Peak deformations in preparative supercritical fluid chromatography due to co-solvent adsorption

Abstract

In supercritical fluid chromatography (SFC) the mobile phase comprises of carbon dioxide (CO₂) as main solvent and smaller amounts of an organic polar solvent (often an alcohol) as co-solvent. The co-solvent is considered to function by changing the overall polarity of the eluent, i.e. by acting as a "modifier". However, recent studies indicate that the co-solvent methanol can also adsorb to some common SFC stationary phases. Hence, the co-solvent should also be able to function as an "adsorbing additive", i.e. an eluent component that competes with the injected solutes about the stationary phase surface. In this study it was found by fitting different mechanistic models to systematic experimental data, that the co-solvent methanol can have both functions: at low co-solvent fractions, methanol acts as an additive whereas at larger fractions it acts as a modifier. Moreover, it was found that when the co-solvent adsorbs more strongly to the stationary phase than the solute, "bizarre" deformations of the preparative band shapes can occur. This is illustrated by a solute that converts from a normal "Langmuirian" band shape to an "anti-Langmuirian" shape when changing from neat carbon dioxide (CO₂) to an eluent containing co-solvent. This peak shape transition is dependent on both (i) the relative retention of the solute and co-solvent to the stationary phase in eluent containing neat CO₂ and on (ii) the relative retention of the additive perturbation peak and the solute peak in eluent containing also co-solvent.

Keywords: Adsorption strength; Anti-Langmuir band shape; Langmuir band shape; SFC; Solvent adsorption; Supercritical fluid chromatography.