Page 103 - Multidimensional Chromatography

Orthogonal GC–GC 95

Figure 4.11 (a) Representation of GC–MS as a two-dimensional analysis method. (b)
Representation of GC GC as a two-dimensional separation, with separation mechanisms
based of different chemical properties in each dimension.

second. We gain no extra separation or identification information from this experi-
ment; there is no orthogonality. Thus, we ensure orthogonality of an analysis by
employing techniques which exploit the chemical properties of the solutes to be
studied, in a manner that gives independent responses in the two dimensions. In the
case of two separation dimensions, we can force orthogonality by varying the analy-
sis conditions of the second separation dimension as the first dimension proceeds.
This can be referred to as tuning of the conditions, and is essentially achieved by
using a second temperature zone for the second column.

4.4.1 HOW CAN WE ENSURE THAT THE TWO COLUMNS
ARE ORTHOGONAL?

The elution of compounds on GC columns is a complex process related to the
volatility of the compound, which results from its boiling point, and the chemical
interactions between the compound and the stationary phase. These interactions are
typically those which arise from polar–polar interactions, dispersion forces,
dipole–dipole interactions, and so forth. Collectively, they are described by the term
the chemical potential, °, which derives from the potential for the compound to