Topics - Carbon dioxide
Carbon dioxide is a gas at normal temperatures and pressures, has a molecular weight of 44.01, and a critical point at 31C and 73 atmospheres (ca 1050 lbs per sq. in.). In chromatography it is often used under super-critical conditions as an extracting solvent and as the mobile phase in supercritical fluid chromatography. The advantages of super critical carbon dioxide as a solvent is its purity (leaves no residue), the high solubility of organic compounds (particularly flavors and fragrances and other essential oils) and the ease of recovery. After extraction the liquid carbon dioxide can be removed by merely reducing the pressure and allowing the gas to be evolved. This can be carried out at relatively low temperatures and so thermally labile materials (such materials being frequently found in essential oils) are not decomposed. The advantage of using supercritical carbon dioxide as the mobile phase in liquid solid chromatography is that it has the characteristics of both a liquid (strong solvating power) and those of a gas (fast exchange kinetics) and, thus, provide improved elution rates and more efficient columns. Although some of these advantages have been realized, they have not shown sufficient improvement to make the technique competitive with normal liquid chromatography. In many examples, the same results could have been obtained by using conventional liquid chromatography, employing a slightly longer column, slightly smaller particle diameters or a different operating temperature.