Separations of Long-chain Hydrocarbons and Mink Oils Using Pressure Programming Supercritical Fluid Chromatography

Abstract

The advantages of supercritical fluid chromatography (SFC) have recently been realized in particular its rapidity, flexibility, and ability to allow the analysis of substances which cannot be analysed by gas chromatography (GC). The separating power of open-tubular column GC is unparalleled, but its applicability is restricted by the limited volatility and thermal stability of many organic compounds; less volatile compounds can be analysed by high performance liquid chromatography (HPLC), but long analysis times and very small column diameters are required for efficient separations because of the limitations of solute diffusion in the mobile phase. SFC overcomes these difficulties and permits high resolution at low temperatures with short analysis times. Above its critical point a substance such as carbon dioxide has properties which make its use as a chromatographic mobile phase very favourable. The ability of supercritical fluids to dissolve substances was first recorded in 1879, when Hannay and Hogarth studied the solubility of cobalt and iron chlorides in supercritical ethanol. Numerous applications of extraction with supercritical solvents have been described, and the underlying theory fully understood. Following a suggestion by Lovelock in 1958 that a supercritical fluid might be used as a mobile phase in chromatography, Klesper et al. first deminstrated SFC by the separation of nickel porphyrins using supercritical chlorofluoromethanes as mobile phases. Sie and Rijnders and Giddings developed the technique further, both practically and theoretically, and many applications have been reported. Supercritical fluids may be defined as fluids that are at temperatures and pressures above their critical temperatures and critical pressures. Supercritical fluids are now widely used in extraction, fraction, and chromatography. Supercritical fluid separations of the triglycerides in mink oils have not been reported yet, although other triglycerides in fish oil, rapeseed oil, soybean oil, and butterfat were separated and those of butterfat were identified using massspectrometry. In addition to their critical temperatures and pressures, the chromatographically important properties of supercritical fluid are the density, viscosity, and the diffusion coefficients of solutes. Above its critical point, a substance has density and solvating power approaching that of a liquid, but viscosity similar to that of a gas, and diffusivity intermediate between those of a gas and liquid. Hence, the supercritical fluids have properties which make their use as chromatographic phase very favourable. As long as intermolecular interactions are sufficiently strong, supercritical fluids are able to dissolve a variety of solutes, even those with high molecular mass and low volatility. The density of supercritical fluids, and hence the solubility and chromatographic retention of solutes, can easily be changed by changing the applied pressure. This paper deals with the supercritical fluid separations of long-chain hydrocarbons and mink oils that are very difficult to be separated by either GC or HPLC, using pressure programming technique.