ASTM E1642-00
(Practice)Standard Practice for General Techniques of Gas Chromatography Infrared (GC/IR) Analysis
Standard Practice for General Techniques of Gas Chromatography Infrared (GC/IR) Analysis
SCOPE
1.1 This practice covers techniques that are of general use in analyzing multicomponent samples, by using a combination of gas chromatography (GC) and infrared (IR) spectrophotometric techniques. The mixture is separated into its individual components by GC, and then these individual components are analyzed by IR spectroscopy.
1.2 The values stated in SI units are to be regarded as the standard.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:E1642–00
Standard Practice for
General Techniques of Gas Chromatography Infrared (GC/
IR) Analysis
This standard is issued under the fixed designation E1642; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 3. Terminology
1.1 Thispracticecoverstechniquesthatareofgeneralusein 3.1 Definitions—Fordefinitionsoftermsandsymbols,refer
analyzing qualitatively multicomponent samples by using a to Terminology E131 and Practice E 355.
combination of gas chromatography (GC) and infrared (IR)
4. Significance and Use
spectrophotometric techniques. The mixture is separated into
its individual components by GC and then these individual 4.1 This practice provides general guidelines for the proper
practice of gas chromatography coupled with infrared spectro-
components are analyzed by IR spectroscopy. Types of GC-IR
techniques discussed include eluent trapping, flowcell, and photometric detection and analysis (GC/IR). This practice
assumes that the chromatography involved in the practice is
eluite deposition.
1.2 The values stated in SI units are to be regarded as the adequate to separate the compounds of interest. It is not the
intentionofthispracticetoinstructtheuserhowtoperformgas
standard.
1.3 This standard does not purport to address all of the chromatography properly.
safety concerns, if any, associated with its use. It is the
5. General GC/IR Techniques
responsibility of the user of this standard to establish appro-
5.1 Three different types of GC/IR technique have been
priate safety and health practices and determine the applica-
used to analyze samples. These consist of analyte trapping,
bility of regulatory limitations prior to use.
flowcell, or lightpipe, and direct eluite deposition and are
2. Referenced Documents
presented in the order that they were first used.
2.1 ASTM Standards: 5.2 The GC eluent must not be routed to a destructive GC
detector (such as a flame ionization detector) prior to reaching
E131 Terminology Relating to Molecular Spectroscopy
E168 Practices for General Techniques of Infrared Quanti- the IR detector as this will destroy or alter the individual
tative Analysis components. It is acceptable to split the eluent so that part of
the stream is directed to such a detector or to pass the stream
E260 Practice for Packed Column Gas Chromatography
E334 Practice for General Techniques of Infrared Mi- back to the detector after infrared analysis if such techniques
are feasible.
croanalysis
E355 Practice for Gas Chromatography Terms and Rela- 5.3 Eluent Trapping Techniques—Analyte trapping tech-
niques are the least elaborate means for obtaining GC/IR data.
tionships
E932 Practice for Describing and Measuring Performance In these techniques, the sample eluting from the chromato-
graph is collected in discrete aliquots to be analyzed. In
of Dispersive Infrared Spectrometers
E1252 Practice for General Techniques for Qualitative utilizing such techniques, it is essential that a GC detector be
employed to allow definition of component elution. If a
Infrared Analysis
E1421 PracticeforDescribingandMeasuringPerformance destructive detector is employed, then post-column splitting to
that detector is required. GC fractions can be trapped in the
of Fourier Transform Infrared (FT-IR) Spectrometers:
Level Zero and Level One condensed phase by passing the GC effluent through a solvent,
a powdered solid, or a cold trap for subsequent analysis (see
E1510 Practice for Installing Fused Silica Open Tubular
3 4
Capillary Columns in Gas Chromatographs PracticeE1252) (1). Vaporphasesamplescanbetrappedina
heated low-volume gas cell at the exit of the GC, analyzed,
then flushed with the continuing GC effluent until the next
This practice is under the jurisdiction ofASTM Committee E13 on Molecular
aliquot of interest is in the gas cell when the flow is stopped
Spectroscopy and is the direct responsibility of Subcommittee E13.03 on Infrared
Spectroscopy.
Current edition approved March 10, 2000. Published June 2000. Originally
published as E1642–94. Last previous edition E1642–94.
