Quantitative determination of rocuronium in human plasma by liquid chromatography–electrospray ionization mass spectrometry

Liquid chromatography–electrospray ionization mass spectrometry (LC–ESI-MS) was used for the quantification of the neuromuscular blocking agent rocuronium in human plasma. Verapamil was used as internal standard. The samples were subjected to a dichloromethane liquid–liquid extraction after ion pairing of the positively charged ammonium compound with iodide prior to LC–MS. Optimized conditions involved separation on a Symmetry Shield RP-18 column (50×2.1 mm, 3.5 μm) using a 15-min gradient from 10 to 90% acetonitrile in water containing 0.1% trifluoroacetic acid at 250 μl/min. Linear detector responses for standards were observed from 25 to 2000 ng/ml. The extraction recovery averaged 59% for rocuronium and 83% for the internal standard. The limit of quantification (LOQ), using 500 μl of plasma, was 25 ng/ml. Precision ranged from 1.3 to 19% (LOQ), and accuracy was between 92 and 112%. In plasma samples, at 20 and 4°C, rocuronium was stable at physiological pH for 4 h; frozen at −30°C it was stable for at least 75 days. The method was found suitable for the analysis of samples collected during pharmacokinetic investigations in humans.