Page 143 - Multidimensional Chromatography

Multidimensional Chromatography
Edited by Luigi Mondello, Alastair C. Lewis and Keith D. Bartle
Copyright © 2002 John Wiley & Sons Ltd
ISBNs: 0-471-98869-3 (Hardback); 0-470-84577-5 (Electronic)

6 Supercritical Fluid Techniques
Coupled with Chromatographic

Techniques

F. M. LANÇAS
University of São Paulo, São Carlos (SP), Brasil

6.1 INTRODUCTION

The first report on the occurrence of a supercritical fluid is usually attributed to
Baron Cagniard de la Tour in 1822 (1). Working with a closed glass container
he observed that for certain materials the gas–liquid boundary disappeared when the
system was heated at a certain temperature. An extension of this observation was the
discovery of the critical point of a chemical substance. Hannay and Hogarth (2, 3)
carried out the next important step in the development of this area. These authors
reported in 1879 (2) the results of their studies on the solubility of metal halides in
ethanol under various experimental conditions. They found that the solubility
increased by increasing the pressure and that decreasing the pressure then caused
precipitation of the dissolved salts. This was the first practical demonstration of the
solvating power of supercritical fluids, later confirmed by Buchner (4), among
others.
There is no consensus about a rigorous definition of a supercritical fluid (SF) (5).
In general, the concept of supercritical fluidity for a given substance is taken from its
phase diagram (Figure 6.1). In this figure, the solid lines define the solid, liquid and
gas states, as well as the possible transitions among them (sublimation, melting and
boiling processes), while the points between the phases (along the lines) define the
equilibrium between the individual phases.
The triple point (TP in Figure 6.1) is the point at which the substance coexists as a
solid, liquid and gas. By increasing the temperature and pressure along the boiling
line, the critical point (CP, Figure 6.1) is reached. The required pressure and temper-
ature to reach the critical point (critical temperature (T c ); critical pressure (P c )) varies
from substance to substance. Any substance above its T c and P c is defined as a super-
critical fluid (SF), while the region above the critical point, where a single phase
exists which presents some properties of both a liquid and a gas, is termed the critical
region. The critical temperature (T c ) is defined as the highest temperature at which a