Chiral separation of phosphine-containing alpha-amino acid derivatives using two complementary cellulosic stationary phases in supercritical fluid chromatography.

Enantiomeric separations of six amino-acid derivatives have been studied using packed-column supercritical fluid chromatography with two polysaccharide-based enantioselective stationary phases: cellulose tris(3,5-dimethylphenylcarbamate) and cellulose tris(3-chloro-4-methylphenylcarbamate) (Lux Cellulose-1 and -2). The effect of analyte structure on retention and separation was studied. Varied mobile phase compositions were investigated: alcohol modifier percentage was increased from 3 to 40% but smaller amounts were most effective in separating these compounds. Besides, ethanol was preferred to methanol or isopropanol as it proved to be a good compromise to achieve sufficient resolution in a reasonable analysis time. Moreover, a carbon dioxide-ethanol mixture allows performing analyses in safe and green conditions. The effect of temperature at constant mobile phase composition was explored between 10 and 40 degrees C. In most cases, increasing the temperature improved the chiral separation, up to an optimum temperature. The results are discussed in line with the structure variation of the racemic derivatives analyzed and the two columns are compared. The two columns were shown to provide complementary selectivities for the investigated solutes: whereas Lux 1 provided separation for five of the six racemates, Lux 2 could resolve the last racemic mixture. Finally, optimized conditions of separation are defined.