Mobile phase pH and organic modifier in reversed-phase LC-ESI-MS bioanalytical methods: assessment of sensitivity,...

The aim of the work described herein was to undertake a systematic investigation of the effect of mobile phase pH and organic modifier in typical reversed-phase LC-MS methods with regard to ESI-MS response, chromatographic performance and correlation of retention time with in silico logD predictions.

For the test set of pharmaceutical analytes investigated, ESI-MS response was generally greater when employing methanol rather than acetonitrile as the organic modifier, and increases of up to tenfold were observed dependent on the pH-buffered mobile phase employed. Deleterious effects on chromatographic performance of protonated basic analyte were observed under conditions of neutral to weakly basic pH. A qualitative correlation between plots of predicted logD and observed retention time against pH was demonstrated.

In the absence of a simple and/or predictive dependence of analyte ESI-MS response on the mobile phase pH, a practical evaluation should be undertaken when absolute sensitivity is paramount. The use of in silico predictions of analyte logD to direct the development of bioanalytical assays is broadly valid, but further scrutiny is recommended in predicting the retention of ionized analyte.