1、Designation: E 1642 00 (Reapproved 2005)Standard Practice forGeneral Techniques of Gas Chromatography Infrared (GC/IR) Analysis1This standard is issued under the fixed designation E 1642; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revis
2、ion, 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 techniques that are of general use inanalyzing qualitatively multicomponent sam
3、ples 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
4、, andeluite 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 sta
5、ndard 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 131 Terminology Relating to Molecular SpectroscopyE 168 Practices for General Techniques of Infrared Quanti-tative Analysis
6、E 260 Practice for Packed Column Gas ChromatographyE 334 Practice for General Techniques of Infrared Mi-croanalysisE 355 Practice for Gas Chromatography Terms and Rela-tionshipsE 932 Practice for Describing and Measuring Performanceof Dispersive Infrared SpectrometersE 1252 Practice for General Tech
7、niques for Obtaining In-frared Spectra for Qualitative AnalysisE 1421 Practice for Describing and Measuring Performanceof Fourier Transform Mid-Infrared (FT-MIR) Spectrom-eters: Level Zero and Level One TestsE 1510 Practice for Installing Fused Silica Open TubularCapillary Columns in Gas Chromatogra
8、phs3. Terminology3.1 DefinitionsFor definitions of terms and symbols, referto Terminology E 131 and Practice E 355.4. 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
9、). This practiceassumes 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 ha
10、ve beenused to 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 I
11、R detector as 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 trap
12、ping tech-niques 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 elutio
13、n. If adestructive 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,a powdered solid, or a cold trap for subsequent analysis (seePractice E 1252) (1).3Vapor phase samples can b
14、e trapped in a1This practice is under the jurisdiction of ASTM Committee E13 on MolecularSpectroscopy and is the direct responsibility of Subcommittee E13.03 on InfraredSpectroscopy.Current edition approved Sept. 1, 2005. Published September 2005. Originallyapproved in 1994. Last previous edition ap
15、proved in 2000 as E 1642 00.2For 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.3The boldface numbers in parenthe
16、ses refer to a list of references at the end ofthe text.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.heated low-volume gas cell at the exit of the GC, analyzed,then flushed with the continuing GC effluent until the nextaliquot of
17、interest is in the gas cell when the 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 in
18、tegrity can be compromised by slow 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 t
19、he flowcell or “light-pipe”technique. 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 tran
20、sfer line. Individualcomponents are 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 comm
21、ercially available.5.4.1 The rapidity 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
22、.2 The transfer line from the GC to 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
23、degradation of the analytes. Typical 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
24、 of analytes. The light-pipe dimen-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
25、 the transfer line. The maximumtemperature 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 ch
26、romatographic separation requiresthat 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
27、 the light-pipe should be reduced 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 ca
28、n react with, and sometimes destroy,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 t
29、o that of a GC detector in the absence 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 br
30、omide windows, but thismaterial is 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
31、 containing water shouldconsider using 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
32、 buildup of organic char on both the 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 i
33、ncreasing amount of infraredradiation. 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
34、observed interferometricACsignal is 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
35、 versus light-pipe temperaturefor 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 col
36、d aperture between thelight-pipe and 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 refoc
37、used at the plane of the coldshield. 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 th
38、ermal energyemitted from the vicinity 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 theoverall condition of the assembly. It is important that all testsbe conducted at a constant temperature because of the eff
39、ect ofthe emitted energy on the detector (see 5.4.3.4). It is recom-mended that records be kept of the interferogram signalE 1642 00 (2005)2strength, single-beam energy response, and the ratio of twosuccessive single-beam curves (as appropriate to the instru-ment used). For more information on such
40、tests, refer toPractice E 1421. 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 commonly used forthese experiments as they provide greater dete
41、ctivity 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 results will be obtained by purgingthe complete optical path
42、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 vapor and carbon dioxide alsoprovide adequate purging of t
43、he 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 the interface when preparingto carry out a GC/IR experiment
44、, 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 Direct Deposition GC/IRThe direct deposition GC/IRtechnique can
45、 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 cold substrate.These methods permit detection as low assubnan
46、ogram 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 effectively stored there. It is possible, therefore,to analyz
47、e 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 maintained at a temperature of about13K. Argon is condensed to f
48、orm 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 surrounded by argon atoms and isisolated from other analyte m
49、olecules. 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 condense analytes from the eluent. The tempera-ture of this
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