Page 367 - Multidimensional Chromatography

358 Multidimensional Chromatography

One problem with SPE–LC is the low selectivity of the precolumn.
Immunosorbents, which have been developed for a few compounds (72), can consid-
erably improve selectivity. The selectivity was high for different pollutants (73, 74).
Figure 13.14 shows that selectivity is higher with an anti-isoproturon cartridge than
with a PLRP-S precolumn.
Some groups of pollutants also have specific problems. For instance, PAHs tend
to adsorb on the walls of the system with which they come into contact and so an
organic solvent or surfactant must be added to the sample. Several solvents have
been tested (66, 67): isopropanol or acetonitrile are the most often used solvents,
while Brij is the most recommended surfactant (66). A very critical parameter in
these cases is their concentration.
Another problem is the determination of ionic substances. Although these can be
analysed by an ionic exchange precolumn, there are some significant limitations and,
in most cases, common sorbents are preferred. In this case, an ion-pair reagent must
be added to the sample to form an ion-pair and so reduce the polarity. This is used,
for instance, to determine naphthalenesulfonic acids in environmental samples.
Tetrabuthylamonium (TBA) is the most common ion-pair reagent (68), although
when LC is coupled to mass spectrometry, it must be replaced by a volatile ion-pair
reagent, e.g. triethylamine (69).
One problem with environmental samples that has already been mentioned con-
cerns humic and fulvic acids which may be retained in the precolumn and co-elute
with the more polar compounds. Of course, this depends on the selectivity of the sor-
bent in the precolumn. A simple solution is to add sodium sulphite to the solution
prior to preconcentration. This approach has led to good results (37, 71).
Figure 13.15 shows the influence of adding sodium sulphite on the chromatogram
of a river water sample.
Another solution to this particular problem is to use a restricted-access-material
(RAM) column prior to the precolumn (54).
For the determination of a wide range of neutral, acidic and basic pollutants, two
precolumns can be coupled in series (75). A PLRP-S precolum is used to trap the
neutral and non-ionized acidic pollutants and a precolumn packed with the same sor-
bent and loaded with a sodium dodecylsulfate (SDS) (to form the ion-pair) is used to
trap the positively charged (basic) pollutants. Each precolumn is coupled to one ana-
lytical column.
Sensitivity is usually higher with SPE–LC than with LC–LC, although both
selectivity and sample throughput are lower (76).

13.4 LIQUID CHROMATOGRAPHY–GAS CHROMATOGRAPHY

13.4.1 INTRODUCTION

The main drawback of GC is sample introduction and this is especially important
when analytes are to be determined at trace levels. Today, however, there is no prob-
lem with introducing 10–100 l of organic solvents such as ethyl acetate or alkanes