Page 352 - Multidimensional Chromatography

Multidimensional Chromatography in Environmental Analysis 343

be taken into account. For example, the differences in the physico-chemical bases of
the separation processes involved may lead to poorly compatible mobile phase sys-
tems, thus requiring complex interfaces. Moreover, the separation obtained in the
first column can, at least partly, be decreased in the second column.
In spite of such limitations, some examples can be found in literature. For exam-
ple, a reversed-phase C 18 column has also been coupled to a weak ion-exchange col-
umn to determine gluphosinate, glyphosate and aminomethylphosphonic acid
(AMPA) in environmental water (28). This method will be described further below.
Zebühr et al. (29) developed an automated system for determining PAHs, PCBs
and PCDD/Fs by using an aminopropyl silica column coupled to a porous graphitic
carbon column. This method gives five fractions, i.e. aliphatic and monoaromatic
hydrocarbons, polycyclic aromatic hydrocarbons, PCBs with two or more ortho-
chlorines, mono-ortho PCBs, and non-ortho PCBs and PCDD/Fs. This method
employed five switching valves and was successfully used with extracts of sedi-
ments, biological samples and electrostatic filter precipitates.
As mentioned above, the most commonly used liquid chromatographic technique
is reverse-phase liquid chromatography (RPLC), which is also the most often used
coupled technique. When two RPLC systems are coupled to analyse aqueous sam-
ples, there is an additional advantage because large sample volumes can be injected
without causing extensive band broadening. This means that there is on-line enrich-
ment. In general, therefore, the less polar the analyte, then the more the sample vol-
ume can be enriched without causing band broadening or breakthrough. However,
when highly polar analytes have to be determined, the enrichment and clean-up
needed to eliminate the interference become more limited. However, results have
been good for some analytes, as we will see later.
Most work on LC–LC in environmental analysis has been developed by the Van
Zoonen group (30, 31) and the Hernández group (32–34).
A commonly used system in environmental analysis is the heart-cutting technique
which uses the separation power of the first column to obtain a higher selectivity
than with the previously described precolumn enrichment. The two columns are
coupled via a switching valve, as shown in Figure 13.5.
Separation in column 1 (C-1) removes early-eluting interference compounds, and
so considerably increases the selectivity. The fraction of interest separated in C-1 is
then transferred to column 2 (C-2) where the analytes of the fraction are separated.
These transfers can be carried out either in forward mode or backflush mode. The
forward mode is preferred because the backflush mode has two disadvantages for
polar to moderately polar analytes. For most polar compounds, it leads to additional
band broadening, while for more retained analytes there is a decrease in the separa-
tion obtained earlier in the process (31).
An important parameter in LC–LC is the transfer volume, i.e. the time that C-1 is
coupled to C-2, since the selectivity is highly dependent on this. In environmental
samples, it is important to remove early-eluting interference in order to ensure selec-
tive analysis. A short analysis time is important for routine analysis of environmental
samples.