Page 354 - Multidimensional Chromatography

Multidimensional Chromatography in Environmental Analysis 345

Depending on the kind of sorbent in the precolumn connected on-line to the
analytical column, the retention of analytes in the precolumn may be more or less
selective.
The sorbents that are most frequently used in environmental analysis are
C 18 -silica based sorbents, polymeric sorbents (usually styrenedivinilbenzene) and
graphitized carbon. In order to increase the selectivity of these sorbents, immunosor-
bents (35, 36) have been developed and used with good results, while recently,
molecularly imprinted polymers have started be to used (35, 36).
Polar compounds present the most problems because of their low breakthrough
volumes with common sorbents. In the last few years, highly crosslinked polymers
have become commercially available which involve higher retention capacities for
the more polar analytes (37, 38). Polymers have also been chemically modified with
polar groups in order to increase the retention of the compounds previously
mentioned (35, 37).
Instead of a sorbent contained in a precolumn, discs can also be used with a spe-
cial device (38, 39) which enables the number of discs to be changed easily, although
this technique is currently limited to the kind of discs that are commercially
available.
The characteristics of the sorbent in the precolumn may lead to problems when
coupling the two systems. Therefore, when the analytes are more retained in the pre-
column than in the analytical column, peak broadening may appear, even when the
analytes are eluted in the backflush mode (40). This has been solved with a special
design in which the analytes retained in the precolumn are eluted with only the
organic solvent of the mobile phase and the corresponding mobile phase is subse-
quently formed (40, 41).
On-line SPE–LC has been widely used in environmental analysis to solve the
problems caused by the low concentrations of the analytes to be detected and also to
automate the analysis (42–44).

13.3.2 EXAMPLES OF MULTIDIMENSIONAL LIQUID
CHROMATOGRAPHY IN ENVIRONMENTAL ANALYSIS

LC–LC is applied to environmental samples with two major aims, i.e. to determine a
single analyte and to determine a group of analytes (by the multiresidue methods) at
the low levels required by legislation in both cases. Some examples of these are dis-
cussed below. In addition, some applications for the particular case of SPE–LC, will
also be described.
The single-residue methodology has been used to determine analytes with differ-
ent characteristics. The main advantage of this technique is the short analysis time.
LC–LC methodology has been applied to various polar analytes, such as bentazone
(46), and less polar compounds such as isoproturon (46) or pentaclorphenol (47).
Most applications refer to water samples (46, 48), although solid samples have also
been studied (31, 49, 50). Soil samples contain more interfering compounds so
clean-up is even more important when analysing soil extracts. Coupled RPLC is