Page 295 - Multidimensional Chromatography
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290                                     Multidimensional Chromatography

                           short time and, therefore, CE in principle can be a very suitable candidate for the
                           second dimension in a comprehensive multidimensional separation system. In a
                           comprehensive LC–CE system, fractions of the LC effluent are continuously
                           injected into a CE system, thus yielding high peak capacities due to the comple-
                           mentary character of the two dimensions. Comprehensive LC–CE has predomi-
                           nantly been investigated by Jorgenson and co-workers (172–175), with most of
                           their research being devoted to the problem of effectively interfacing the two
                           techniques. Both RPLC–CE and SEC–CE have been reported for the separation
                           of mixtures of (serum) proteins and peptides (tryptic digests). Even a three-
                           dimensional system (combining SEC, RPLC and CE) has been designed in which
                           a SEC separation of peptides, with an analysis time over several hours, is repeat-
                           edly sampled into a rapid two-dimensional RPLC–CE system with an analysis
                           time of 7 min (173).




                           11.9  CONCLUSIONS

                           The applications described in the preceding sections of this chapter clearly demon-
                           strate that coupled-column chromatography shows very good potential for the selec-
                           tive and sensitive determination of biomedically and pharmaceutically interesting
                           compounds in biological samples. For many bioanalytical queries, a multidimen-
                           sional chromatographic approach is indispensable to prevent unwanted co-elution of
                           analytes with other sample components, and also to incorporate on-line sample
                           preparation in the analytical procedure. Nowadays, well-developed methodologies
                           exist for multidimensional modes such as LC–LC, SPE–LC, NPLC–GC and
                           SPME–GC, and these have been used routinely for quite a number of real-life appli-
                           cations. Multidimensional systems can yield high selectivities for a single analyte,
                           although during the past few years there has been a growing interest in multi-target
                           analysis systems which can deal with several analytes at the same time. This often
                           means that in the first dimension a larger fraction has to be selected at the expense of
                           a lower attainable selectivity.
                             Selectivity enhancement can be achieved by coupling a relatively simple chro-
                           matographic system to (tandem) MS. Today, easy-to-operate bench-top MS(–MS)
                           systems (single/triple quadrupole or ion-trap) with robust atmospheric-pressure ion-
                           ization (API) interfaces are widely available, and LC–MS(–MS) has become the
                           method of choice in many stages of drug development, e.g. for quantitative bioanaly-
                           sis (176). It has even been suggested that the high selectivity (and sensitivity) of MS n
                           techniques might be sufficient to directly detect and quantify target compounds in
                           complex mixtures by MS alone. However, it is clear now that in most cases some
                           kind of sample preparation and pre-separation remains necessary in order to avoid
                           serious reduction of analyte response (ion-suppression) by matrix effects and/or ion-
                           source contamination (177). Generally, the use of LC–MS(–MS) as a routine tech-
                           nique for the analysis of pharmaceutical and biomedical samples will certainly
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