Determination of pharmaceutical compounds in aqueous dimethyl sulfoxide by electrospray ionization mass spectrometry

Standard solutions of reserpine, dextromethorphan, imipramine and amitriptyline in dimethyl sulfoxide (DMSO), DMSO containing 0.1% formic acid, and DMSO/H(2)O (1:1, v/v) containing 0.1% formic acid were analyzed by positive electrospray ionization mass spectrometry (ESI-MS). A triple-quadrupole mass spectrometer equipped with a TurboIonspray (TIS) interface was used for the ESI-MS analyses. The samples dissolved in the respective DMSO solution were infused directly into the mass spectrometer at 10 microL/min using an infusion pump. The positive Q1 full-scan (m/z 50-650) mass spectrum of DMSO (MW = 78) showed three main peaks at m/z 79, 101 and 179, corresponding to the protonated molecule [DMSO+H](+), and the sodiated adducts [DMSO+Na](+) and [2DMSO+Na](+), respectively. The ESI of the compounds in DMSO and DMSO containing 0.1% formic acid was promoted by using the TIS gas (GS2) combined with the nebulizer gas (GS1), and TIS source temperature set to 250 degrees C. In contrast, samples dissolved in DMSO/H(2)O (1:1, v/v) containing 0.1% formic were sprayed at a lower temperature (100 degrees C) using only the nebulizer gas. The TIS voltage (V) was optimized in order to establish the lowest voltage necessary to achieve optimum ESI of each pharmaceutical compound with the voltage maintained below the onset potential required to produce a corona discharge at the TIS probe (sprayer). Detection limits of 10 ng/mL were achieved for reserpine, dextromethorphan, imipramine and amitriptyline in each solvent composition.