Supercritical Fluid Chromatography:

During the last quarter of the twentieth century, another type of chromatography with an entirely new type of supercritical fluid, mobile phase, has come up on the forefront. It is basically partition chromatography. Where, alter of a liquid or a gas, we use a supercritical fluid. In order to understand a supercritical fluid we will have to know critical temperature and pressure of a substance. The critical temperature of a substance is the temperature above which a distinct liquid phase cannot exist regardless of pressure. A vapour pressure of a substance at its critical temperature is its critical pressure. At pressure and temperature above its critical temperature and pressure (critical point), a substance is called as supercritical fluid. The properties like density, viscosity and other characteristics of supercritical fluids are intermediate between the gaseous and liquid state. A most well known compound used as a supercritical fluid in chromatography is CO₂. The other compounds in use are nitrous oxide, ammonia and n- butane. There are about two dozen compounds which have been used as mobile phases in supercritical chromatography. The critical temperature and pressure at these temperatures is well within the operating conditions used in HPLC.