A comparison of methanol and isopropanol in alcohol/water/CO2 mobile phases for packed column supercritical fluid chromatography

Isopropanol and methanol were compared as alcohol/water modifiers of supercritical carbon dioxide (SC-CO2) in packed column supercritical fluid chromatography (pSFC). Isopropanol was found to allow 5 times more mobile phase water content in pSFC than methanol. Of note, a 1:1 isopropanol/water ratio was attainable in SC-CO2 relative to a maximum 9:1 ratio realized for methanol/water. In subsequent pSFC test separations of polar analytes, the methanol system easily eluted compounds from a non-polar PRP-1 column, but failed to elute the most polar analytes from a polar silica gel or diol column. By comparison, isopropanol systems could elute all of the analytes from each of these columns. While an increase in column backpressure was noted when using 60% of 1:1 isopropanol/water in SC-CO2, this was reduced by instead using 4:1:5 isopropanol/methanol/water, which maintained water content and elution strength. Thus, isopropanol can assist in increasing the water capacity of SC-CO2 and potentially facilitate the analysis of various polar hydrophilic analytes by pSFC.