Separation of basic compounds of pharmaceutical interest by using nano-liquid chromatography coupled with mass spectrometry

Nano-liquid chromatography–mass spectrometry (nano-LC–MS) was evaluated for the separation of basic compounds of pharmaceutical interest. The separation of selected β-blockers, namely nadolol, oxprenolol, alprenolol and propranolol in the presence of terbutaline was performed using two 75 μm I.D. capillaries packed with two different RP18 stationary phases (SP). The best results concerning resolution and efficiency were achieved using the SP where free silanol groups were not present. As expected, this latter SP proved to be very efficient and symmetry factors were observed mainly in the case of the more retained analytes. Baseline resolution of all studied basic compounds was achieved with the Cogent® bidentate C18 silica phase (CBC18) eluting analytes at 800 nL/min with a mobile phase containing 500 mM ammonium acetate pH 4.5–water–methanol (1:8:91, v/v/v). The separated basic compounds were revealed using on-column UV detector at 205 nm and electrospray-ion-trap mass spectrometer (ESI-MS). The packed capillary was connected to the MS through a commercial sheath liquid interface or a sheathless nano-spray interface and in both cases the sensitivity was studied and the results compared. Limit of detection (LOD) as low as 0.1 ng/mL was measured for nadolol using the sheathless nano-spray interface and the capillary column packed with the CBC18 stationary phase.