from a gas chromatograph at atmospheric pressure into the vacuum inlet of a
mass spectrometer.
There are two alternative strategies for analyzing samples by multiple tech-
niques.● Several representative samples are taken from the specimen and each is
analyzed using optimum conditions for the techniques selected. This is
referred to as a multidisciplinary approach. For example, a pharmaceutical
sample may need to be analyzed by UV, IR, NMR, MS and other spectro-
metric techniques.
● A single representative sample is taken and analyzed by an instrument
designed to perform more than one analytical technique at the same time.
For example, a sample that is chemically or thermally unstable could be
studied by gas chromatography linked to an infrared spectrometer. If the
instruments for GC and IR spectrometry are combined so that the analyses
are done essentially at the same time, this is called a simultaneous approach
and is often written with a hyphen, so that they may be referred to as
hyphenated techniques, for example GC-MSand GC-IR.Problem solving The procedure for the analysis of each sample must be optimized at all stages to
take account of the information required, as detailed in Topic A1.
The sample handling and the amount of sample available may determine
which techniques are possible. The quantitative accuracy and the components
sought govern the choice of method. Whether the exact structure of an
unknown component is required, or merely an indication of its chemical class,
may control the time required for the analysis and preparation of the final
analytical report.
The time factor is of vital importance in a busy analytical laboratory schedule.
Any method that saves time, or that can be operated automatically under
computer control on a sequence of different samples, as described in Section H,
is of value. Similarly, if this can be linked to a computer search facility to aid
structure determination, then this offers a further time saving.
Advantages The advantages are summarized below.
● The combined approach often provides more information than could be
obtained by using the individual techniques in isolation.
● Multidisciplinary analysis is a necessary tool in the work of the analytical
chemist, otherwise misleading or incorrect results may be obtained.
● Hyphenated techniques provide a time saving because two or more analyses
can be run at the same time, since the instrumentation combines the features
of both, and sample preparation time is reduced.
● Hyphenated techniques provide analytical information on exactly the same
sample, provided that the sample is truly representative (Topic A4) of the
material to be analyzed. If comparisons between samples are required, then it
is essential that cross-contamination does not occur.Another advantage of using a combined approach is that of obtaining time-
resolved (or temperature-resolved) spectra or chromatograms. If changes
occur, due to reaction, evaporation or phase separation, these changes may
be studied by recording the analytical data at specific time (or temperature)
intervals.284 Section F – Combined techniques