I. Chromatography with supercritical fluids. Investigation of operating parameters for liquid solid fluid chromatography. II. Performance of a sulfur sensitive flame photometric detector. Application to sulfur compounds in cigarette smoke

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1973

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Although Supercritical Fluid Chromatography SFC has been introduced only in the last decade, it has shown promise for the analytical separation of high molecular weight compounds which could not be achieved in comparable time and manner by any other technique. SFC has not become very popular because of the difficulties involved in regulating and maintaining pressures within close limits and the lack of a suitable detector which can directly measure physical changes in the fluid. Although a definite hazard is involved in the handling of supercritical fluids, the safety requirements are often overestimated. High pressure, high resolution liquid chromatography HPLC as the closest competitor has experienced a tremendous improvement within the last few years and will prevent SFC from becoming a widespread and universal analytical technique, although there is still room for special applications. This work deals with the investigation of the operating parameters for SFC. The elution behavior of a substance was found to be primarily a function pf the pressure and temperature of the fluid and temperature of the fluid and to a far smaller extent on the column used. Small differences in the adjustments of pressure, especially close to the critical conditions yielded significantly large differences in elution behavior. The distribution coefficient k can be readily changed by an increase or decrease in pressure of only a few psi, especially in the neighborhood of the critical conditions. It is favorable to work some 10 or 20[degrees] C above the critical temperature, since each column has a certain pressure drop and a partition gradient would exist along the column. Although small particle size packings decrease the height of an equivalent plate HETP, high pressure drops result. It was found that a compromise between particle size and pressure drop is necessary. Programming techniques comparable to temperature programming in gas chromatography and solvent polarity gradient techniques in liquid chromatography are technically difficult to do in SFC. Although inverse temperature programming should have a similar effect as an increase of pressure i.e. accelerated migration, severe baseline drift occurs. The addition of a small amount of a polar moderator to a nonpolar carrier fluid gives similar results as in HPLC. Polar compounds are eluted faster with less tailing. SFC can be regarded as a bridge between GC. and HPLC and represents a very useful separation method for a wide variety of high molecular weight compounds.

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