| OBSERVATION'S |
SFC does not necessarily provide an advantage in enantioselectivity compared with other liquid chromatography (LC) methods, points out Karen W. Phinney, research chemist at the National Institute of Standards & Technology, in a recent review article (Anal. Bioanal. Chem. 2005, 382, 639). But the reduced viscosities and increased diffusivities of supercritical fluids, compared with typical organic liquids, allow for higher flow rates, rapid column equilibration, and faster resolution. As a result, SFC using columns half the size or smaller than in LC is often many times more productive. This reduces solvent use and waste, speeds up method development, and makes large-scale separations more feasible. A major benefit of SFC is its simple mobile-phase composition and the chance to replace solvents such as hexane with inexpensive, readily available, less flammable, and low-toxicity mobile phases, Phinney tells C&EN. The nomenclature is often loosely applied, but what is generally called SFC employs carbon dioxide above or near its critical temperature of 31 °C and pressure of 73 bar, combined with an organic modifier, such as methanol or ethanol. At concentrations typically much less than 50% by volume, the modifier increases the solvent strength of the system and elutes most compounds. Modifier choice can alter retention times, selectivity, and resolution, Phinney explains. Small amounts of polar substances are sometimes incorporated as additives to help elution. Columns can contain one of many possible CSPs to achieve the desired enantioselectivity. All these factors can be tweaked to find the best separation conditions. "IT WASN'T until about a decade ago that commercial SFC equipment similar enough in operation to LC instruments became available and made the transition easy," Phinney says. The equipment could even use many existing CSPs. With SFC, after reducing the pressure and vaporizing the eluent, compounds are collected in small volumes of solvent (the modifier). This is unlike LC, where very large amounts of solvent must be removed and then disposed of or recycled. "So SFC for chiral separations really started to take off," she adds. "It was just the perfect application and what the chiral community had been waiting for." |
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