Page 164 - Tandem Techniques

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Additionally, a significant amount of spectral data on the double bond configuration of mono-
unsaturated compounds have been reported by Attygalle et al. [5], which would also be useful for
substance identification in similar types of sample. This procedure obviously greatly simplifies the
quantitative estimation of the individual aromatic hydrocarbons present in a hydrocarbon mixture. It has
been used in a very similar manner to determine the aromatics present in exhaust fumes. Samples of car
exhaust were trapped onto a suitable absorbent, and then thermally desorbed onto a capillary column.
The separation was monitored by IR absorption using the Perkin Elmer light pipe interface. The results
obtained are shown in figure 4.13. The upper chromatogram depicts all the hydrocarbons as they are
eluted, the lower chromatogram was obtained by plotting the absorption at the characteristic
wavelength where aromatics exhibit maximum absorption. It is seen that in the lower chromatogram,
the aromatic hydrocarbons are clearly and unambiguously depicted and the other hydrocarbons appear
absent.

The Cryostatic Interface

The cryostatic interface was introduced as an alternative to the light pipe which, due to the catalytic
nature of the gold surface, can cause some decomposition or molecular rearrangement with certain
samples. In contrast, the total system in the cryostatic interface is sensibly inert to the sample, as it is
contained in a solid argon matrix. A diagram of the Cryolect manufactured by Mattson Instruments
TM
Inc. is shown in Figure 4.14. Unfortunately, this instrument is no longer commercially available
although a large number are in use at this time. The device is extremely sensitive and provide excellent
spectra, for this reason, although not commercially available, the instrument will be discussed in some
detail. The carrier gas is helium and contains a small amount of argon (ca 0.5 %), although nitrogen can
also be used. On leaving the column, the carrier gas is allowed to impinge onto a rotating gold plated
drum, situated in a evacuated box, thermostatted at about 12°K by a liquid helium thermostat. The
drum, as well as rotating, also moves very slowly in an axial direction, so the samples are deposited as a
thin helical deposit on the outer walls of the drum. The frozen sample on the drum surface is trapped in
a