Page 334 - Multidimensional Chromatography

324 Multidimensional Chromatography

12.8 SFC–GC AND NORMAL PHASE-LC–SFC APPLICATIONS

Supercritical fluid chromatography (SFC) is an intermediate mode between gas and
liquid chromatography which combines the best features of each of these techniques.
This is a chromatographic technique in which the mobile phase is neither a liquid or
a gas but a supercritical fluid, with physical properties that are intermediate between
those of gases and liquids. Supercritical chromatography is of importance either in
the first or second dimension since it permits the separation and determination of
compounds that cannot be analyzed by HPLC or GC. Such compounds could be
either non-volatile, of high molecular mass, reactive or thermally labile, thus making
them unsuitable for GC. Thermally labile compounds could include polymer addi-
tives, with the low operating temperatures of SFC being important for the analysis of
such compounds. In addition, one of the advantages that SFC has over HPLC is that
when using CO 2 as a mobile phase it can be easily interfaced to a multitude of detec-
tion systems, including UV, FID, MS and FTIR. SFC could also be an attractive
alternative to normal phase LC and SEC for the analysis of high-molecular-mass
species such as polymers and fossil fuels since SEC analyses are hindered by poor
efficiency, while normal phase LC analysis lacks a universal detection system.
Hence, SFC with FID detection allows the analysis of high-boiling petroleum frac-
tions, group-type analysis of saturates, olefins and aromatics in petroleum fuels (21).

12.9 NORMAL PHASE-LC– SFC APPLICATIONS

The use of microcolumn LC generates small peak volumes and allows the coupling
of SFC by using a solvent-venting injection procedure. The preseparation of poly-
mer additives from various polymers was carried out on a 320 m i.d. fused-silica
column packed with 5 m polystyrene particles, using THF as the eluent (22). The
low-molecular-weight additives (MW 1200) were eluted at the total permeation
volume since they were more retained than their excluded polymer counterparts.
The evaporation of the solvent from the collected fraction was carried out by flush-
ing the loop with nitrogen gas, in turn leaving the polymer additives coated on the
wall of the retention gap. The additives were then transferred to the SFC column by
switching the liquid CO 2 flow via the retention gap to the SFC system. The addi-
tives were focused by the effect of the higher temperature, while their separation
was achieved by gradient elution by systematically changing the density of the
supercritical fluid.
An application of an LC–SFC system has been demonstrated by the separation of
non-ionic surfactants consisting of mono- and di-laurates of poly (ethyleneglycol)
(23). Without fractionation in the precolumn by normal phase HPLC (Figure 12.18
(a)) and transfer of the whole sample into the SFC system, the different homologues
coeluted with each other. (Figure 12.18(b)). In contrast with prior fractionation by
HPLC into two fractions and consequent analysis by SFC, the homologues in the
two fractions were well resolved (Figures 12.18(c) and 12.18(d)).