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58 Multidimensional Chromatography

isomers yield highly similar mass fragmentation patterns. In these cases, it is only
through high resolution separations that these goals may be achieved.

3.3.1 PETROLEUM, FUELS, FEEDSTOCKS AND COMBUSTION
ANALYSIS METHODOLOGIES

As mentioned at the start of this chapter, it was the analysis of crude oil fractions that
was at the forefront of multidimensional GC development. The need to accurately
quantify components within such mixtures was of tremendous importance not only
commercially, but also in diagnosing environmental consequences and impact. As
mixtures, fuel type samples are obvious candidates for multidimensional chromatog-
raphy. They naturally contain very large numbers of aliphatic compounds, with often
a staggering degree of sample complexity and isomeric abundance. There are, for
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example, around 4 10 potential isomers (14) in the diesel fraction range C 10 –C 25 .
Within matrices such as these, however, there exist large numbers of groups of com-
pounds of particular interest to the analyst, such as heterocompounds containing
nitrogen, sulfur and oxygen, as well as amounts of polycyclic aromatic materials.
For example, their presence may be significant in reducing catalytic convertor per-
formance, or in increasing the emission of NO x ,SO 2 or harmful carcinogenic com-
pounds to the atmosphere.
In almost all fuel and feedstock analysis, the peak capacity of a single column is
far from sufficient to resolve all of the species present and analysis of the PIONA
(paraffins; isoparaffins; olefins; naphalenes; aromatics) fraction is a particular chal-
lenge. It was PIONA analysis that highlighted the first significant industrial applica-
tion in 1971 (15) of two-dimensional GC, with a method that has now found general
acceptance (16). The ASTM standard methods of PIONA analysis can be highly
complex in nature, using between two to five parallel secondary GC columns, with
valve transfers, independent temperature controls and multiple re-injections. These
standard GC–GC methods still do not produce a complete baseline separation of the
mixtures, but do offer a highly targeted analysis of a large number of key species. In
general, this type of GC–GC analysis system is optimized for condensed-phase, rel-
atively involatile material. More volatile mixtures, such as exhaust emissions or
refinery gases, are still sufficiently complex to display incomplete resolution on a
single column. In cases such as these it is essential to isolate the gas-phase species
from the less volatile material in order to avoid column and detector contamination.
Early GC–GC applications used packed columns in combination with liquid-film
capillary columns (17, 18). More recently, backflushed liquid-film pre-columns have
been used with PLOT-type secondary columns (19). In these configurations, the
high-retention PLOT columns require, that involatile material is completely
excluded, or otherwise irreversible adsorption may occur.
The production of petrochemicals from feedstocks has also been an area of wide
application of two-dimensional GC. A detailed knowledge of the composition of
feedstocks and intermediates in the manufacturing process is central to obtaining