Effect of Helium in Helium Headspace Carbon Dioxide Cylinders on Packed-Column Supercritical Fluid Chromatography

Abstract

Supercritical fluid chromatography of PAHs was performed with pure carbon dioxide and helium headspace carbon dioxide at various cylinder fill levels. The retention times of the PAHs increased when helium headspace carbon dioxide was used as a carrier fluid relative to pure carbon dioxide. The increased retention times were affected by the level of the liquid phase present in the helium headspace carbon dioxide cylinder. As more liquid phase was removed from the cylinder, the effect of helium on the solvating power of CO(2) was reduced because the relative amount of helium dissolved in the liquid phase decreased. Furthermore, the effect of helium headspace carbon dioxide was investigated with methanol-modified carbon dioxide mobile phases for the analysis of steroids. We observed that the relative solubility of helium in carbon dioxide resulted in longer retention times when compared to pure carbon dioxide as the liquid level of carbon dioxide decreased.

Helium Head Pressure Carbon Dioxide in Supercritical Fluid Extraction and Chromatography: Thermodynamic Analysis of the Effects of Helium

“…Consequently, it appears that, in both SFE and SFC, the complications induced by the use of HHPCO 2 can actually outweigh its benefits. Therefore, the results of this study provide computational support for the conclusion of Leichter et al 8 and Zhang and King 10 that the use of HHPCO 2 should be avoided, at least in the cases when reproducibility of solubilities in SFE and/or reproducibility of retention times in SFC is of primary concern.…”

“…King et al 7 found that the presence of helium in the extraction fluid resulted in a substantial reduction in soybean oil solubility relative to that in pure CO 2 . Leichter et al 8 used HHPCO 2 as a mobile-phase fluid in SFC of polycyclic aromatic hydrocarbons and observed a systematic decrease in retention as the liquid phase was depleted from the storage tank. They also found an analogous effect in SFC separation of steroids with helium-pressurized, methanolmodified CO 2 .…”

Polymerizations in Supercritical Carbon Dioxide

“…Furthermore, the small percentage of helium in the reaction mixture decreases the solvent strength of the medium, as determined by solvatochromic investigations. Other scientists have noted the increase in retention times in SCF chromatography − and the reduced extraction rate in SCF extractions when helium is present in the CO 2 . An acoustic technique, used by Kordikowski and co-workers, measured the composition of helium in CO 2 tanks and showed the content to be unpredictable, presumably due to common lack of equilibration in the cylinder .…”

Determination of thermodynamic properties by supercritical fluid chromatography

“…Also, in the course of usage of HHPCO 2 tanks, it has become apparent that the assumption mentioned above is not valid. Experimental evidence has been accumulated of irreproducible retention times in SFC [126][127][128] and extraction efficiencies in SFE [129][130][131] suggesting that the entrained helium acts as a negative modifier (antisolvent). Other unexpected deleterious effects that might occur with HHPCO 2 were discussed by Parcher and Xiong [132] and by Wells et al [133].…”

“…5 shows the density differences between pure CO 2 and HHPCO 2 as obtained from acoustic measurements [135]. Consequently, a growing number of authors have concluded that the use of HHPCO 2 tanks should be discouraged [128,131,132,[134][135][136]. Wells et al [136] utilised the high-pressure behaviour of He-CO 2 system (type III phase behaviour) in a remarkable demonstration of gas-liquid chromatography with a dynamic stationary phase formed by the CO 2 -rich liquid at temperatures below the critical point of the He-CO 2 mixtures and pressures above the vapour pressure of pure CO 2 .…”