Page 291 - Multidimensional Chromatography

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sample loadability (typically 1–10 nl of sample only) and, when optical detection
is used, to the small path length of the capillary (50–100 m). Interferences are
often related to salts, proteins and other endogenous substances present in biologi-
cal samples such as urine and plasma. Hence, in order to effectively apply CE in
biomedical analysis, appropriate sample pretreatment procedures which provide
both analyte enrichment and sample clean-up, are required. To this end, some mul-
tidimensional approaches such as SP(M)E–CE and LC–CE have been explored
during the last few years.

11.8.1 SPE–CE AND SPME–CE
Today, off-line SPE is still one of the most frequently used sample preparation tech-
niques for CE (155). In order to overcome the laboriousness of the off-line approach,
Veraart et al. (156–158) coupled SPE and CE in an at-line fashion by using a robotic
SPE device (Figure 11.17(a)). These authors used disposable ODS cartridges for
direct extraction of urine and plasma samples, and applied the SPE–CE system to
the determination of non-steroidal anti-inflammatory drugs (156), sulfonamides
(157) and the anti-coagulant phenprocoumon (158). The automated SPE procedure
was performed parallel to the CE analysis, thus yielding desalted, deproteinated and
concentrated extracts (Figure 11.17(b) and (c)). Good linearity and detection limits
of ca. 100 ng/ml were obtained when UV detection was used, but, as is true for most
SPE–CE combinations, only a very small fraction of the final extract was actually
analysed by CE.
On-line SPE–CE using a separate SPE column was investigated in the early
1990s, but has never become really successful for biological samples (155). More
recently, micro-SPE units have been developed in order to incorporate SPE in a CE
capillary (159) (see Figure 11.18). The unit sits between two capillary pieces and
contains a small amount of sorbent which is held stationary by micro-frits or by a
membrane. With such a module, the introduced sample volume can be increased
considerably and up to 1000-fold enrichment of analytes can be achieved. Up until
now, a limited number of bioanalytical applications of these so-called in-line
SPE–CE systems have been reported and in most of these cases additional sample
preparation prior to analysis was still required. These applications include the deter-
mination of doxepin (160) and Bench Jones proteins (161) in urine, haloperidol in
urine and liver (161, 162), metallothionein in liver (163), 3-phenylamino-1,2-
propanediol metabolites in liver cells (164), peptides from cell surfaces (165) and
proteins in humor (166). In-line SPE–CE has also been used for protein identi-
fication via peptide mapping. Peptides resulting from protein digests were concen-
trated on an micro-SPE device and subsequently separated and identified by
CE–MS–MS (167–169). The in-line SPE–CE approach is still promising and sig-
nificantly enhances the concentration sensitivity, but it should be added that the CE
performance is frequently compromised due to detrimental effects of the packing
material, frits, connectors and relatively large volumes of desorbing solvent. Matrix