Page 309 - Chiral Separation Techniques

12.2 Sub- and Supercritical Fluid Chromatography 301

below T and pressures above P , the fluid becomes a liquid. However, many of the
c c
desirable properties of supercritical fluids, including low viscosity and high diffu-
sivity, are retained under these subcritical conditions. At temperatures above T and
c
pressures below P , the fluid becomes a gas. Therefore, supercritical fluids can be
c
viewed as part of a continuum between liquids and gases [13].

12.2.2 Supercritical Fluids as Mobile Phases

The use of supercritical fluids as eluents for chromatographic separations was first
reported by Klesper et al. in 1962. They reported the separation of porphyrin mix-
tures using supercritical chlorofluoromethanes as eluents [14]. Several years later,
Giddings et al. demonstrated the separation of solutes such as carotenoids and sterols
that were not amenable to separation by gas chromatography (GC). Compression of
certain gases reportedly produced eluents with liquid-like solvent properties [15].
Much of the early work in SFC utilized either ammonia (NH ) or carbon dioxide
3
(CO ), but compressed ammonia proved to be too hazardous for most users. Carbon
2
dioxide has been the primary component of most eluent systems in SFC because of
its modest critical parameters, moderate cost, and low toxicity. Early work greatly
overestimated the elution power of pure CO . Giddings proposed that the solvent
2
strength of supercritical CO approximated that of isopropyl alcohol [16], a belief
2
which led to considerable confusion and disappointment in early applications of
SFC. A number of studies with solvatochromic dyes have now revealed that pure
CO is actually similar to pentane or hexane in solvent strength [17] and is, there-
2
fore, not a suitable eluent for most polar compounds.
The elution power of CO and other fluids can be altered through the incorpora-
2
tion of an organic modifier. Most common organic solvents, such as methanol, ace-
tonitrile, and dichloromethane, can be used as modifiers in SFC [18]. The addition
of modifiers increases the critical parameters for the fluid. At the near-ambient tem-
peratures (T < T ) used for many SFC separations, the modified eluent may actually
c
be in the subcritical (liquid) state. However, solute diffusion coefficients remain
higher in subcritical fluids than in traditional liquids [19]. In binary eluent systems,
separation of the eluent into two phases is possible. Phase diagrams for many binary
systems such as methanol-CO are available, and chromatographic parameters that
2
result in phase separation are easily avoided [20]. In some instances, a very polar
substance may be added to the modifier to form a ternary eluent system. These addi-
tives are generally acids or bases and are used to elute certain components or to
improve chromatographic peak shape [21, 22]. Common additives include isopropy-
lamine and trifluoroacetic acid.
The use of both sub- and supercritical fluids as eluents yields mobile phases with
increased diffusivity and decreased viscosity relative to liquid eluents [23]. These
properties enhance chromatographic efficiency and improve resolution. Higher effi-
ciency in SFC shifts the optimum flowrate to higher values so that analysis time can
be reduced without compromising resolution [12]. The low viscosity of the eluent
also reduces the pressure-drop across the chromatographic column and facilitates the