Page 24 - Tandem Techniques

Page 6

Today, unless techniques are used that are specific to the compound of interest, any successful analysis
entails three distinctly different procedures. In the first instance the sample must be separated into its
constituents so that the pertinent substances are isolated from the main bulk of the sample. This will
probably involve the use of an appropriate separation technique such as chromatography. Secondly, the
components that have been isolated will need to be identified, involving techniques such as
spectroscopy or X-ray crystallography. Finally the amount of each component isolated must be
quantitatively determined, ideally by the use of primary measurements such as those provided by
gravimetric or volumetric techniques. A more practical alternative for quantitative assessment is to
employ secondary methods of measurement, using devices that have been calibrated by primary
procedures. An example of the latter is the chromatographic detector that has been calibrated for
response and linearity against known concentrations of standard substances.

The three basic requirements, separation, identification and quantitative measurement, coupled with the
availability of reliable and appropriate separating and identifying instrumentation evoked the
combination of two types of analytical systems. The first efficient tandem system to be developed was
the combination of the GC with the mass spectrometer (MS) and the first effective GC/MS instrument
was assembled in the very early 1960s. This evolution was natural, as both the gas chromatograph and
the mass spectrometer had reached a reasonably advanced stage of development and, although not
particularly straightforward, interfacing the chromatograph with the mass spectrometer was one of the
easier tandem systems to fabricate. GC can only separate volatile substances and so it was the needs of
the analysts of the petroleum, essential oil and solvent industries that were the first to be satisfied by a
separation/identification combination of instruments. It should be remembered that the gas
chromatography detector gives a linear response with concentration and thus the GC/MS combination
satisfies all the demands of the analyst. It follows that for the satisfactory use of tandem instruments,
the analyst requires an armory of separation and identification techniques from which to choose. The
choice of available