Page 159 - 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)

7 Unified Chromatography:

Concepts and Considerations for
Multidimensional Chromatography

T. L. CHESTER
The Procter & Gamble Company, Cincinnati, OH, USA

7.1 INTRODUCTION

Multidimensional chromatography brings together separations often based on differ-
ent selectivity mechanisms. Although the forms of the mobile phase are not required
to be different in the individual steps of a multidimensional separation, we usually
strive to achieve orthogonal selectivity of these individual separation steps (1).
This is usually not a problem when bringing together two techniques with very
different mobile phases, such as liquid chromatography (LC) and gas chromatogra-
phy (GC), for example, to carry out a two-dimensional LC–GC separation. In GC,
the only significant intermolecular forces are between the solutes and the stationary
phase, and there are no attractive forces of any consequence involving the mobile
phase. Rather, it is just an inert carrier that transports the gaseous fraction of the
solute through the interparticle volume of the column whenever the solute is not
associated with the stationary phase. Once a GC column is chosen, control of solute
partitioning between the stationary and mobile phases is simply a function of tem-
perature and nothing else.
However, in LC solutes are partitioned according to a more complicated balance
among various attractive forces: solutes interact with both mobile-phase molecules
and stationary-phase molecules (or stationary-phase pendant groups), the stationary-
phase interacts with mobile-phase molecules, parts of the stationary phase may inter-
act with each other, and mobile-phase molecules interact with each other. Cavity
formation in the mobile phase, overcoming the attractive forces of the mobile-phase
molecules for each other, is an important consideration in LC but not in GC.
Therefore, even though LC and GC share a considerable amount of basic theory, the
mechanisms are very different on a molecular level. This translates into conditions
that are very different on a practical level; so different, in fact, that separate instru-
ments are required in modern practice.