Modeling the competitive adsorption of sample solvent and solute in supercritical fluid chromatography

Uncommon retention behavior of a series of n-alkylbenzene homologues as well as the effect of different sample solvents on chromatographic efficiency were studied in supercritical fluid chromatography. After testing various columns, an alkylamide stationary phase was selected for detailed studies. The results showed that even a small amount of methanol originating only from the sample, overloaded the column and competitive adsorption was induced between the analytes and the sample solvent for adsorption on the stationary phase. This was indicated by the changes in column efficiency, retention and peak widths. The concentration of the analytes in the sample was negligible compared to the amount of methanol - but their adsorption was influenced by the solvent - while the adsorption of methanol remained unaffected by the n-alkylbenzenes. First, the competition was described by determining the single-component adsorption isotherms for both the analytes and their solvent, then competitive isotherms were calculated. Based on the peak profiles, bi-Langmuir and competitive bi-Langmuir isotherms were assumed. The solvent effect was modeled by a numerical method created in-house where the differential mass balance equation given by the equilibrium-dispersive (ED) model was integrated using the Rouchon algorithm. The experimental observations were confirmed by in silico experiments and additional cases involving two hypothetical analytes were studied as well.