The Role of Mobile Phase Additives in Developing and Optimising Separations of Water-Soluble Enantiomers by High-Performance Liquid Chromatography

Summary

Chiral separations are presented which demonstrate the role of mobile phase additives for enantiomer separations by high-performance liquid chromatography (HPLC).

Salts of relatively hydrophobic (log P > 2.2) aminoalcohol racemates are often resolved as free bases on chiral stationary phases (CSPs) using organic eluents modified with a base. However, efficient chromatographic separations for a number of more hydrophilic molecules of this type have been precluded by their low solubilities in organic solvents. Using a CSP we have developed acidic eluents which generate organically soluble ion pairs. The optimum acid has been identified. For a range of compounds the selectivity of acidic and basic eluents was compared and related to log P. Acidic eluents offered significant advantages for polar aminoalcohols.

In a second example a substituted 1,3-dioxan was resolved using a Cyclobond I (β) CSP. The efficiency, selectivity and capacity of this separation were measured and a comparison with results obtained using a nitrile (CN) column with ‘β’-cyclodextrin as an eluent additive was made. Data are presented to illustrate eluent/column selection. Enhanced parameters were obtained using the modified eluent.

Finally the application of several CSPs for the optical purity measurement of a methotrexate analogue is discussed. In this case it was also perceived that the solute anion would bind to serum proteins. A separation was achieved using bovine serum albumin (BSA) in a buffered eluent on a Diol column. The effects of pH, bonded phase and organic modifier are presented.

The use of eluent additives has proved to be a flexible technique for the separation and isolation of polar water-soluble enantiomers which are poorly resolved on CSPs.