Immobilisation helps broaden extended choice of solvents

Tong Zhang and Pilar Franco examine extended solvent versatility of immobilised polysaccharide-derived chiral stationary phases for the separation of enantiomers by chromatography.

The immobilisation of polysaccharide derivatives on a silica support has been considered as a direct approach to confer a universal solvent compatibility to these highly selective chiral supports. The immobilisation broadens the choice of solvents to be used as mobile phases with these chiral stationary phases (CSPs).

In this context, Daicel Chemical Industries Ltd. has recently developed a new series of CSPs for HPLC using improved immobilisation technologies. CHIRALPAKIA, a 3,5-dimethylphenylcarbamate derivative of amylose immobilised onto silica gel, is the first of this innovative generation of CSPs. CHIRALPAKIB, the second member of this series, is now available. The chiral selector in CHIRALPAKIB is of the same nature as in CHIRALCELOD, but immobilised onto the silica gel matrix.

The main innovation of these new immobilised supports is the unprecedented possibility of using an extended series of solvents as mobile phase components and/or sample solvents without compromising the CSP stability.

These columns can be used with all kinds of miscible organic solvents, ranging from the traditional mobile phases used with Daicel non-immobilised CSPs (typically alcohols, acetonitrile, mixtures of alkane/alcohol) to mobile phases containing ethyl acetate, tetrahydrofuran (THF), methyl tert-butyl ether (MtBE), dichloromethane and chloroform, among many others.

The option to use the anon-standard' solvents in the mobile phase with these polysaccharide-based selectors allows the enhancement of chiral separations in terms of enantioselectivity, resolution degree, and analysis efficiency.

Owing to the different natures of their chiral selectors, CHIRALPAKIA and CHIRALPAKIB can offer complementary separations of enantiomers: a racemate which is not resolved into enantiomers on CHIRALPAKIA may be well separated on CHIRALPAKIB and vice versa. All these features of the immobilised CSPs allow the enlargement of their application domain at analytical scale. Two examples of chiral separations on CHIRALPAKIA and CHIRALPAKIB are shown in Fig.1.

With these immobilised CSPs, there no longer are limitations on the sample injection solvent. Solvents such as dichloromethane, acetone, THF, dimethylformamide (DMF) or even dimethyl sulphoxide (DMSO) can be safely used as sample diluents. This is highly beneficial for the automation of injections for samples issuing directly from various synthetic media and for the samples that have limited solubility in certain solvents.

The solvent robustness of these CSPs, combined with their high enantioselective performance and high loading capacity, makes them a powerful tool for the preparative separation of enantiomers.

Some enhancements in productivity with these new CSPs have already been demonstrated for certain compounds in single-column chromatography. The market introduction of the 20µm immobilised-CHIRALCELOD before the end 2004 and the preparative version of CHIRALPAKIA in Spring2005 will undoubtedly enforce the industrial applications of chiral chromatography in continuous processing mode, such as simulated moving bed (SMB).

 

Tong Zhang and Pilar Franco are with Chiral Technologies Europe, Illkirch, France. www.chiral.fr

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