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Title: THE EFFECT OF DISSOLVED HELIUM ON THE DENSITY AND SOLVATION POWER OF SUPERCRITICAL CARBON DIOXIDE

Author
item Zhang, Zhouyao
item KING, JERRY

Submitted to: Journal of Chromatographic Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/29/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: The extraction of trace chemicals, such as pesticides, as well as fat and oil from food and agricultural products is routinely practiced for the purpose of analyzing these substances. Traditionally, such extractions are accomplished using liquid solvents which may be dangerous or environmentally harmful. Recently a new technique, called analytical supercritical fluid extraction (SFE) has been developed by our laboratory and by others to replace the use of the objectable solvents. By far the most commonly employed supercritical fluid is simple carbon dioxide which is safe and environmentally compatible. This fluid is normally delivered to the extraction apparatus from a gas cylinder. We have discovered recently that cylinders which have been overpressurized with helium inside such cylinders; to facilitate the easier pumping of the extraction fluid, reduce the extraction effectiveness of the carbon dioxide (that is to say, the chemicals or fat/oil cannot be as easily extracted in the presence of helium dissolved in the carbon dioxide. The study reported on in this manuscript verifies that the presence of helium impedes the extraction of the above substances and that it should be avoided by an analyst using SFE for chemical analysis. Use of helium to overpressure carbon cylinders is particularly disadvantageous to the analyst extracting oil/fat from agricultural products such as meat, cereal grains, oilseeds, etc.

Technical Abstract: Carbon dioxide (CO2) cylinders, pressurized with helium in their headspace, are widely used in supercritical fluid chromatography and extraction (SFC and SFE). A few percent of helium are dissolved in the liquid CO2 phase of the cylinder, which can result in such problems as retention time shift and poor reproducibility in SFC, as well as reduced solubility and extraction rate in SFE. In this study, a high precision density meter and a gas chromatography equipped with a thermal conductivity detector were used to monitor the density and composition of the supercritical fluid generated from a helium headspace (HHS) CO2 cylinder over the duration of its use. These measurements were related to the solubility of soybean oil and cholesterol in the fluid. The density measurements (accurate to 10**-4 g/mL) showed that the density of the fluid was linearly proportional to the helium content of the CO2. However, the significant drop of solute solubility in helium/CO2 mixtures cannot be explained by the reduction in fluid density alone and may involve a disruption of the solvent (CO2) shell around the dissolved solute.