Page 348 - Multidimensional Chromatography

Multidimensional Chromatography in Environmental Analysis 339

conformations and 19 form stable enantiomers due to the restricted rotation around
the central carbon–carbon bond at ambient temperatures (14). There are at least nine
of the conformationally stable chiral PCBs which are present in commercial formu-
lations and these are expected to accumulate in the environment. In order to analyse
them, an achiral column was coupled to a chiral column (14, 25). The gas chromato-
graphic separation of most of the chiral PCB congeners was achieved on different
cyclodextrin phases (14).
MDGC has been used for separating commercial formulations of PCBs (11, 12,
22, 23, 26) although it is not widely used on real samples. In some examples, MDGC
has been applied to determine PCBs in sediment samples (13, 14, 27) and water
samples (14, 24).
For PCB analysis, Glausch et al. used an achiral column coated with DB-5 and a
chiral column coated with immobilized Chirasil-Dex (14). The column was switched
with a pneumatically controlled six-port valve and peak broadening was minimized
by cooling the first part of the second column with air precooled with liquid nitro-
gen, thus focussing the cut fraction. These authors determined the chiral polychlori-
nated biphenyls 95, 132 and 149 in river sediments, using microsimultaneous steam
distillation–solvent extraction as the sample treatment technique. Figure 13.2 pre-
sents the MDGC-ECD chromatograms of PCB fractions from sediment samples,
where it can be seen that the separation of the PCB enantiomers is good.
In many environmental extracts, the analytes of interest overlap with other ana-
lytes or matrix components. MDGC is therefore essential for improving accuracy
when identifying non-target methods. In a typical example (20), contaminated water,
clay and soil samples have been analysed. While water and clay extracts can be anal-
ysed by GC-IR–MS, soil samples, because of their greater complexity, need MDGC-
IR–MS to identify the pollutants present in the sample. In such cases, an
MDGC-IR–MS system with multiple parallel cryogenic traps and sample recycling
is used (see Figure 13.3).
The chromatogram from the first column was divided into five areas of five heart-
cuts each. Some peaks identified in the chromatogram from the first column were
used as heart-cut markers. This method has some limitations which mainly concern
contamination of the system, and also with the determination of less volatile pollu-
tants. However, such a system is able to detect and accurately identify about 40
pollutants.
Another interesting application of MDGC is in the rapid determination of
isoprene (the most reactive hydrocarbon species) and dimethyl sulfide (DMS) (the
major source of sulfur in the marine troposphere and a precursor to cloud formation)
1
in the atmosphere (16). The detection limits were 5 and 25 ng l , respectively.
A programmed temperature-vaporization (PTV) injector (with a sorbent-packed
liner) was used to preconcentrate and inject the sample. Thermal desorption was per-
formed and the analytes were passed to a primary column (16 m 0.32 mm i.d.,
film thickness 5 m, 100% methyl polysiloxane) and separated according to analyte
vapour pressure. Selected heart-cuts were transferred to a second column (15 m
0.53 mm i.d., Al 2 O 3 /Na 2 SO 4 layer, open tubular column with 10 m stationary
phase) where final separation was performed according to chemical functionality.