Page 170 - Multidimensional Chromatography

162 Multidimensional Chromatography

(28), especially when the solutes have some degree of volatility. However, attention
to mass-transfer is necessary when non-volatile or even low-volatility solutes must
be transported to and detected by low-pressure detectors.
All of these changes to the underlying HPLC instrument are already practiced
very successfully in the technique we call packed-column SFC. It is possible to per-
form conventional HPLC on an SFC instrument just by choosing HPLC mobile-
phase components, setting the temperature to a value appropriate for conventional
HPLC, and setting the outlet pressure to ambient. Of course, this is not a surprise to
us if we are convinced that the SFC instrument actually has all of the essential capa-
bilities of a unified chromatograph and has simply been misnamed, and if we agree
that HPLC is just one specific application of unified chromatography.
GC is the most abbreviated form of unified chromatography. GC requires the least
and cheapest equipment, and provides the fastest analyses for small, volatile solutes.
Therefore, it is likely to continue flourishing as a technique practiced separately
from the others.

7.4 ADVANTAGES OF AND CHALLENGES FOR
UNIFIED CHROMATOGRAPHY TECHNIQUES
IN MULTIDIMENSIONAL SYSTEMS

Here we will limit the scope of discussion to non-ionic solutes. Interactions of
solutes with stationary and mobile phases are determined by fundamental inter-
molecular forces. Orientation forces, when present, are important contributors and
often influence selectivity between solutes. These forces include hydrogen bonding
(subdivided into hydrogen-bond donating and accepting) and dipole–dipole interac-
tions not involving hydrogen bonding. On many bonded stationary phases, the orien-
tation influence is provided by specific functional groups occupying a small fraction
of the entire stationary phase. An example is the hydrogen-bonding interaction of a
cyano group at the end of a cyanopropyl chain substituted onto a silicone polymer.
Induction forces, such as between a permanent dipole and a polarizable group, are
weaker but are often encountered, for example when a phenyl-containing stationary
phase interacts with an alcohol. The dispersion force, arising from the temporary
dipoles occurring randomly in all molecules, is the weakest intermolecular force, but
often dominates retention since all parts of the stationary phase contribute.
In thinking about performing multidimensional separations within the framework
of unified chromatography, we must think about using all available tuning opportuni-
ties to maximize the differences in the separation mechanisms in the successive parts
of the process. The following is just one example.
Normal-phase LC tends to separate according to solute polarity since the station-
ary phase is polar and retention is often dominated by hydrogen bonding. Thus,
normal-phase LC is useful in sorting out classes of materials according to the polar-
ity of the solutes. Fatty acids are easily separated from monoglycerides, but the sepa-
ration of individual saturated fatty acids from each other on the basis of their carbon