higher than themaximum temperature reached during the chromatography).5.4.3 The light-pipe is normally gold-coated on the interiorto give maximum optical throughput and at the same timeminimize decomposition of analytes. The light-pipe dimen
24、-sions are typically optimized so that the volume accommodatesthe corresponding eluent volume of a sharp chromatographicpeak at the peaks full width at half height (FWHH). Thelight-pipe is heated to a constant temperature at or slightlyhigher than the temperature of the transfer line. The maximumtem
25、perature recommended by the manufacturer should not beexceeded. In general, sustained light-pipe temperatures above300C may degrade the gold coating and the life of the coatingdrops quickly with successively higher temperatures. It shouldbe pointed out that, if a chromatographic separation requirest
26、hat the GC temperature be raised above this level, it may benecessary to temporarily raise both the temperature of thelight-pipe and transfer line to maximum temperature of thechromatography to avoid condensation of the eluent. If this isthe case, the temperature of the light-pipe should be reduced
27、toa safe level as soon as possible. It must be noted that repeatedtemperature changes to the light-pipe and transfer line willcause a more rapid aging of the seals and may cause leaks.5.4.3.1 It should be noted that any metal surface inside thelight-pipe assembly can react with, and sometimes destro
28、y,some specific materials (for example, amines) as they elutefrom the GC. Consequently, it is possible to fail to identify thepresence of such a compound in the mixture. This situation canbe identified by comparing the response of the GC detectorafter the flowcell to that of a GC detector in the abs
29、ence of aflowcell, or by comparing the GC/IR detector output to theresults of a suitable alternate analytical technique.5.4.3.2 The ends of the light-pipe are sealed with infraredtransmissive windows. The optimum optical transmission isobtained by using potassium bromide windows, but thismaterial is
30、 very susceptible to damage by water vapor. As thelight-pipe is used, small amounts of water vapor will etch thewindow surfaces, and the optical throughput of the windowswill drop. Eventually these windows will have to be changed.Users who expect to analyze mixtures containing water shouldconsider u
31、sing windows made of a water-resistant materialsuch as zinc selenide, but this will result in a noticeable drop inoptical transmission due to optical reflection properties of suchmaterials.5.4.3.3 Usage of the light-pipe at high temperatures mayresult in the gradual buildup of organic char on both t
32、he cellwalls and end windows. As this occurs the optical throughputwill drop correspondingly. Eventually the light-pipe assemblywill have to be reconditioned (see 5.4.3.5).5.4.3.4 As the temperature of the light-pipe is raised aboveambient, the light-pipe emits an increasing amount of infraredradiat
33、ion. This radiation is not modulated by the interferometerand is picked up by the detector as DC signal. The DCcomponent becomes large at the normal working temperatures(above 200C), and lowers the dynamic range of the detector.The result of this effect is that the observed interferometricACsignal i
34、s reduced in size as the temperature increases and theobserved spectral noise level increases correspondingly. Byraising the temperature from room temperature to 250C, thenoise level typically doubles; it is recommended that the usercreate a plot of signal intensity versus light-pipe temperaturefor
35、reference purposes. As a consequence of this behavior, itmay be advantageous to record data using relatively lowtemperatures for both temperature and transfer line for thoseGC experiments that only use a limited temperature ramp.Some instrument designs include a cold aperture between thelight-pipe a
36、nd the detector to minimize the amount of radiationreaching the detector (see Note 1) (6,7).NOTE 1A cold aperture is a metal shield, maintained at roomtemperature, sited between the light-pipe and the detector. The infraredbeam diverging from the light-pipe is refocused at the plane of the coldshiel
37、d. The cold shield has a circular hole (aperture) of the same diameteras the refocused beam. After passing through the aperture and movingaway from this focal point, the beam is again focused onto the detectorelement. This small aperture shields the detector from thermal energyemitted from the vicin
38、ity of the hot light-pipe.5.4.3.5 The optical throughput of the light-pipe should beperiodically monitored since this is a good indicator of the3The boldface numbers in parentheses refer to a list of references at the end ofthis standard.E1642 00 (2010)2overall condition of the assembly. It is impor
39、tant that all testsbe conducted at a constant temperature because of the effect ofthe emitted energy on the detector (see 5.4.3.4). It is recom-mended that records be kept of the interferogram signalstrength, single-beam energy response, and the ratio of twosuccessive single-beam curves (as appropri
40、ate to the instru-ment used). For more information on such tests, refer toPractice E1421. These tests will also reveal when the MCTdetector is performing poorly due to loss of the Dewar vacuumand consequent buildup of ice on the detector face. MCTdetectors, as discussed in this text later, are commo
41、nly used forthese experiments as they provide greater detectivity and fasterdata acquisition.5.4.3.6 Care must be taken to stabilize or, preferably,remove interfering spectral features resulting from atmosphericabsorptions in the optical beam path of the spectrometer andthe GC/IR interface. Best res
42、ults will be obtained by purgingthe complete optical path with dry nitrogen gas. Alternatively,dry air can be used for the purge gas which will lead tointerferences in the regions of carbon dioxide absorption (2500to 2200 cm1and 720 to 620 cm1). Commercially availableair scrubbers that remove water
43、vapor and carbon dioxide alsoprovide adequate purging of the spectrometer and GC inter-face. In some instruments, the beam path is sealed in thepresence of a desiccant, but invariably interferences from bothcarbon dioxide and water vapor (1900 to 1400 cm1) will befound. If the purge is supplied to t
44、he interface when preparingto carry out a GC/IR experiment, the atmosphere must beallowed to stabilize before data collection commences. Atmo-spheric stability inside the instrument can be judged byrecording the single-beam energy response and the ratio of twosuccessive single-beam spectra.5.5 Direc
45、t Deposition GC/IRThe direct deposition GC/IRtechnique can follow either of two methods, that of matrixisolation (8) or continuous subambient temperature analytetrapping (9). In both of these methods, the gas chromatographiceffluent is passed through a heated transfer line and is depositedonto a col
46、d substrate.These methods permit detection as low assubnanogram amounts of material. The subambient tempera-ture of the substrate necessitates the use of an evacuatedchamber to avoid condensation of atmospheric gases. Byfreezing the eluite onto the cold substrate, the components ofthe sample are eff
47、ectively stored there. It is possible, therefore,to analyze the sample after the GC/IR experiment has finished,as well as perform real-time analyses.5.5.1 In the matrix isolation method, a small amount ofargon is added to the helium carrier gas. The column effluent isdirected onto a substrate mainta
48、ined at a temperature of about13K. Argon is condensed to form a solid matrix while thehelium carrier gas is pumped away. It is important that anycomponent eluting from the chromatograph is entrained in thisargon matrix at a concentration (0.2 %) sufficiently low suchthat each analyte molecule is sur
49、rounded by argon atoms and isisolated from other analyte molecules. An instrument has beendevised in which the beam from the FT-IR spectrometer passesthrough the track of argon, is reflected from the gold surface, istransmitted a second time through the argon, and is finallyfocused onto the detector (8). Additionally, other matrixisolation interface devices are available from vendors.5.5.2 In the case of the continuous subambient temperaturetrapping method, the sample is deposited directly onto aninfrared transmissive plate maintained at the temperaturessufficient to c
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