Comparison of helium head pressure carbon dioxide and pure carbon dioxide as mobile phases in supercritical fluid chromatography

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 .…”

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

“…More recent work, however, presents mounting indications that the above assumption is questionable at best and that the entrained helium acts as a "negative modifier" in both SFE and SFC. Görner et al 4 observed a considerable increase in SFC retention times when using HHPCO 2 as compared with pure CO 2 at the same temperature, pressure, and flow rate. They ascribed their observation to a decrease in the density of HHPCO 2 relative to that of pure CO 2 and used the Lee-Kesler equation of state 5 (EOS) to model the density decrease.…”

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].…”