ROLE OF ADDITIVES IN PACKED-COLUMN SUPERCRITICAL FLUID CHROMATOGRAPHY - SUPPRESSION OF SOLUTE IONIZATION

Tailing of polycarboxylic acids in packed column supercritical fluid chromatography (SFC) is primarily due to solute ionization. Very acidic additives improve the peak shape of acidic solutes by suppressing solute ionization. Coverage of active sites appears to be a secondary function of additives. The sorption of acidic additives was measured on five stationary phases used in packed column SFC. Surface coverages (at constant mobile phase composition) varied by more than 50-fold (0.4 to 21%), depending on the stationary phase identity. Within the group of additives used, coverage was independent of additive identity or concentration but was inversely proportional to modifier concentration. Chromatographic peak shapes of polyfunctional acidic solutes were also observed under the same conditions as used for the surface coverage measurements. Solute peak shapes depended on the acid strength of the additives but were unrelated to the amount of additive retained on the column.