Insights into chiral recognition mechanisms in supercritical fluid chromatography V. Effect of the nature and proportion of alcohol mobile phase modifier with amylose and cellulose tris-(3,5-dimethylphenylcarbamate) stationary phases.

In enantioselective supercritical fluid chromatography (SFC) with chiral stationary phases (CSP), the elution strength of carbon dioxide is usually modulated by the use of polar organic solvents, also called modifiers. Alcohols like methanol, ethanol and isopropanol are the most commonly used co-solvents. While most applications of chiral SFC are optimized through a process of varying the co-solvent nature and proportion, only a limited number of thorough investigations have been carried out to unravel the effects of the co-solvent on the enantioseparation process. In an attempt to clarify the effect of the mobile phase co-solvent on enantioselective SFC separations, a wide range of compounds (achiral and chiral) were analyzed on an amylosic (Chiralpak AD-H) and a cellulosic (Lux cellulose-1) CSP. The influence of the modifier polarity and steric hindrance must be considered thus several different alcoholic solvents were evaluated: methanol, ethanol, 1-propanol, 2-propanol and 1-butanol, with a proportion of 10% in carbon dioxide. A selected group of racemates was further analyzed with varying proportions of each alcohol ranging from 5 to 25%. Besides, because mixtures of solvents were sometimes reported to produce unexpected results, a 50:50 mixture of methanol and ethanol was also evaluated. Chemometric methods provide some insight into the enantio-separation process and help identifying the differences between the mobile phase conditions.