Page 178 - 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)

8 Multidimensional Planar
Chromatography

Sz NYIREDY
Research Institute for Medicinal Plants, Budakalász, Hungary

8.1 INTRODUCTION

Unquestionably, most practical planar chromatographic (PC) analytical problems
can be solved by the use of a single thin-layer chromatographic (TLC) plate and for
most analytical applications it would be impractical to apply two-dimensional (2-D)
TLC. One-dimensional chromatographic systems, however, often have an inade-
quate capability for the clean resolution of the compounds present in complex bio-
logical samples, and because this failure becomes increasingly pronounced as the
number of compounds increases (1), multidimensional (MD) separation procedures
become especially important for such samples.
Multidimensional planar separation exploits combinations of different separation
mechanisms or systems (2, 3); such methods can generally be developed by combin-
ing almost any of the different chromatographic mechanisms or phases (stationary
and/or mobile), electrophoretic techniques, and field-flow fractionation sub-tech-
niques (4). According to Giddings (5), the correct definition of multidimensional
chromatography includes two conditions. “First, it is one in which the components
of a mixture are subjected to two or more separation steps in which their displace-
ments depend on different factors. The second criterion is that when two components
are substantially separated in any single step, they always remain separated until the
completion of the separative operation.” This latter condition, therefore, precludes
simple tandem arrangements in which compounds separated in the first separation
system can re-merge in the second (3). The following modes have most frequently
been used for multidimensional separations (6) involving planar chromatography:
• 2-D development on the same monolayer stationary phase with mobile phases
characterized by different total solvent strength (S T ) and selectivity values (S V );
• 2-D development on the same bilayer stationary phase either with the same
mobile phase or with mobile phases of different composition;
n
• multiple development ( D) in one, two, or three dimensions on the same mono-
layer stationary phase with mobile phases characterized by different solvent
strengths and selectivity values;