Elution behavior of drugs in high-speed counter-current chromatography using on-column complexation with metal ions

In this study, determination of (nitrogen containing) drugs by on-column complexation with metal ions in high-speed counter-current chromatography (HSCCC) was investigated. Bromazepam (BMP) was strongly retained in the organic upper stationary phase (UP) of the two-phase solvent system composed of tert-butyl methyl ether-acetonitrile–water (2:2:3, v/v/v) by eluting the aqueous lower mobile phase (LP) at a flow rate of 2 mL min−1. On the other hand, BMP (200 µg mL−1) was eluted faster without retention to the organic UP with the two-phase system containing 100 μg mL−1 of copper ions (CuCl2) because a very polar BMP-Cu2+ complex was immediately formed in the aqueous LP. The dramatic change in the retention behavior of BMP resulted from on-column complexation. The on-column complexation in HSCCC was further investigated for five (nitrogen containing) drugs and seven metal ions. In the result, tizanidine and phentolamine formed complexes with Al3+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+, ambroxol formed complexes with Al3+, Fe2+, and Cu2+, but voriconazole formed no complexes with all metal ions tested.