Page 167 - Multidimensional Chromatography

Unified Chromatography 159

Figure 7.7 The Hyperbaric Chromatography region, at temperatures above the 2-phase
region and pressures below the locus of critical pressures. Reproduced by permission of the
American Chemical Society.

As we continue lowering the pressure, GC is the final limiting case when the
mobile phase has zero solvent strength over the entire column length and where tem-
perature is the only effective control parameter. Gas chromatography is shown in
Figure 7.3.
Our view of unified chromatography simply involves realizing that the technique
names and the boundaries defining them are arbitrary definitions lacking any real
meaning with respect to fluid behavior, except perhaps when ambient conditions are
invoked by default. In order to perceive unified chromatography, we simply elimi-
nate the arbitrary boundaries separating the adjacent techniques and merge or unify
all these techniques into one, as shown in Figure 7.8. We are also now completely
free to select the conditions that produce the best separation for a particular problem
from anywhere within this continuum of behavior without regard to naming conven-
tions or arbitrary restrictions. We will probably want to avoid the two-phase region
under most circumstances, and are limited in temperature and pressure only by the
capabilities and stability of the instrument, stationary phase and solutes.

7.3 INSTRUMENTATION

The basic instrument required for packed-column unified chromatography is shown
schematically in Figure 7.9. This is essentially a two-pump HPLC instrument utiliz-
ing high-pressure mixing with just a few new components. At least one pump must