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Multidimensional High Resolution Gas Chromatography 57

(a) Columns with high-temperature phases can be used in combination with those
of limited stability. This is most relevant when a coupling is made between
highly stable cross-linked methyl polysiloxane and high-polarity (but low ther-
mal stability) wax or porous layer open tubular (PLOT)-type columns.
(b) The ability to maintain the ovens at two independent isothermal temperatures
allows accurate retention index behaviour to be established.
(c) Optimization of stationary phase selectivity by temperature adjustment of
coupled columns.
(d) Analyte refocusing on transfer from the primary to the secondary column.

3.3 PRACTICAL EXAMPLES OF
TWO-DIMENSIONAL CHROMATOGRAPHY

It is through observing examples of actual applications that the best understanding of
GC–GC separation principles can be achieved. Over the past 30 years, there have
been essentially three main areas where two-dimensional gas chromatography has
been applied:
• petroleum, fuels, feedstocks and combustion analysis;
• flavours, fragrance and food;
• environmental contaminants.

Multidimensional techniques are used extensively in all of these areas, with
GC–GC being only one of the many commonly used hyphenated methods. Reviews
of each of these application areas is discussed in greater detail in Chapters 10, 13 and
14. The remaining sections of this present chapter, however, will use some selected
GC–GC applications to demonstrate how such techniques in particular have been
applied in practice.
What is common to all of these areas is that the relevant number of published
GC–GC papers is very small when compared to those concerning single-column
and GC–MS methods. While approximately 1000 papers per year are currently
published on single-column GC methods and, in recent years, nearly 750 per year
on GC–MS techniques, only around 50 per annum have been produced on two-
dimensional GC. Of course, this may not be a true reflection of the extent to which
two-dimensional GC is utilized, but it is certainly the case that research interest in its
application is very much secondary to that of mass spectrometric couplings. A num-
ber of the subject areas where two-dimensional methods have been applied do high-
light the limitations that exist in single-column and MS-separation analysis.
A large amount of fuel and environmentally based analysis is focused on the
determination of aliphatic and aromatic content. These types of species are often
notoriously difficult to deconvolute by mass spectrometric means, and resolution at
the isomeric level is almost only possible by using chromatographic methods.
Similarly, the areas of organohalogen and flavours/fragrance analysis are dominated
by a need to often quantify chiral compounds, which in the same way as aliphatic