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all samples. One of the few pesticide samples analysed by comprehensive gas chro-
matography, using an early design system, indicated the promise that GC GC held,
but few more recent follow-up studies have been reported (40). The alternative capa-
bilities of the cryogenic modulation system, with methods such as targeted MDGC,
will hopefully be seen as another reason to investigate this new GC direction.

4.6.2 MASS SPECTROMETRY DETECTION

There is much excitement, but at the moment few results, on the use of mass spec-
trometry (MS) combined with GC GC analysis. Such a system represents a three-
dimensional analytical procedure, since it provides three orthogonal dimensions
with each contributing to the identification of components. The MS final stage
enables identification of the 2D separated peaks, and hence brings additional under-
standing to the unique structured chromatograms. The main criterion that decides if
MS can be used with fast GC peaks is the scanning speed of the spectrometer, i.e. the
number of scans per second. Since GC GC peaks may be as narrow as 100 ms or
less, and since reliable spectra or peak reproduction requires at least five scans
(preferably ten) per peak, then a scan rate of 50–100 scans/s are needed. Time-of-
flight (TOF) MS is the only practical technology that can deliver this speed, and
since there are few such instruments for GC available, and even fewer in those labo-
ratories with experience in GC GC, therefore little has been published on this
hyphenated technique. One approach that has been taken, however, for coupling
quadrupole MS with GC GC analysis, is to use a flow splitter before the modula-
tor, so that at least some indication of the possible components which constitute the
second-dimension result might be known (41). Another approach is to slow down the
second dimension so that maybe two or three spectra can be acquired, and then hope
that one suitable spectrum for library matching might be obtained (42). This study
employed a long second-dimension column (14 m in length) which gave peaks about
1 s wide. The total GC analysis was developed over 440 min. This is unlikely to be
very useful for routine analysis! More recently, the Centres for Disease Control have
used a GC–TOFMS system for GC GC, and the results of this work are eagerly
anticipated.
Even though it appears that the technology has not been adopted yet, it is
expected that TOF MS will be useful to validate the power of the GC GC separa-
tion experiment by proving the separate identities of the vast number of resolved
peaks and so show that the analyst who does not use GC GC is missing valuable
chemical compositional information on their samples. In addition, it is just as
significant to TOFMS that GC GC becomes a widespread separation tool, since
this will then provide a demand for the powerful capabilities of TOFMS for
identification. The GC community must wait for this to be demonstrated, and those
who are working in GC GC development are convinced that the wait will
be worth it!