ASTM E1642-00(2010)
(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
SIGNIFICANCE AND USE
This practice provides general guidelines for the proper practice of gas chromatography coupled with infrared spectrophotometric detection and analysis (GC/IR). This practice assumes that the chromatography involved in the practice is adequate to separate the compounds of interest. It is not the intention of this practice to instruct the user how to perform gas chromatography properly.
SCOPE
1.1 This practice covers techniques that are of general use in analyzing qualitatively 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. Types of GC-IR techniques discussed include eluent trapping, flowcell, and eluite deposition.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this 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 (Reapproved2010)
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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope E1421Practice for Describing and Measuring Performance
of Fourier Transform Mid-Infrared (FT-MIR) Spectrom-
1.1 Thispracticecoverstechniquesthatareofgeneralusein
eters: Level Zero and Level One Tests
analyzing qualitatively multicomponent samples by using a
E1510Practice for Installing Fused Silica Open Tubular
combination of gas chromatography (GC) and infrared (IR)
Capillary Columns in Gas Chromatographs
spectrophotometric techniques. The mixture is separated into
its individual components by GC and then these individual
3. Terminology
components are analyzed by IR spectroscopy. Types of GC-IR
techniques discussed include eluent trapping, flowcell, and
3.1 Definitions—Fordefinitionsoftermsandsymbols,refer
eluite deposition. to Terminology E131 and Practice E355.
1.2 The values stated in SI units are to be regarded as
4. Significance and Use
standard. No other units of measurement are included in this
standard.
4.1 This practice provides general guidelines for the proper
practice of gas chromatography coupled with infrared spectro-
1.3 This standard does not purport to address all of the
photometric detection and analysis (GC/IR). This practice
safety concerns, if any, associated with its use. It is the
assumes that the chromatography involved in the practice is
responsibility of the user of this standard to establish appro-
adequate to separate the compounds of interest. It is not the
priate safety and health practices and determine the applica-
intentionofthispracticetoinstructtheuserhowtoperformgas
bility of regulatory limitations prior to use.
chromatography properly.
2. Referenced Documents
5. General GC/IR Techniques
2.1 ASTM Standards:
E131Terminology Relating to Molecular Spectroscopy
5.1 Three different types of GC/IR technique have been
E168Practices for General Techniques of Infrared Quanti-
used to analyze samples. These consist of analyte trapping,
tative Analysis
flowcell, or lightpipe, and direct eluite deposition and are
E260Practice for Packed Column Gas Chromatography
presented in the order that they were first used.
E334Practice for General Techniques of Infrared Micro-
5.2 The GC eluent must not be routed to a destructive GC
analysis
detector (such as a flame ionization detector) prior to reaching
E355PracticeforGasChromatographyTermsandRelation-
the IR detector as this will destroy or alter the individual
ships
components. It is acceptable to split the eluent so that part of
E932PracticeforDescribingandMeasuringPerformanceof
the stream is directed to such a detector or to pass the stream
Dispersive Infrared Spectrometers
back to the detector after infrared analysis if such techniques
E1252Practice for General Techniques for Obtaining Infra-
are feasible.
red Spectra for Qualitative Analysis
5.3 Eluent Trapping Techniques—Analyte trapping tech-
niques are the least elaborate means for obtaining GC/IR data.
This practice is under the jurisdiction ofASTM Committee E13 on Molecular
In these techniques, the sample eluting from the chromato-
Spectroscopy and Separation Science and is the direct responsibility of Subcom-
graph is collected in discrete aliquots to be analyzed. In
mittee E13.03 on Infrared and Near Infrared Spectroscopy.
utilizing such techniques, it is essential that a GC detector be
Current edition approved March 1, 2010. Published April 2010. Originally
employed to allow definition of component elution. If a
approved in 1994. Last previous edition approved in 2005 as E1642–00(2005).
