Page 370 - Multidimensional Chromatography

Multidimensional Chromatography in Environmental Analysis 361

LC–GC is a very powerful analytical technique because of its selectivity and sen-
sitivity in analysing complex mixtures and therefore it has been used extensively to
determine trace components in environmental samples (2, 5, 77). LC allows presepa-
ration and concentration of the components into compound types, with GC being
used to analyse the fractions. The advantages of on-line LC–GC over the off-line
system are, first, the less sample which is required and, secondly, that there is less
need for laborious sample pretreatment because the method is automated (78).
Important developments in LC–GC have been made by Grob and co-workers
(79–81) and by the Brinkman group (82–87), who have mainly studied the applica-
tion of this technique to environmental analysis. This coupled technique has usually
been applied to water, although air and soil extracts have also been analysed.
On-line coupling of normal-phase liquid chromatography (NPLC) and gas chro-
matography is today a well developed and robust procedure and has been regularly
applied to environmental analysis. When a fraction of the NPLC sample is intro-
duced in to the GC unit, a large-volume interface (LVI) is needed but, due to the
volatility of the organic solvent used in NPLC, this does not present such a great
problem.
However, for water analysis, reverse-phase liquid chromatography is more suit-
able but its coupling with GC has some drawbacks because of the partly aqueous
effluent. Several systems have been developed (88, 89) and applied to determine pol-
lutants in water.
An alternative way of eliminating water is to pass the aqueous sample through a
small LC column, also called a precolumn or an SPE column, where the analytes are
retained and the water is mostly eliminated. Subsequent elution of these analytes
with an organic solvent (usually ethyl or methyl acetate) and more compatible han-
dling with GC systems enables the sample to be significantly concentrated and low
levels of analytes to be determined. However, an additional step is needed prior to
the elution of the retained analytes in order to eliminate the small amounts of water
in the LC column. This is usually carried out by drying the LC column with nitrogen
(89) or by adding a small drying column containing sodium sulfate or silica (89, 90).
The former approach is normally used and we can find several applications to envi-
ronmental analysis in the literature (91–93).
These small columns,(usually 10 mm 1–4.6 mm i.d.) are normally packed
with 10–40 m sorbents such as C 18 -bonded silica, C 8 -bonded silica or
styrene–divinylbenzene copolymer. These sorbents are not very selective and more
selective sorbents, such as the immunosorbent (94), have also been used with good
results. Coupling of SPE–gas chromatography is in fact the one most often used in
environmental analysis because it reaches a high level of trace enrichment, elimi-
nates water and elutes retained compounds easily with an organic solvent that can be
injected into the gas chromatograph.
In order to achieve the widest application range, partially concurrent solvent
evaporation (PCSE) with an on-column interface is normally used during the transfer
of analytes from the LC-type precolumn to the GC system. Fully concurrent solvent
evaporation (FSCE), with a loop-type interface, is used in some cases, although the