Modifier effects in open tubular capillary column supercritical fluid chromatography: Retention mechanisms from linear solvation energy relationships

Abstract

Recently, the solvatochromic comparison method was successfully applied to determine the effects of adding a modifier to the mobile phase in packed capillary columns, and we have extended this method to understand the modifier effect in open tubular capillary column supercritical fluid chromatography (SFC). SFC retention data for a wide variety of compounds at different modifier concentrations were obtained using a 10 m×50 μm I.D. SB-methyl-100 polydimethylsiloxane capillary column. Supercritical carbon dioxide at a pressure of 260 atm was used as mobile phase and methanol as modifier. The LSER regression equation used was log k′=SP 0+ l log L 16+ s π 2 H+ a α 2 H+ b β 2 H+ r R 2, where k′ represents the capacity factor and L 16 represents the partition coefficient of the solute between the gas phase and n-hexadecane at 298 K, and l, s, a, b and r are the regression coefficients. For open tubular column SFC, variation of the l and s coefficients showed a similar pattern to the packed capillary column SFC. However, the b coefficient showed a different pattern from packed capillary SFC, i.e., a gradual decrease with increasing modifier concentration. Based on these observations, the effects of a small amount of a modifier in the supercritical mobile phase and its effects on solute retention mechanisms have been discussed. For open tubular column SFC, the swelling of the chromatographic stationary phase by a modifier and carbon dioxide and the increase of dipolarity of the mobile phase are the most important contributing factors in substantiating the modifier effect on solute retention.