Page 310 - Chiral Separation Techniques

302 12 Sub- and Supercritical Fluid Chromatography for Enantiomer Separations

use of long columns or multiple columns connected in series [24]. Manipulation of
mobile phase composition, temperature, and pressure can be used to achieve the
desired selectivity in SFC [25]. Rapid column equilibration in SFC is an additional
advantage over liquid chromatographic methods [26]. Replacement of liquid eluents
with sub- and supercritical fluids also reduces solvent consumption.

12.2.3 Instrumentation for SFC

SFC has been performed with either open capillary columns similar to those used in
GC or packed columns transferred from LC, and the instrumentation requirements
differ for these two approaches [12]. This chapter will focus on the use of packed
column technology because of its dominance in the area of pharmaceutical com-
pound separations. Current commercial instrumentation for packed column SFC uti-
lizes many of the same components as traditional LC instruments, including pumps,
injection valves, and detectors. In fact, most modern packed column SFC instru-
ments can also be used to perform LC separations, and many of the same stationary
phases can be used in both LC and SFC [9].
Certain modifications to the chromatographic system are necessary to accommo-
date the compressibility of the eluent in SFC [27]. Carbon dioxide and other fluids
comprising the bulk of the mobile phase are liquefied gases supplied in cylinders.
Chilling of the pump head is necessary to ensure that the fluid remains in the liquid
state. A second pump can be used to deliver a modifier if binary eluents are desired.
When necessary, mobile phase additives are incorporated by adding them to the
modifier. Automatic injection systems are available for SFC, but injection valves
must be modified to permit introduction of the sample into a high-pressure environ-
ment. Injection volumes of 5–20 µL are common. The most noticeable difference
between LC and SFC is the addition of a back-pressure regulator after the detector
to control outlet pressure and prevent expansion of the eluent into a gas. Electronic
back-pressure regulators allow independent control of pressure and mobile phase
composition [12]. Measurement of UV absorbance remains the most common detec-
tion method for packed column SFC. The position of the back-pressure regulator
necessitates the use of detection cells capable of withstanding high pressure.

12.3 Advantages of SFC for Chiral Separations

The high diffusivity and low viscosity of sub- and supercritical fluids make them
particularly attractive eluents for enantiomeric separations. Mourier et al. first
exploited sub- and supercritical eluents for the separation of phosphine oxides on a
brush-type chiral stationary phase [28]. They compared analysis time and resolution
per unit time for separations performed by LC and SFC. Although selectivity (α) was
comparable in LC and SFC for the compounds studied, resolution was consistently