DOI: 10.1520/E1642-00R10.
destructive detector is employed, then post-column splitting to
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
that detector is required. GC fractions can be trapped in the
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
condensed phase by passing the GC effluent through a solvent,
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. a powdered solid, or a cold trap for subsequent analysis (see
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1642 − 00 (2010)
Practice E1252) (1). Vapor phase samples can be trapped in a a safe level as soon as possible. It must be noted that repeated
heated low-volume gas cell at the exit of the GC, analyzed, temperature changes to the light-pipe and transfer line will
then flushed with the continuing GC effluent until the next cause a more rapid aging of the seals and may cause leaks.
aliquot of interest is in the gas cell when the flow is stopped 5.4.3.1 It should be noted that any metal surface inside the
again for analysis (2). Since the analyte of interest is static
light-pipe assembly can react with, and sometimes destroy,
when employing an analyte trapping technique, the spectrum some specific materials (for example, amines) as they elute
can be recorded using a long co-addition time to improve the
from the GC. Consequently, it is possible to fail to identify the
signal-to-noise (SNR) ratio. However, in analyte trapping, presenceofsuchacompoundinthemixture.Thissituationcan
sample integrity can be compromised by slow decomposition.
be identified by comparing the response of the GC detector
A spectrum should be obtained with a short co-addition time after the flowcell to that of a GC detector in the absence of a
first, to create a reference spectrum to ensure the integrity of
flowcell, or by comparing the GC/IR detector output to the
the spectrum obtained after long co-addition. results of a suitable alternate analytical technique.
5.4.3.2 The ends of the light-pipe are sealed with infrared
5.4 Flowcell Detection of Vapor Phase Components—The
transmissive windows. The optimum optical transmission is
most common GC/IR technique is the flowcell or “light-pipe”
obtained by using potassium bromide windows, but this
technique. The GC eluent stream is monitored continuously in
material is very susceptible to damage by water vapor.As the
the time frame of the chromatography (real-time) by the IR
light-pipe is used, small amounts of water vapor will etch the
spectrometer with the use of a specially designed gas cell
window surfaces, and the optical throughput of the windows
called a light-pipe. In this design, the light-pipe is coupled
will drop. Eventually these windows will have to be changed.
directly to the GC by a heated transfer line. Individual
Users who expect to analyze mixtures containing water should
components are analyzed in the vapor phase as they emerge
consider using windows made of a water-resistant material
from the transfer line. This technique typically yields low
suchaszincselenide,butthiswillresultinanoticeabledropin
nanogram detection limits for most analytes (3-5). Instruments
opticaltransmissionduetoopticalreflectionpropertiesofsuch
that include the IR spectrometer, the gas chromatograph,
materials.
heatedtransfer-line,andlight-pipearecommerciallyavailable.
5.4.3.3 Usage of the light-pipe at high temperatures may
5.4.1 The rapidity with which spectra of the individual
result in the gradual buildup of organic char on both the cell
components must be recorded requires a Fourier-transform
walls and end windows. As this occurs the optical throughput
infrared (FT-IR) spectrometer. Such instruments include a
will drop correspondingly. Eventually the light-pipe assembly
computer that is capable of storing the large amount of
will have to be reconditioned (see 5.4.3.5).
spectroscopic data generated for subsequent evaluation.
5.4.3.4 As the temperature of the light-pipe is raised above
5.4.2 ThetransferlinefromtheGCtothelight-pipemustbe
ambient, the light-pipe emits an increasing amount of infrared
made of inert, non-porous material (normally fused silica
radiation.Thisradiationisnotmodulatedbytheinterferometer
tubing) and be heated to prevent condensation. The tempera-
and is picked up by the detector as DC signal. The DC
ture of the transfer line is normally held constant during a
component becomes large at the normal working temperatures
complete analysis at a level chosen to avoid both condensation
(above 200°C), and lowers the dynamic range of the detector.
and degradation of the analytes. Typical working temperatures
TheresultofthiseffectisthattheobservedinterferometricAC
are about 100 to 300°C (normally 10°C higher than the
signal is reduced in size as the temperature increases and the
maximum temperature reached during the chromatography).
