Addition of methanol to the mobile phase in packed capillary column supercritical fluid chromatography Retention mechanisms from linear solvation energy relationships

Linear solvation energy relationships (LSERs) were used to verify previously proposed retention mechanisms when an organic modifier is used with an octyldecylsilane bonded phase in packed capillary column supercritical fluid chromatography. Supercritical fluid chromatographic retention data for a wide variety of compounds at different modifier concentrations were obtained using a 60 cm×200 μm I.D. ODS packed 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_o+llog L¹⁶+sπ₂^H+aα₂ ^H+bβ₂^H+rR₂, where k′ represents the capacity factor and L¹⁶ 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. By increasing the percentage of modifier added to the mobile phase, the l coefficient decreased gradually. The b and s coefficients were greatly decreased on moving from neat carbon dioxide mobile phase to 2% methanol modifier, thereafter showing a gradual decrease with increasing modifier concentration. From these observations, we substantiate the conclusion that when a small amount of modifier (up to 2%) is added, the predominent modifier effect on the ODS bonded phase is dynamic coating of free silanols on the surface of the stationary phase, and when more modifier is added, another effect, i.e. an increase in the density of the mobile phase leading to an increase in elution strength of the eluent, becomes more important.