Page 144 - Multidimensional Chromatography

136 Multidimensional Chromatography

Figure 6.1 Phase diagram of a pure substance: cp, critical point; tp, triple point.

gas can be converted to a liquid by increasing its pressure, while the critical pressure
(P c ) is the highest pressure at which a liquid can be converted to a gas by increasing
its temperature (6–8).
Although several other early studies were conducted using Supercritical Fluids
(SFs), also termed at that time as ‘dense gases’, this field did not receive the attention
it deserved during the first half century after its discovery. Even then, the small num-
ber of investigators dedicated to further explore the scientific and technological
potential of supercritical fluids concentrated their efforts on industrial rather than
analytical applications (9, 10).
In spite of their tremendous potential, most of the analytical applications of SFs
investigated after 1980 were concentrated in two areas, i.e. Supercritical Fluid
Extraction (SFE) and Supercritical Fluid Chromatography (SFC). In the first case
(SFE), the supercritical fluid replaces the use of organic solvents during the sample
preparation step prior to further analytical determinations, which usually employ
chromatographic techniques. In the second case, the supercritical fluid is used as the
mobile phase in chromatographic analysis. The instrumentation used in both tech-
niques presents several common features, with the major difference being that the
analytical column used in SFC is replaced by an extraction cell in SFE, in most cases
an open tube where the sample is placed for extraction. General schematic diagrams
of the basic instrumentation for SFE and SFC are displayed in Figure 6.2. In both
cases, a high-pressure system (usually a high-pressure pump) is used to deliver the
fluid at the desired pressure, and an oven is then used to set and maintain the target
temperature; a restrictor regulates the pressure inside the extraction cell (SFE) or
inside the column (SFC), and the stream containing the supercritical fluid and the
analytes is either vented (SFC) or collected (SFE).