Page 174 - Tandem Techniques

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Thermal degradation products could also be examined using the light pipe interface which, although not
nearly as sensitive or as fast, is much less expensive. Furthermore it can yield spectral data that is at
least as accurate and precise as that obtained from the cryogenic interface. About 5 mg of a
polyethylene film, 50 µm thick, dispersed on the surface of a platinum coil pyrolysis probe, was placed
at the inlet of an open tubular column and was heated to a temperature of about 1000°C for a period of
10 seconds. The column was 25 m long, 0.53 mm I.D., and carried a 5 µm film of methyl phenyl
silicone stationary phase It was operated at a flow rate of 4 ml/min, split 2:1 in a split/splitless injector.
The separation monitored by a flame ionization detector is shown in Figure 4.22.

The column oven was initially held at 45°C for 3 min., and then programmed up to 280°C at 4°/min.
10% of the eluent passed to the detector and the remainder passed through the light pipe. The
chromatogram obtained by reconstruction of the IR absorption spectra between 16 and 30 minute
together with some of the associated spectra are shown in Figure 4.23. It is seen that a more than
adequate sensitivity is achieved and well defined spectra are obtained from the four closely eluted
peaks. The quality of the spectra are sufficient for solute identification, providing reference spectra
were available. The spectra would also aid in the structure elucidation of a completely unknown
substance providing complementary mass spectra were also available.

Thermal gravimetric analysis, in conjunction with a GC/FTIR tandem system utilizing a light pipe
interface, has also been used to determine the pyrolysis and oxidation products of an
alkylbenzimidazole. The system was operated both as a tandem TGA/FTIR combination and, by
employing a suitable trapping procedure, as a triplet system TGA/GC/FTIR. The pyrolysis and
oxidation process was carried out in a Perkin Elmer thermogravimetric analyzer and the FTIR spectra
obtained from a PE 2000 FTIR instrument. Samples from the TGA instrument were either taken
directly by a light pipe interface, to the FTIR spectrometer or, alternatively, the products were collected
in a Tenax trap. The products were regenerated by rapidly heating the trap to 275°C in the capillary