Page 85 - Multidimensional Chromatography

Multidimensional Chromatography
Edited by Luigi Mondello, Alastair C. Lewis and Keith D. Bartle
Copyright © 2002 John Wiley & Sons Ltd
ISBNs: 0-471-98869-3 (Hardback); 0-470-84577-5 (Electronic)

4 Orthogonal GC–GC

P. J. MARRIOTT
Royal Melbourne Institute of Technology, Melbourne, Australia

4.1 INTRODUCTION TO MULTIDIMENSIONAL
GAS CHROMATOGRAPHY
A multidimensional gas chromatography (MDGC) separation involves two columns
in the separation process. However, simply the joining together of two columns is
not sufficient to produce the MDGC process. The direct coupling of two different
columns– called multichromatography (1) – is able to improve separation, but is
essentially the same as mixing stationary phases (in this case, the two phases on the
coupled columns) to obtain an improved selectivity in the separation. In Figure 4.1,
the interface or valve at the confluence of the two columns is simply a column join in
this instance. There is no increase in capacity (total number of available separation
plates) in the system, but merely a shifting of the peak relative retentions. This exper-
iment has been improved in its implementation by locating a valve between the two
columns, thus allowing the mid-point pressure to be altered (the stimulus for the
valve in Figure 4.1 is variable pressure). This is termed a pressure-tuning experiment
(2), and by varying the pressure the relative contribution of each column to the sepa-
ration can be varied, and so a wide range of apparent retention factors can be
obtained. By optimization of the pressure and other experimental settings (3, 4), the
chromatographic separation can be consequently optimized according to the mixture
being studied. This method still does not qualify to be called MDGC. The MDGC
definition must be cast to allow differentiation between the accepted MDGC experi-
ment and the above coupled-column approaches. It must incorporate the process of
isolation of zones of effluent from column 1 (also called dimension 1 (1D)) and then
passing them to column 2 (also called dimension 2 (2D)). One could believe that this
must involve extra-column couplings, but that has been obviated by recent develop-
ments (see below). Thus, the simplest MDGC arrangement will be to use a valve
switching system that can pass zones of effluent from 1D to 2D. This will be the sub-
ject of other chapters in this book and so will not be treated any further here. In the
case of the system shown in Figure 4.1, the interface or valve will be a mechanical
valve or a pressure switching valve that can be switched to pass effluent from 1D to
2D, while the stimulus will be the electronic valve drive or balancing pressure sup-
ply. Those zones not passed to 2D will be directed to the detector at the end of 1D.