produce ring compounds, alkane isomers and shorter chain alkanes and alkenes. The octane rating of
the product is dependent upon the proportion of aromatic compounds synthesized while the low boiling
cracked products are wasted. To establish optimum reaction conditions for a catalyst it is necessary to
make an extensive series of studies in which the composition of the product is monitored as the
temperature, pressure, flow rate, etc., are varied. Rapid, on line analysis of a complex mixture of
hydrocarbons is needed. Gas chromatography offers the best solution to this problem, as it provides for
ready separation of the volatile constituents, their identification from retention times and their
quantitative analysis from peak areas. However, with complex mixtures it becomes necessary to obtain
independent confirmation of the peak identities and to establish that the peaks obtained correspond to
single components. Integrated gas chromatographic and mass spectrometric analysis can provide this
additional information.
In a series of experiments with silica supported platinum catalysts the feedstock (n-hexane) was passed
continuously over the catalyst for 30 minutes, and the products collected in a liquid nitrogen trap. Ten
samples of the products were injected into a gas chromatograph interfaced with a mass spectrometer.
Good separations were obtained on an alumina, PLOT column, and a typical chromatogram is shown in
Figure 12.4. A synthetic mixture of the expected components was also analysed under identical
conditions. Peak identities were tentatively assigned on the basis of retention times in this standard
chromatogram and confirmed by the mass spectra. Figures 12.5(a) and 12.5(b) show typical spectra for
peaks 11 and 12 which were identified as methyl cylopentane and benzene. Subsequent assessments of
the proportions of the products were made by the comparison of integrated peak areas.
This example illustrates the power of a modern analytical procedure to reduce a complicated problem to
a set of relatively straightforward measurements. Furthermore it highlights the effectiveness of
interfacing two techniques to produce a method which has a resolution unmatched by either
individually.
12.3—
The Automation of Analytical Procedures
One area of analytical chemistry which is currently developing rapidly is the automation of methods.
Some degree of automation has been used for a number of years in instruments such as automatic
burettes coupled to absorptiometric or electrometric end-point detectors, and in data output devices
which provide continuous pen recording or signal integration facilities. The major features of recent
developments include the scope for instrumental improvements provided by solid-state electronic
circuits and the increasing application of digital computers (Chapter 13).
A fully automated analytical method has a number of significant advantages over the manual version. It
can be used to carry out a very large number of similar analyses or to provide continuous monitoring of
an