Application of Chiralcel OJ in Supercritical Fluid Chromatography for the Resolution of Different Groups of Frequently Used Drug Racemates AN VAN OVERBEKE,* PAT SANDRA, 2 ANDREI MEDVEDOVICI, 2 WILLY BAEYENS, 1 AND HASSAN Y. ABOUL-ENEIN 3 1 Laboratory for Drug Analysis, Faculty of Pharmaceutical Sciences, University of Ghent, Belgium 2 Department of Organic Chemistry, University of Ghent, Belgium 3 Biological and Medical Research, King Faisal Specialist Hospital and Research Centre, Saudi Arabia ABSTRACT Supercritical fluid chromatography was applied to evaluate the direct chiral discriminative properties of ChiralcelOJ, a tris(4-methylbenzoate)cellulose col- umn towards frequently administered drugs. Two groups ofacidic drugs,profen and barbiturate derivatives, and a few basic drugs of the benzodiazepine type were ana- lysed.The effect on enantioselectivity of carbon dioxide when using methanol or ace- tonitrile as primary modifier was studied. Acetonitrile proved to be a good alternative for methanol, especially for the profen compounds that were not well resolved using metha- nol.The results were compared to normal phase chromatographic applications on the same column. SFC was not necessarily superior to high-performance liquid chromatog- raphy. Chirality 9:126–132, 1997. © 1997 Wiley-Liss, Inc. KEY WORDS: supercritical fluid chromatography; profens; 2-arylpropionic acids; barbi- turates;benzodiazepines; tris(4-methylbenzoate) cellulose Supercritical fluid chromatography (SFC) is a valuable additionaltechnique to gas chromatography (GC) and high-performance liquid chromatography (HPLC). SFC is often promoted as an alternative to normal phase liquid chromatography (LC) because of its increased resolving power within shorter analysis times in many cases. As su- percritical fluids possess higher diffusion coefficients than liquids,the increased diffusivity gives generally sharper chromatographic peaks and,thus, better sensitivity. Higher linear solvent velocities can be applied, resulting in faster analyses. 1–3 Because ofthe low polarity of carbon dioxide, the pri- mary SFC eluent, normalphase LC columns are obvious candidates for SFC application. Different derivatised cellu- lose chiralstationary phases (CSP; packed columns from Daicel Co., Tokyo, Japan) have already been applied in super-or subcritical FC. 3–12 The tribenzoate derivative of cellulose (Chiralcel OB) has been the subject of several comparative studies between LC and SFC. 3,4 The wide ap- plicability of tris(3,5-dimethylcarbamate) derivatives of cel- lulose (Chiralcel OD) and amylose (Chiralpak AD) in di- rect HPLC separations was confirmed by their success in resolving various racemates in SFC. 6–12 In this preliminary study,Chiralcel OJ,a tris(4-methylbenzoate) cellulose de- rivative coated on a silica support, has been evaluated in SFC. The marked stereoselectivity of the cellulose poly- mers results from a complex ensemble of interactions be- tween the solute and both achiral and chiralsites ofthe CSP. Small alterations in its steric structure may exert sig- nificanteffects on the interaction mechanism of one or both enantiomers of a racemic solute. The chiral recogni- tion processes responsible for the direct separation of such a wide and varied range of racemates have not been eluci- dated in detail thus far for this type of polymeric phase. Cellulose CSPs generally exist as b-polymeric chains of derivatised D-glucose residues with b-1,4-linkages. These chains lie side by side, possess a certain degree of rigidity, and assume an extended helical structure. The recognition mechanism is determined by the formation of interstrand and intrastrand inclusion complexes of (part of) the analyte in between the chiral cavities in the cellulose polymers. These cavities have a high affinity with the aromatic groups.The formation of reversible diastereomeric com- plexes between the analytes and the cellulose polymer is simultaneously governed by different points of interaction at the specific derivatives of the glucose units, 4-methyl- benzoate moieties in the case of ChiralcelOJ. The main adsorbing site is considered to be the polar ester function. These groups can interact with the solutes via hydrogen- bonding and dipole-dipole interaction. The benzoylmoi- eties provide interaction possibilities through charge trans- fer (p-p) formation, e.g.,with aromatic groups of solutes. The steric requirements giving successful chiral resolution of a racemic solute are assumed to be very strict. 13–16 *Correspondence to: A. Van Overbeke, Laboratory for Drug Analysis, Uni- versity of Ghent, Harelbekestraat 72, B-9000 Ghent, Belgium. Received for publication 25 July 1996; Accepted 10 October 1996 CHIRALITY 9:126–132 (1997) © 1997 Wiley-Liss, Inc.