Three-dimensional SF chromatogram. SF chromatographywas performed using SF carbon dioxide containing methanol (100 ll/min) at a total flow rate of 2.2 I/min, pressure of 160 bar and temperature of 60%. Samples: racemic N-acetyl-tert-butyl esters of S-benzylcysteine (N-Ac-Cys(S-Bzl)OBuf) and Nind-terf-butyltryptophan (N-Ac-Trp(Nind-But) OBu'). Column: stainless-steel tubing, 20 cm length and 4 mm i.d., packed with N-formyl-L-valyl-aminopropylsilanized silica.
Enantiomeric pairs of cysteine derivatives eluted first, followed by those of the tryptophan derivatives. l h e o-isomers of both racemic samples eluted faster than the L-isomers. The elution order was identical with that of liquid chromatographic separations using binary solvents consisting of a diluent and stronger solvent. The chromatographic process responsible for recognition of the enantiomers in SF is assumed to be similar to that in organic solvent phase systems [6,7]. In either case, the enantiomeric retention order should be related to the stability difference of the diastereomeric complex formed by the solutes and chiral stationary phase in the liquid or SF.
Fast optical resolution of the enantiomers was also performed by preparative scale SFchromatography.The resultswill be reported in a future paper.