Page 308 - Chiral Separation Techniques

300 12 Sub- and Supercritical Fluid Chromatography for Enantiomer Separations

first reported more than 30 years ago, but most of the growth in SFC has occurred
very recently. In fact, the dramatic increase in enantiomeric separations provided the
perfect opportunity for a resurgence of SFC [8]. Numerous reports have now illus-
trated the advantages that can be realized by utilizing SFC as an alternative to LC for
chiral separations, including increased efficiency, simplified method development,
and reduced analysis time [9]. This revival has also been facilitated by the re-emer-
gence of commercial instrumentation for SFC.

12.2 Sub- and Supercritical Fluid Chromatography

Myths and misconceptions about the characteristics of supercritical fluids have
slowed their application to chromatographic separations. While these fluids do have
interesting properties, they are not “super” fluids, and they are not suitable for all
types of separations. An understanding of the fundamental behavior of supercritical
fluids is key to identifying appropriate applications [10].

Table 12-1. Physical parameters of selected supercritical fluids.
Critical temperature Critical pressure
Fluid T (°C) P (MPa)
c
c
CO 2 31.3 7.39
N O 36.5 7.34
2
NH 3 132.5 11.40
SF 45.5 3.76
6
C H 12 196.6 3.37
5
CHF 25.9 4.75
3

12.2.1 Properties of Supercritical Fluids

By definition, a supercritical fluid exists when both the temperature and pressure of
the system exceed the critical values, T and P . The critical parameters of some flu-
c c
ids are listed in Table 12-1 [11]. Supercritical fluids have physical properties that
position them between liquids and gases. Like gases, supercritical fluids are highly
compressible, and properties of the fluid including density and viscosity can be
manipulated by changes in pressure and temperature. Under the conditions used for
most chromatographic separations, solute diffusion coefficients are often an order of
magnitude higher in supercritical fluids than in traditional liquids, and viscosities are
lower than those of liquids [12]. Supercritical fluids can be comprised of a single
component, but binary and ternary fluid systems are also possible. At temperatures