Page 275 - Handbook of Instrumental Techniques for Analytical Chemistry

Infrared Spectroscopy 265

on a rotating circular gold-coated metal disk cooled at 12 °K, the helium gas is evacuated by pumping.
The components separated by the chromatograph are dissolved and trapped in the argon matrix. After
the GC run is completed, the argon track is rotated into the IR beam and the reflection–absorption IR
spectra are obtained for each component on the cooled surface. Cryogenic temperatures are maintained
while the spectra are acquired.
In GC/MI/FTIR, the components are isolated in small areas. Because IR measurements are not
made in real time, these components can be held in the IR beam for longer periods, allowing improved
signal-to-noise ratios through averaging of multiple scans. Thus, GC/MI/FTIR offers significant sensi-
tivity improvement over light-pipe GC/FTIR.
Gas chromatography/direct deposition/Fourier transform infrared (GC/DD/FTIR) spectroscopy is
another sensitive technique that permits a usable spectrum to be obtained with 100 pg of component
compound. The separated components are directly deposited in a track of 100 µm width on a liquid-
nitrogen–cooled, IR-transmitting disk such as zinc selenide. Transmission IR spectra can be taken in
real time. Alternatively, the isolated components can be repositioned in the IR beam after the run is
completed to gain the multiscan signal-averaging advantages.
High-performance liquid chromatography/Fourier transform infrared (HPLC/FTIR) spectroscopy
uses the same approach as the GC/DD/FTIR to eliminate the mobile phase and gain satisfactory sensi-
tivity.
Conventional flow cells for HPLC chromatograph generally do not provide adequate sensitivity,
due to the IR absorption of all HPLC mobile phases. Instead, an HPLC/FTIR interface using the direct
deposition technique has been designed and delivers subnanogram sensitivity (1). The interface con-
sists of two concentric fused silica tubes of different internal diameters. While the eluate stream from
an HPLC column flows through the inner tube, a sheath of heated gas (helium or air) passes through
the outer tube. The nebulized spray is directed to a rotating sample collection disk onto which the com-
ponent compounds are deposited. During the process the mobile phase is evaporated. The resulting dep-
ositions of sample components can then be positioned in the IR beam and their IR spectra collected.
Other techniques involving IR spectrometers, including supercritical fluid chromatography/Fouri-
er transform infrared (SFC/FTIR) spectroscopy, thermogravimetry/Fourier transform infrared (TGA/
FTIR) spectroscopy and, gas chromatography/Fourier transform infrared/mass spectrometry (GC/
FTIR/MS) have also become available commercially. They generally use more cost-effective flow-
through interfaces.

Analytical Information

Qualitative

The combination of the fundamental vibrations or rotations of various functional groups and the subtle
interactions of these functional groups with other atoms of the molecule results in the unique, generally
complex IR spectrum for each individual compound. IR spectroscopy is mainly used in two ways:
structural elucidation and compound identification.

Structural Elucidation
Because of complex interactions of atoms within the molecule, IR absorption of the functional groups
may vary over a wide range. However, it has been found that many functional groups give characteristic