Nonaqueous capillary electrophoresis-mass spectrometry for separation of venlafaxine and its phase I metabolites

Aqueous and nonaqueous capillary electrophoresis (NACE) were investigated f or separation of venlafaxine, a new second-generation antidepressant, and i ts three phase I metabolites. Working at basic pH, around the venlafaxine p K(a) value, was effective in resolving the investigated drugs, but created considerable peak tailing. To overcome electrostatic interactions between a nalytes and silanol groups, investigations were also carried out at acidic pH. However, despite the addition of up to 50% v/v of organic solvents (e.g ., methanol or acetonitrile), complete separation of the studied compounds was not possible. NACE was found to be an appropriate alternative to resolv e venlafaxine and its metabolites simultaneously. Using a conventional capi llary (fused-silica, 64.5 cm length, 50 mum inner diameter), and a methanol -acetonitrile mixture (20/80 v/v) containing 25 mM ammonium formate and 1 M formic acid, complete resolution of these closely related compounds was pe rformed in less than 3.5 min. Selectivity, efficiency and separation time w ere greatly affected by the organic solvent composition. As the electric cu rrent generated in nonaqueous medium was very low, the electric field was f urther increased by reducing the capillary length. This allowed a baseline resolution of venlafaxine and its three metabolities in 0.7 min. Selectivit y was compared in aqueous and nonaqueous media in relation to the acid-base properties of the analytes as well as to the solvation degree. Finally, th e method successfully coupled on-line to mass spectrometry with electrospra y ionization interface allowed significant sensitivity enhancement.