Page 79 - Multidimensional Chromatography

70 Multidimensional Chromatography

in isolating several important pairs of atropisomers (55), and quantitation has been
possible since peaks have been baseline-separated by using simple well character-
ized detectors such as ECDs. Examples of quantitative two-dimensional separations
which have been used to identify non-racemic distributions of PCBs in river and
marine samples have been reported recently (56).
Chlorinated dioxins and benzofurans are perhaps the most toxic of all persistent
organic compounds found in the environment. Their toxicity is by no means univer-
sal, and variations between apparently similar molecules can vary by several orders
of magnitude. Because of this specificity, analysis must be similarly species-specific.
This group of compounds suffer from the common problem of being present in a
matrix of hydrocarbon species at a far higher concentration. In order to simplify the
separation, a very extensive sample clean-up is often required, and this can aid in
gaining reasonably high resolution even on a single column. The use of two-dimen-
sional techniques reduces the degree to which interfering compounds must be
removed at the off-line clean-up stage, thus increasing reproducibly and reducing
sample handling. Work carried out in the mid 1980s demonstrated the two-dimen-
sional separation of several polychlorinated dibenzo-p-dioxins (PCDDs) congeners,
and also illustrated how the period of heart-cut transfer was critical in obtaining a
baseline-resolved secondary chromatogram (57). In addition, for PCDDs and PCBs,
the use of ECDs is widespread. This is for a number of reasons, related to cost, ease
of quantitation and sensitivity, which all favour the use of ECDs as opposed to mass
spectrometric detectors. It is important, however, since identification is based on
retention index alone, that the timing of heart-cuts in the primary dimension are
extremely precise. Recent developments in electronic pressure and flow rate control
has greatly improved this reliability.
Chlorinated camphene and bornane compounds (referred to generally as
toxaphenes) are a further group of anthropogenically produced persistent organic
pollutants (58). While most of the 200 reported derivatives are chiral in nature, they
are manufactured and subsequently occur in the environment as racemates. The
degradation of such species within organisms, however, occurs non-racemically and
this phenomenon may be used to study the exact metabolic processes involved in
their degradation. In a similar way to PCBs, this places an analytical requirement
that any determination is both isomeric- and enantiomeric-specific. Two-dimen-
sional GC had been applied to the study of toxaphenes in marine mammal samples
(59), by using an apolar primary column and a chiral secondary dimension. The
chromatogram shown in Figure 3.8 illustrates several heart-cuts from the primary
column, demonstrating that in many instances a non-racemic mixture of enantiomers
is obtained, as a result of metabolic degradation. The separation was enabled by
using non-polar and chiral columns, with independent temperature programming of
both of these. While a large number of target compounds with potentially differing
enantiomeric ratios were identified, each pair required a heart cut and subsequent
analysis. The authors noted that although the separations were baseline, and thus
only possible through this analytical route, the number and length of analyses
required to characterize large number of species in a complex mixture is still a major
limitation of GC–GC.