2 4
Annual Book of ASTM Standards, Vol 03.06. The boldface numbers in parentheses refer to a list of references at the end of
Annual Book of ASTM Standards, Vol 14.02. the text.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1642
again for analysis (2). Since the analyte of interest is static presenceofsuchacompoundinthemixture.Thissituationcan
when employing an analyte trapping technique, the spectrum be identified by comparing the response of the GC detector
can be recorded using a long co-addition time to improve the after the flowcell to that of a GC detector in the absence of a
signal-to-noise (SNR) ratio. However, in analyte trapping, flowcell, or by comparing the GC/IR detector output to the
sample integrity can be compromised by slow decomposition. results of a suitable alternate analytical technique.
A spectrum should be obtained with a short co-addition time 5.4.3.2 The ends of the light-pipe are sealed with infrared
first, to create a reference spectrum to ensure the integrity of transmissive windows. The optimum optical transmission is
the spectrum obtained after long co-addition. obtained by using potassium bromide windows, but this
material is very susceptible to damage by water vapor. As the
5.4 Flowcell Detection of Vapor Phase Components—The
light-pipe is used, small amounts of water vapor will etch the
most common GC/IR technique is the flowcell or “light-pipe”
window surfaces, and the optical throughput of the windows
technique. The GC eluent stream is monitored continuously in
will drop. Eventually these windows will have to be changed.
the time frame of the chromatography (real-time) by the IR
Users who expect to analyze mixtures containing water should
spectrometer with the use of a specially designed gas cell
consider using windows made of a water-resistant material
called a light-pipe. In this design, the light-pipe is coupled
suchaszincselenide,butthiswillresultinanoticeabledropin
directly to the GC by a heated transfer line. Individual
opticaltransmissionduetoopticalreflectionpropertiesofsuch
components are analyzed in the vapor phase as they emerge
materials.
from the transfer line. This technique typically yields low
5.4.3.3 Usage of the light-pipe at high temperatures may
nanogram detection limits for most analytes (3-5). Instruments
result in the gradual buildup of organic char on both the cell
that include the IR spectrometer, the gas chromatograph,
walls and end windows. As this occurs the optical throughput
heatedtransfer-line,andlight-pipearecommerciallyavailable.
will drop correspondingly. Eventually the light-pipe assembly
5.4.1 The rapidity with which spectra of the individual
will have to be reconditioned (see 5.4.3.5).
components must be recorded requires a Fourier-transform
5.4.3.4 As the temperature of the light-pipe is raised above
infrared (FT-IR) spectrometer. Such instruments include a
ambient, the light-pipe emits an increasing amount of infrared
computer that is capable of storing the large amount of
radiation.Thisradiationisnotmodulatedbytheinterferometer
spectroscopic data generated for subsequent evaluation.
and is picked up by the detector as DC signal. The DC
5.4.2 ThetransferlinefromtheGCtothelight-pipemustbe
component becomes large at the normal working temperatures
made of inert, non-porous material (normally fused silica
(above 200°C), and lowers the dynamic range of the detector.
tubing) and be heated to prevent condensation. The tempera-
TheresultofthiseffectisthattheobservedinterferometricAC
ture of the transfer line is normally held constant during a
signal is reduced in size as the temperature increases and the
complete analysis at a level chosen to avoid both condensation
observed spectral noise level increases correspondingly. By
and degradation of the analytes. Typical working temperatures
raising the temperature from room temperature to 250°C, the
are about 100 to 300°C (normally 10°C higher than the
noise level typically doubles; it is recommended that the user
maximum temperature reached during the chromatography).
create a plot of signal intensity versus light-pipe temperature
5.4.3 The light-pipe is normally gold-coated on the interior
for reference purposes. As a consequence of this behavior, it
to give maximum optical throughput and at the same time
may be advantageous to record data using relatively low
minimize decomposition of analytes. The light-pipe dimen-
temperatures for both temperature and transfer line for those
sionsaretypicallyoptimizedsothatthevolumeaccommodates
GC experiments that only use a limited temperature ramp.
the corresponding eluent volume of a sharp chromatographic
Some instrument designs include a cold aperture between the
peak at the peak’s full width at half height (FWHH). The
light-pipeandthedetectortominimizetheamountofradiation
light-pipe is heated to a constant temperature at or slightly
reaching the detector (see Note 1) (6,7).
higher than the temperature of the transfer line.The maximum
NOTE 1—A cold aperture is a metal shield, maintained at room
temperature recommended by the manufacturer should not be
temperature, sited between the light-pipe and the detector. The infrared
exceeded. In general, sustained light-pipe temperatures above
beam diverging from the light-pipe is refocused at the plane of the cold
300°Cmaydegradethegoldcoatingandthelifeofthecoating
shield.The cold shield has a circular hole (aperture) of the same diameter
drops quickly with successively higher temperatures. It should
as the refocused beam. After passing through the aperture and moving
be pointed out that, if a chromatographic separation requires
away from this focal point, the beam is again focused onto the detector
that the GC temperature be raised above this level, it may be element. This small aperture shields the detector from thermal energy
emitted from the vicinity of the hot light-pipe.
