Efficiency Studies in Supercritical Fluid Chromatography: Importance of Thermal Diffusivity Near the Critical Point

Abstract

Fast efficient chromatography is important in order to resolve the individual components of complex biological mixtures. Classically, thermal effects that decrease chromatographic efficiency accompany both HPLC and SFC. In HPLC, viscous flow causes the mobile phase temperature to increase from the inlet to the outlet of steel analytical scale (2mm-5mm ID) columns. In SFC, a pressure drop is accompanied by isenthalpic expansion and cooling of the mobile phase. As a result, radial temperature gradients form across the column due to heat exchange with the column surroundings. These dynamic radial gradients form a radial distribution of retention properties and excess chromatographic band spreading. Thermal effects can be minimized in HPLC at the cost of narrow bore capillary columns which help to dissipate the heat formed at high flow rates. In SFC however, commercially available instruments are capable of handling the increased flow rate due to the high diffusivity of supercritical CO2. In addition, retention near the critical point is sensitive to variations in temperature and pressure. This allows chemists the ability to tune retention properties without changing mobile phase composition. Recent research has shown that efficiency in this region can be significantly improved by operating the SFC column under near-adiabatic conditions. The results presented here suggest that the thermal diffusivity of the mobile phase near the column outlet may be an important factor in controlling chromatographic efficiency near the critical point. In order for chromatographers to be able to take full advantage of the speed and tunability of operating near the critical point, a better understanding of the factors causing excess efficiency loss in this region is needed.