Page 274 - Handbook of Instrumental Techniques for Analytical Chemistry

264 Handbook of Instrumental Techniques for Analytical Chemistry

tion was not widely used until the fast-scanning, sensitive FTIR spectrometers became available com-
mercially.
The most commonly used GC/FTIR interface is a light pipe flow cell. The light pipe is typically a
piece of glass tubing 10 to 20 cm long, approximately 1 mm inside diameter, gold coated on the inside,
with IR-transmitting windows on each end. This design provides a long path length and low dead volume
(90 to 300 µL), resulting in high IR absorbance with minimal peak broadening. The light pipe is connected
to the effluent port of the gas chromatograph by a heated transfer line. The gas flow assembly can be heat-
ed up to 350 °C to prevent sample components from condensing onto the light pipe and transfer line. Fig-
ure 15.12 illustrates the optical design of a GC/FTIR interface.
Eluents from a capillary gas chromatograph flow through the transfer line into the light pipe, where
the IR spectra are acquired in real time with a rate up to 20 spectra per second. The light-pipe GC/FTIR
offers nanogram-level sensitivity. Typically, a usable spectrum can be obtained from 5 to 20 ng of com-
ponent compound. The flow emerging from the chromatograph is often split between the light pipe and
a conventional GC detector (flame ionization, thermal conductivity, or mass spectrometer). This per-
mits the simultaneous generation of a normal chromatogram and the corresponding IR spectra for each
chromatographic peak. Alternatively, the total flow after the light pipe can be routed into a conventional
detector to provide in-line detection by a flame ionization or mass spectrometer detector. In fact, the
combination of a gas chromatograph with an FTIR and mass spectrometer (GC/FTIR/MS) is available
commercially.
Although common GC/FTIR spectroscopy is not as sensitive as gas chromatography/mass spec-
trometry (GC/MS), GC/FTIR offers a major advantage over GC/MS: the ability to identify structural
isomers. In addition, the sensitivity of GC/FTIR can be further improved by matrix isolation or direct
deposition techniques.
Gas chromatography/matrix isolation/Fourier transform infrared (GC/MI/FTIR) spectroscopy pro-
vides subnanogram sensitivity, but is a very expensive technique. The helium carrier gas of a gas chro-
matograph is mixed with a small amount of argon. While argon is condensed in a track of 300 µm width

Figure 15.12 Schematic diagram of a GC/FTIR interface.
(Reprinted by permission of Nicolet Instrument Corporation.)