the central compartment to form water by replacing Na+ ions or methyl sulphonate anions (MSA),
respectively. These pass outwards into the cathode or anode compartment where they are removed,
along with the oxygen and hydrogen, in the regenerant stream of deionized water. The device is
essentially maintenance free and provides superior sensitivity and operational stability compared to
earlier designs.
Ion chromatography has been used mainly for the separation of inorganic anions such as chloride,
bromide, fluoride, sulphate, nitrate, nitrite and phosphate at ppm levels in surface waters, industrial
effluents, food products, pharmaceuticals and clinical samples. However, separations of organic acids
and bases, alkali, alkaline earth and transition metal cations are becoming more widespread. Organic
solvents such as methanol or acetonitrile can be added to the mobile phase when organic species, e.g.
low molecular weight amines or acids, are to be separated. Ion-exchange resins which have a proportion
of the ionic sites replaced with hydrophobic reversed phase groups commonly used in HPLC columns,
typically octadecylsilane (ODS), enable mixtures of both non-ionic and ionic species to be separated.
Some examples of IC separations are included in Figure 4.47. Like HPLC, IC has become micro-
computer controlled and fully automated.
An alternative means of detection involves UV spectrophotometry, the mobile phase containing the
strongly absorbing phthalate ion which gives a constant high absorbance baseline signal that displays
negative peaks as the sample components elute from the separator column. No suppressor column is
therefore needed and the sensitivity is comparable to conductometric detection.
4.3.3—
Supercritical Fluid Chromatography
Supercritical fluid chromatography (SFC) is a recently developed technique that is a hybrid between gas
and liquid chromatography employing a supercritical fluid as the mobile phase. Supercritical fluids are
produced when a gas or liquid is subjected to a temperature and pressure that both exceed critical
values. Under these conditions the properties of the fluid, including density, viscosity and solute
diffusion coefficients are intermediate between those of a gas and a liquid and vary with pressure.
Increasing the pressure further makes a supercritical fluid more like a liquid, i.e. higher density and
viscosity, smaller solute diffusion coefficients, whilst reducing the pressure causes it to become more
like a gas, i.e. lower density and viscosity and larger solute diffusion coefficients. The only substance to
be extensively used for SFC so far is carbon dioxide, for which the critical temperature is 31.1°C and
the critical pressure is 72.9 bar. Xenon, with values of 16.5°C and 57.6 bar respectively, may prove
useful, if expensive, for SFC-FT-IR (cf. GC-FT-IR, p. 117) as it is completely transparent to IR
radiation. Other substances such as ammonia, hexane and nitrous oxide have critical values that could
easily be exceeded in practice but are hazardous, toxic and/or corrosive.