Gas chromatography and HPLC commonly use inert gases or liquid mobile phases. With the introduction of supercritical fluid chromatography which is a hybrid of the two techniques the scope of applications has expanded due to its several inherent features.
In the first place it is important to understand what is a supercritical fluid. A supercritical fluids result when a gas is raised to a temperature above its critical temperature or it is compressed to a pressure higher than its critical pressure. A substance cannot exist in liquid state irrespective of pressure above the critical temperature. Its vapour pressure at critical temperature is referred to as its critical pressure.
Supercritical fluid chromatography permits the isolation and estimation of components that are not conveniently handled by GC and HPLC techniques.
Several gases have been used as supercritical mobile phases such as lower alkanes, ammonia, nitrous oxide, and tetrahydrofuran, However, the most popular is carbon dioxide because of its low critical temperature (31°C), low toxicity, reactivity and lower cost. The advantages offered by supercritical fluid chromatography are briefly discussed in the present article
Cost of Operation
HPLC analysis requires high purity solvents as mobile phases which further require filtration and degassing for removal of dissolved air and solid suspended impurities. In comparison carbon dioxide which is commonly used as a supercritical fluid is available in required purity and does not require such additional steps as in the case of HPLC.
Commonly used mobile phase organic solvents are toxic and their usage and disposal require special safety precautions. Such steps are not necessary for supercritical fluid chromatography studies
Low back pressure
Low density and viscosity of supercritical fluids do not present flow related problems during operation. These properties also contribute to low column back pressures.
On isolation the supercritical fluid can be easily removed from the isolated fractions for large-scale fraction collections Simply lowering the temperature and/or pressure can help recover the isolated fractions. There is no need for adopting tedious steps for removing of liquid impurities
The resolving power of supercritical fluid chromatography separations is higher in comparison with HPLC or GC separations. Resolving power is at least 5-10 times greater than conventional HPLC separations
Separation of vast range of molecular species
Supercritical fluid chromatography can be used for analysis of nonvolatile high molecular weight molecules, thermally labile molecules and polar molecules without the need for derivatization.
Supercritical fluid chromatography system does not require any major instrumental changes. Commonly used HPLC or GC having similar electronics, columns and detectors are used in super fluid chromatography. The only major change in the system configuration is inclusion of a restrictor which maintains the pressure of the fluid higher than the critical pressure.
To summarize supercritical fluid chromatography has contributed to the ease of handling, time and cost saving for common separations and has led to several new applications in diverse areas such as pharmaceuticals, foods, chiral separations and pesticides,etc.