Quantitative analysis of clindamycin in human plasma by liquid chromatography/electrospray ionisation tandem mass spectrometry using d 1 ‐ N ‐ethylclindamycin as internal standard

A new method for the quantitative analysis of clindamycin in human plasma by liquid chromatography/electrospray ionisation tandem mass spectrometry (LC/ESI‐MS/MS) is presented. Recently published methods possess a disadvantage because of their use of internal standards with extraction and ionisation properties different from those of clindamycin. To avoid these problems, d 1 ‐ N ‐ethylclindamycin was synthesised for use as internal standard by N‐demethylation and subsequent d 1 ‐N‐ethylation. Plasma sample preparation was done by an easy and rapid liquid–liquid extraction using ethyl acetate. The method was validated in the expected concentration range for a pharmacokinetic study. Calibration graphs were linear within the range 0.05–3.2 μg/mL plasma. Intra‐day precision was between 0.90% (2.8 μg/mL) and 3.25% (0.05 μg/mL), inter‐day variability was found to be between 1.33% (0.7 μg/mL) and 2.60% (0.05 μg/mL). Inter‐day accuracy showed deviations between 0.4% (0.05 μg/mL) and −4.8% (0.2 μg/mL). The method is simple and robust, and has been applied to the batch analysis of clindamycin during a pharmacokinetic study. Copyright © 2002 John Wiley & Sons, Ltd.