Comprehensive investigation of the influence of acidic, basic, and organic mobile phase compositions on bioanalytical assay sensitivity in positive ESI mode LC/MS/MS

The sensitivity and accuracy of a bioanalytical method is critical in defining the pharmacokinetic (PK) parameters of a potential new chemical entity (NCE). Inhaled therapeutics and low dose NCEs present one of the most significant analytical challenges to the bioanalyst, due to their low systemic concentration. The sensitivity of a bioanalytical LC/MS/MS based assay can be influenced by multiple parameters, including: mobile phase composition, extraction efficiency and chromatographic performance. In this work, we discuss the influence of acidic (pH 3), and basic (pH 10) aqueous mobile phases in conjunction with the two most common organic modifiers used in HPLC, acetonitrile and methanol, on the assay sensitivity of twenty-four probe pharmaceuticals in solvent and biological fluid extract. The study showed that when the test probe pharmaceuticals were analyzed with basic aqueous mobile phases compared to standard acidic conditions the following results were observed: increases in chromatographic peak area ranging from 1.2 to 9.6 fold for twenty-one of the test compounds as well as increased signal-to-noise for greater than seventy percent of the compounds. This observed increase in the MS response was not necessarily related to the later elution of the analyte in a higher organic composition under basic conditions. This was demonstrated as seven out of the twenty-four (approximately thirty percent) of the probe pharmaceuticals tested, eluted earlier, or with the same retention time, under basic conditions, and still produced a greater signal-to-noise when analyzed under these basic conditions. Also observed were decreases in chromatographic peak width, and increases in the retention time of very hydrophilic pharmaceutical compounds. The effect of the mobile phase combinations on the retention and MS response of the choline-containing phospholipids present in precipitated plasma was also investigated, as these analytes are a major source of interference when developing a bioanalytical assay.

► We investigate mobile phase modification and its benefit to LC/MS/MS based bioanalytical assays.
► Increase in the signal-to-noise was observed for twenty-two out of the twenty-four probe pharmaceuticals tested under basic versus acidic conditions.
► An increase in the chromatographic retention of poorly retained compounds was observed with basic mobile phase.
► Phospholipids present in protein precipitated rat plasma were slightly affected by a change in mobile phase pH however greatly affected by the organic modifier.
► Mobile phase optimization is critical for LC/MS/MS based bioanalytical assays.