observed spectral noise level increases correspondingly. By
5.4.3 The light-pipe is normally gold-coated on the interior
raising the temperature from room temperature to 250°C, the
to give maximum optical throughput and at the same time
noise level typically doubles; it is recommended that the user
minimize decomposition of analytes. The light-pipe dimen-
create a plot of signal intensity versus light-pipe temperature
sionsaretypicallyoptimizedsothatthevolumeaccommodates
for reference purposes. As a consequence of this behavior, it
the corresponding eluent volume of a sharp chromatographic
may be advantageous to record data using relatively low
peak at the peak’s full width at half height (FWHH). The
temperatures for both temperature and transfer line for those
light-pipe is heated to a constant temperature at or slightly
GC experiments that only use a limited temperature ramp.
higher than the temperature of the transfer line.The maximum
Some instrument designs include a cold aperture between the
temperature recommended by the manufacturer should not be
light-pipeandthedetectortominimizetheamountofradiation
exceeded. In general, sustained light-pipe temperatures above
reaching the detector (see Note 1) (6,7).
300°Cmaydegradethegoldcoatingandthelifeofthecoating
drops quickly with successively higher temperatures. It should
NOTE 1—A cold aperture is a metal shield, maintained at room
be pointed out that, if a chromatographic separation requires
temperature, sited between the light-pipe and the detector. The infrared
that the GC temperature be raised above this level, it may be
beam diverging from the light-pipe is refocused at the plane of the cold
shield.The cold shield has a circular hole (aperture) of the same diameter
necessary to temporarily raise both the temperature of the
as the refocused beam. After passing through the aperture and moving
light-pipe and transfer line to maximum temperature of the
away from this focal point, the beam is again focused onto the detector
chromatography to avoid condensation of the eluent. If this is
element. This small aperture shields the detector from thermal energy
thecase,thetemperatureofthelight-pipeshouldbereducedto
emitted from the vicinity of the hot light-pipe.
5.4.3.5 The optical throughput of the light-pipe should be
periodically monitored since this is a good indicator of the
The boldface numbers in parentheses refer to a list of references at the end of
this standard. overall condition of the assembly. It is important that all tests
E1642 − 00 (2010)
beconductedataconstanttemperaturebecauseoftheeffectof 5.5.2 In the case of the continuous subambient temperature
the emitted energy on the detector (see 5.4.3.4). It is recom- trapping method, the sample is deposited directly onto an
mended that records be kept of the interferogram signal infrared transmissive plate maintained at the temperatures
strength, single-beam energy response, and the ratio of two sufficient to condense analytes from the eluent. The tempera-
successive single-beam curves (as appropriate to the instru- ture of this substrate is maintained by Peltier cooling or with
ment used). For more information on such tests, refer to liquid nitrogen.The transmissions mode of infrared analysis is
Practice E1421. These tests will also reveal when the MCT used to obtain the spectroscopic data.
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.
data acquisition. However, slow sublimation of the analyte recrystallization of
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
air scrubbers that remove water vapor and carbon dioxide also
available, the manufacturer’s name and model numbers for the
provide adequate purging of the spectrometer and GC inter-
total GC/IR system, or the individual components, should be
face. In some instruments, the beam path is sealed in the
given. The various instrumental and software parameters
presence of a desiccant, but invariably interferences from both
which need to be recorded are listed and discussed in this
−1
carbon dioxide and water vapor (1900 to 1400 cm ) will be
section. In addition, any modifications made to a commercial
found. If the purge is supplied to the interface when preparing
instrument that affect the instrument’s performance must be
to carry out a GC/IR experiment, the atmosphere must be
clearly noted.
allowed to stabilize before data collection commences. Atmo-
6.2 Instrumental Parameters (IR):
spheric stability inside the instrument can be judged by
6.2.1 Detector—The detectors typically used for GC/IR are
recordingthesingle-beamenergyresponsea
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