Determination of ivermectin B(1a) in animal plasma by liquid chromatography combined with electrospray ionization mass spectrometry.

A novel, sensitive and specific method for the quantitative determination of ivermectin B(1a) in animal plasma using liquid chromatography combined with positive electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) is presented. Abamectin was used as the internal standard. Extraction of the samples was performed with a deproteinization step using acetonitrile. Chromatographic separation was achieved on a Nucleosil ODS 5 microm column, using gradient elution with 0.2% (v/v) acetic acid in water and 0.2% (v/v) acetic acid in acetonitrile. The method was validated according to the requirements defined by the European Community. Calibration curves using plasma fortified between 1 and 100 ng ml(-1) showed a good linear correlation (r > or = 0.9989, goodness-of-fit coefficient < or =8.1%). The trueness at 2 and 25 ng ml(-1) (n = 6) was +4.2 and -17.1%, respectively. The trueness and between-run precision for the analysis of quality control samples at 25 ng ml(-1) was -4.0 and 11.0%, respectively (n = 16). The limit of quantification of the method was 1.0 ng ml(-1), for which the trueness and precision also fell within acceptable limits. Using a signal-to-noise ratio of 3 : 1, the limit of detection was calculated to be 0.2 ng ml(-1). The specificity was demonstrated with respect to ivermectin B(1b). The method was successfully used for the quantitative determination of ivermectin B(1a) in plasma samples from treated bovines, demonstrating the usefulness of the developed method for application in the field of pharmacokinetics. Copyright 2002 John Wiley & Sons, Ltd.