necessary to temporarily raise both the temperature of the
light-pipe and transfer line to maximum temperature of the
5.4.3.5 The optical throughput of the light-pipe should be
chromatography to avoid condensation of the eluent. If this is
periodically monitored since this is a good indicator of the
thecase,thetemperatureofthelight-pipeshouldbereducedto
overall condition of the assembly. It is important that all tests
a safe level as soon as possible. It must be noted that repeated
beconductedataconstanttemperaturebecauseoftheeffectof
temperature changes to the light-pipe and transfer line will
the emitted energy on the detector (see 5.4.3.4). It is recom-
cause a more rapid aging of the seals and may cause leaks.
mended that records be kept of the interferogram signal
5.4.3.1 It should be noted that any metal surface inside the strength, single-beam energy response, and the ratio of two
light-pipe assembly can react with, and sometimes destroy, successive single-beam curves (as appropriate to the instru-
some specific materials (for example, amines) as they elute ment used). For more information on such tests, refer to
from the GC. Consequently, it is possible to fail to identify the Practice E1421. These tests will also reveal when the MCT
E1642
detectorisperformingpoorlyduetolossoftheDewarvacuum 5.5.3 Direct deposition techniques provide the advantage of
and consequent buildup of ice on the detector face. MCT greater sensitivity for real-time measurements. Additionally,
detectors,asdiscussedinthistextlater,arecommonlyusedfor extended co-addition of spectra post-run permits further im-
theseexperimentsastheyprovidegreaterdetectivityandfaster provement of the signal-to-noise ratio of spectral results.
However, slow sublimation of the analyte recrystallization of
data acquisition.
the sample or ice formation, or both, may occur with direct
5.4.3.6 Care must be taken to stabilize or, preferably,
depositiontechniques.Itisprudenttoobtainaspectrumwitha
removeinterferingspectralfeaturesresultingfromatmospheric
short co-addition time initially to create a reference spectrum.
absorptions in the optical beam path of the spectrometer and
This will ensure the integrity of the spectrum obtained after
the GC/IR interface. Best results will be obtained by purging
longer co-addition times.
the complete optical path with dry nitrogen gas.Alternatively,
dry air can be used for the purge gas which will lead to
6. Significant Parameters for GC/IR
interferencesintheregionsofcarbondioxideabsorption(2500
−1 −1
to 2200 cm and 720 to 620 cm ). Commercially available
6.1 Where the instrumentation used is commercially avail-
air scrubbers that remove water vapor and carbon dioxide also
able,themanufacturer’snameandmodelnumbersforthetotal
provide adequate purging of the spectrometer and GC inter-
GC/IR system, or the individual components, should be given.
face. In some instruments, the beam path is sealed in the
The various instrumental and software parameters which need
presence of a desiccant, but invariably interferences from both to be recorded are listed and discussed in this section. In
−1
carbon dioxide and water vapor (1900 to 1400 cm ) will be
addition, any modifications made to a commercial instrument
found. If the purge is supplied to the interface when preparing that affect the instrument’s performance must be clearly noted.
to carry out a GC/IR experiment, the atmosphere must be
6.2 Instrumental Parameters (IR):
allowed to stabilize before data collection commences. Atmo-
6.2.1 Detector—The detectors typically used for GC/IR are
spheric stability inside the instrument can be judged by
the mercury-cadmium-telluride (MCT) narrow band photode-
recordingthesingle-beamenergyresponseandtheratiooftwo
tectors of high sensitivity, that have a lower frequency limit of
−1
successive single-beam spectra.
approximately 700 cm . It is possible to measure spectra to
−1
5.5 Direct Deposition GC/IR—The direct deposition GC/IR
frequencies lower than 700 cm by using an MCT detector
technique can follow either of two methods, that of matrix that has a broader band spectral response, but the sensitivity of
isolation (8) or continuous subambient temperature analyte
such detectors is significantly lower.The MCTdetector should
trapping(9).Inbothofthesemethods,thegaschromatographic notbeoperatedinalightsaturatingconditionsoastomaintain
effluentispassedthroughaheatedtransferlineandisdeposited
linearity of sig
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