Reproducibility of packed-column supercritical-fluid chromatography with helium head-pressure carbon dioxide

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 .…”

Acoustic Determination of the Helium Content of Carbon Dioxide from He Head Pressure Cylinders and FT-IR Studies of the Density of the Resulting Supercritical CO2: Implications for Reproducibility in Supercritical Experiments

“…In most analytical applications of supercritical fluids, the composition of the fluid is a vital factor in determining reproducibility. In recent years, there have been several investigations into the effects of He on the chromatographic performance of supercritical CO 2 solutions. − The He derives from the so-called He head pressure cylinders, used by many analytical groups to improve the pumping efficiency of their syringe pumps. Initially, it was believed that the He pressure would have no effect on the physicochemical properties of the CO 2 and that He would be completely immiscible with CO 2 .…”

“…show that about 3 mol % He is soluble in CO 2 at room temperature and 100 bar pressure. It has now been shown that the presence of He substantially affects the results of chromatographic separations and supercritical extractions using He head pressure CO 2 (HHPCO 2 ). − Additionally, it has been reported that, under some conditions, the density of HHPCO 2 can be reduced by more than 25% compared to pure CO 2 at the same temperature and pressure.…”

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].…”