Page 371 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN0011A-541 July 25, 2001 17:27

474 Organic Chemistry, Compound Detection

VII. SUPERCRITICAL FLUID ons, ethylene, pentane, hexane, isopropanol, and carbon
CHROMATOGRAPHY dioxide. It has been observed that the solubility of vari-
ous solutes increases in supercritical phases. The column
In supercritical fluid chromatography (SFC), the mobile packings that are used in SFC are essentially the same as
phase is maintained at a temperature somewhat above its those used in HPLC.
critical point. Since the physical properties of a substance The instrumentation in SFC is quite similar to that of
in the supercritical state near the critical point are interme- modern high-resolution liquid chromatography. A number
diate between those of liquids and gases at ambient con- of modifications are made for LC to be suitable for opera-
ditions, it is designated as a fluid. For chromatographic tion with a supercritical fluid. The separation column can
purposes such a fluid has more desirable transport prop- be coupled to online detectors other than UV, including
erties than a liquid. SFC is superior to LC in separating MS, FTIR, FID, and other GC detectors.
efficiency and speed. In comparison to a gas, a fluid shows Carbon dioxide offers many advantages; it is inexpen-
about a 1000-fold increase in solution capabilities. The sive, available in high purity, and innocuous. Its near-
resultant enhancement in the migration rate of solutes is ambient critical temperature makes it attractive for use
especially valuable in the analysis of higher-molecular- with thermally labile compounds. The UV absorbance
weight compounds. Furthermore, some ionic solutes are of CO 2 is minimal, thus allowing spectrometric detection
soluble in a supercritical fluid. This suggests that SFC down to 190 nm.
may be applicable to the analysis of compounds such as The advantages of SFC are high resolution per unit time,
the phospholipids, which cannot be volatalized for GC orthogonalcolumnselectivitycomparedtoGCandHPLC,
without decomposition. ease of fraction collection, and analysis of thermally labile
The number of compounds that can be analyzed by SFC molecules. It is likely that 20–40% of the solutes presently
6
is potentially enormous. Out of the 10 known compounds separated by HPLC are amenable to SFC separation with
which are currently more or less well characterized, only supercritical CO 2 . It is possible that most separations car-
about 15% can be volatalized without decomposition. ried out by normal-phase HPLC may be handled with good
Compounds such as proteins, synthetic and natural poly- advantage by SFC.
mers, lipids, carbohydrates, vitamins, synthetic drugs, and Recent developments in SFC have not been inspired by
metal organic compounds may well be analyzed by SFC. any revolutionary new ideas. There has been progress in
When a liquid and its vapor in equilibrium with each two directions in the area of SFC. One direction parallels
other are heated in a confined space, the intensive prop- the development of capillary GC, the second direction is
erties of the two coexisting phases become increasingly similar to the development in HPLC. Because capillary
similar until, at the critical temperature, the two phases co- SFC is a high efficiency separation method, it is most
alesce into a fluid and acquire the same properties. When useful for the separation of complex mixtures. In combi-
this substance is heated beyond the critical temperature, a nation with universal (FID) detection, it can be used as a
supercritical phase is obtained, the substance is then called “fingerprint” technique.
supercritical fluid. Packed column SFC with particles of 3 to 10 µm can
For comparative purposes some physical properties of be used with mobile phases of relatively high densities.
a gas, a liquid, and a supercritical fluid are shown in At least 20,000 theoretical plates can be achieved within
Table III. The data in Table III show that the viscosity a few minutes on a routine basis.
of a supercritical fluid is comparable to that of a gas and A further advantage of SFC with respect to LC and
its diffusibility is between that of a gas and a liquid. GC is the separation of nonvolatile components and to in-
Any compound which is thermally stable to somewhat troduce them to a mass spectrometer in such a way that
beyond its critical point can theoretically be used as the both molecular weight information and reproducibly frag-
mobile phase. The mobile phases used in SFC include fre- mented spectra can be obtained.

TABLE III A. Interfacing Supercritical Fluid
Chromatography with Fourier
Supercritical
Property Units Gas Liquid fluid Transform IR (SFC–FTIR)
Supercritical fluids have different coefficients which are
Density g/ml 10 −3 1 0.3
about 100 times greater than those of liquids and with very
3
Diffusibility cm /sec 10 −1 5 · 10 −6 10 −3
low viscosities. When open tubular columns are used in
Dynamic poise 10 −4 10 −2 10 −4 5 6
SFC, overall efficiencies on the order of 10 to 10 the-
viscosity (g/cm sec)
oretical plates are obtained in reasonable analysis times.