Defining a generic column set for achiral supercritical fluid chromatography applied to pharmaceuticals or natural products

When starting a method development in supercritical fluid chromatography (SFC), the first step is usually to screen several stationary phases based on previous experience or simply based on what is available in the laboratory. However, as there are now a large number of stationary phases available for SFC, the choice of an adequate set of columns to rapidly achieve a satisfying result can be difficult. In this project, 16 columns comprising a wide diversity of stationary phases and polarities ranging from the most polar (like bare silica gel) to the least polar (like octadecylbonded-silica) were compared, based on the gradient analysis of 129 probe compounds. The set mostly comprised active pharmaceutical ingredients, natural products and a few metabolites. The columns were ranked with the help of Derringer desirability functions taking account of (i) the number of compounds eluted from the column, (ii) the elution time in a suitable time frame, (iii) the average peak width, (iv) the average peak symmetry and (v) the spreading of retention along the gradient time. The five criteria selected showed no correlation. Overall, it appeared that those columns that had a high overall score were good for several reasons, like bare silica gel, propanediol-bonded silica or pentabromobenzyloxy-bonded silica. Initially, the columns had been screened with a gradient elution starting from 5% co-solvent and ending with 50% co-solvent in CO2. However, for some most retentive columns like amide-bonded silica, too many compounds remained non-eluted from the column. To examine this column more fairly, a second elution gradient was applied that ended with 100% co-solvent. This proved effective in restoring good overall performance through the elution of the most polar compounds.