The effects of liquid chromatography mobile phase buffer contents on the ionization and fragmentation of drug molecules in liquid chromatographic/ionspray tandem mass spectrometric (LC/MS/MS) determination were evaluated for simvastatin (SV) and its hydroxy acid (SVA). The objective was to improve further the sensitivity for SV by overcoming the unfavorable condition caused by the formation of multiple major adduct ions and multiple major fragment ions when using ammonium as LC mobile phase buffer. Mobile phases (70:30 acetonitrile-buffer, 2 mM, pH 4.5) with buffers made from ammonium, hydrazine or alkyl (methyl, ethyl, dimethyl or trimethyl)-substituted ammonium acetate were evaluated. Q1 scan and product ion scan spectra were obtained for SV in each of the mobile phases under optimized conditions. The results showed that, with the alkylammonium buffers, the alkylammonium-adducted SV was observed as the only major molecular ion, while the formation of other adduct ions ([M + H](+), [M + Na](+) and [M + K](+)) was successfully suppressed. On the other hand, product ion spectra with a single major fragment ion were not observed for any of the alkylammonium-adducted SVs. The affinity of the alkylammoniums to SV and the basicity of the alkylamines are believed to be factors influencing the formation and abundance of molecular and fragment ions, respectively. Methylammonium acetate provided the most favorable condition among all the buffers evaluated and improved the sensitivity several-fold for SV in LC/MS/MS quantitation compared with that obtained using ammonium acetate buffer. Better precision for SV in both Q1 and SRM scans was observed when using methylammonium buffer compared with those using ammonium buffer. The mobile phase buffer contents did not seem to affect the ionization, fragmentation and chromatography of SVA. The results of this evaluation can be applied to similar situations with other organic molecules in ionspray LC/MS/MS determination.
Copyright 2002 John Wiley & Sons, Ltd.