Investigation of retention and selectivity in high‐temperature, high‐pressure, open‐tubular supercritical fluid chromatography with CO2 mobile phase

We studied solute retention for n‐hydrocarbons, (low‐molecular‐weight) polystyrenes, an ethoxylated surfactant, and a selectivity test mix on open‐tubular columns with methyl‐, biphenyl‐, and cyanopropyl‐substituted stationary phases for pressures up to 680 atm and temperatures up to 240°C. The solute elution range varied tremendously with column choice, with the least retentive stationary phases providing the highest elution range. However, the best resolution and largest analysis range were obtained with the most retentive stationary phase and the highest pressures. Increasing the temperatures above 160°C did not cause a large increase in elution pressure for the solutes used in this study. Exceptionally large shifts in selectivity with temperatures up to about 160°C occurred for the biphenyl and cyanopropyl stationary phases. The availability of pressures higher than 680 atm, coupled with adequately retentive stationary phases used at optimal temperatures, would further increase the analysis scope of opentubular supercritical fluid chromatography.