Anomalous effects of helium head pressure carbon dioxide in supercritical fluid chromatography and extraction

Helium head pressure carbon dioxide cylinders are commonly used to facilitate the delivery of liquid CO 2 to supercritical fluid extraction and chromatographic pumps. It is usually tacitly assumed that the helium used to increase the delivery pressure of the CO 2 cylinders is completely insoluble in liquid CO 2 and thus remains isolated in the head space of the delivery cylinder. This assumption is invalid because up to 5 mol% helium can be entrained in the liquid CO 2 delivered from helium head pressure cylinders. Significantly, contamination of liquid CO 2 with even small amounts of helium can cause many unforeseen and usually deleterious effects in supercritical fluid chromatography and extraction schemes. The observed anomalies include decreased density of the fluid phase, irreproducible extraction and retention, ghost peaks, and even phase separation within the column or extraction vessel.