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Multidimensional Chromatography in Environmental Analysis 341

Figure 13.3 Schematic diagram of the parallel cryogenic trap MDGC-IR–MS system:
A, splitless injection port; B, Rt x -5 non-polar first-stage separation column; C, HP 5970B
MSD; D, HP 5965B IRD; E, four-port two-way valve (300 °C maximum temperature); F,
external auxiliary carrier gas; G, six-port selection valve (300 °C maximum temperature); H,
stainless-steel cryogenic traps; I, three-port two- way valve (300 °C maximum temperature);
J, Rt x -5 intermediate polarity column. Reprinted from Journal of Chromatography A, 726,
K. A. Krock and C. L. Wilkins, ‘Qualitative analysis of contaminated environmental extracts
by multidimensional gas chromatography with infrared and mass spectral detection
(MDGC–IR–MS)’, pp. 167–178, copyright 1996, with permission from Elsevier Science.

13.3 MULTIDIMENSIONAL LIQUID CHROMATOGRAPHY

13.3.1 INTRODUCTION

Liquid chromatography (LC) is a good alternative to GC for polar or thermolabile
compounds. While polar compounds need to be derivatized for GC analysis, this is
therefore not necessary for LC analysis.
When environmental samples are analysed by reverse-phase liquid chromatogra-
phy, the most widely used technique, polar interferences usually appear (ions, plus
humic and fulvic acids). This makes it difficult to determine more polar compounds
that elute in the first part of the chromatogram. This is specially important when
detection is not selective, e.g. UV detection, which is one of the most common tech-
niques in routine analysis. In such cases, multidimensional chromatography plays an
important role.
The application range of coupled-column technology is determined by the separa-
tion power of the first column. In general, it can be said that low resolution favours
multiresidue methods (MRMs), while high resolution leads to methods for a single
analyte or for a group of analytes with similar properties.
Multidimensional LC–LC, using two high-resolution columns with orthogonal
separation mechanisms, has only a few applications in environmental analysis. The
limitations that such a multidimensional system has with regard to selectivity must