Improved selectivity in detection of polar basic drugs by liquid chromatography-electrospray ionization mass spectrometry. Illustration using an assay method for the determination of famotidine in human plasma.

It is well to assume that bioanalytical chromatographic methods for the determination of polar basic drugs are developed and optimised according to a standardised procedure which involves two alternatives: (a) modifications in the sample preparation procedures, and (b) changes in the stationary phase of the chromatographic system. In this paper, a simple and rapid chromatographic procedure using a specific analytical detection method (ESI tandem mass spectrophotometric detection) in combination with a fast and efficient sample work-up procedure, protein precipitation, is presented. A demonstration of the entire chromatographic procedure is given for an HPLC method for the determination of famotidine in human plasma, a basic polar drug with poor solubility in organic solvents. In order to optimize the mass detection of famotidine, several parameters such as ionization mode, fragmentor voltage, m/z ratios of ions monitored, type of organic modifier and eluent additive, were investigated. Each analysis required 5 min. The calibration curve of famotidine in the range 1-200 ng/ml was linear with a correlation coefficient of 0.9992 (n = 6), and a detection limit a signal-to-noise ratio of 3 was approximately 0.2 ng/ml. The within- and between-day variations in the famotidine analysis were 5.2 (n = 6) and 6.7% (n = 18), respectively. The applicability of this method was also demonstrated for the analysis of plasma samples in a Phase-I human pharmacokinetic study.