Page 179 - Multidimensional Chromatography

172 Multidimensional Chromatography

• coupled layers with stationary phases of decreasing polarity developed with a
mobile phase of constant composition;
• a combination of at least two of the above-mentioned modes;
• automated coupling of two chromatographic techniques in which PC is used as
the second dimension and another separation method, e.g. gas chromatography
(GC), high-performance liquid chromatography (HPLC), etc., as the first.
In the following discussion, two-dimensional and multidimensional planar chro-
matography are defined, the above-mentioned possibilities are discussed in detail,
and attention is drawn to the different possibilities, as well as the advantages and
limitations of the various modes of multidimensional planar chromatography.

8.2 TWO-DIMENSIONAL OR MULTIDIMENSIONAL
PLANAR CHROMATOGRAPHY?

Multidimensional (or coupled) column chromatography is a technique in which frac-
tions from a separation system are selectively transferred to one or more secondary
separating systems to increase resolution and sensitivity, and/or to reduce analysis
time. The application of secondary columns is illustrated schematically in Figure
8.1. The smaller the t r value applied, then the greater is the resolution and number
of runs needed to check a certain portion of the sample (5).
In planar chromatography, the fractions are not always transferred to another sep-
aration system, but rather a secondary separation is developed, orthogonally on the
same chromatographic plate. Therefore, for all substances not completely separated
it is possible that baseline separation can be achieved by means of a second separa-
tion process with an appropriate mobile (stationary) phase. Figure 8.2 shows that in
the second dimension a theoretically unlimited number of secondary ‘columns’ can
be applied. Because of this, the terminology ‘two-dimensional PC’ is not sufficiently

Figure 8.1 Coupled column liquid chromatographic system in which compounds are
passed from the primary column to secondary columns in a time t r .