Comparison of reversed-phase and hydrophilic interaction liquid chromatography for the separation of ephedrines

The separation of highly basic solutes is an ongoing challenge, especially in achieving suitable retention and peak shapes for compounds such as ephedrines that have both high pK(a) values (≥ 9.3) and low lipophilicity (log P ≤ 1.74). In this study we investigate the application of HILIC as a potential alternative approach for the fast separation of the ephedrines phenylpropanolamine, cathine, ephedrine, pseudoephedrine and methylephedrine in doping control analysis. Using sub-2 μm bare silica bridged-ethylene hybrid (BEH) HILIC material, we evaluate the effects of organic modifier, buffer pH and concentration and column temperature on the retention and selectivity of these compounds. Highly symmetrical peak shapes for all ephedrines were achieved under HILIC conditions (A(s0.1) ≤ 1.1). We also compare the kinetic performance of the optimised HILIC separation with a previously developed high pH reversed-phase approach. van Deemter curves and kinetic plots for the two approaches are constructed and illustrate the kinetic benefits of HILIC over the reversed-phase approach. Improved mass transfer characteristics and enhanced diffusion with HILIC offers lower C-term coefficients of 1.46 and 5.68 for ephedrine with HILIC and RPLC, respectively.