Highly sensitive bioassay of lidocaine in human plasma by high-performanceliquid chromatography-tandem mass spectrometry

A very sensitive HPLC-tandem mass spectrometric (LC-MS-MS) method for assay ing lidocaine in human plasma was set up and fully validated. Lidocaine and an internal standard (bupivacaine) were extracted from 1 ml of alkalinized plasma with tert.-butylmethyl ether, back-extracted to a H3PO4 acidified s olution and injected into a C-18 column. Acetonitrile-26 mmol/l ammonium ac etate, pH 4.5 (70:30, v/v) was the mobile phase at a flow-rate of 1 ml/min. The effluent was detected by PE Sciex API 365 LC-MS-MS system in positive ion mode. Ionisation was performed using an atmospheric pressure chemical i onization ion source operating at 400 degrees C. The multi reaction monitor ing transition 235-->86 was monitored. Linearity was ascertained in the 0.2 -30 ng/ml range with a limit of quantitation of 0.2 ng/ml. Intra- and inter -assay precision and accuracy were less than or equal to 3.8%. The high sen sitivity and specificity achieved by the method allowed concentrations of l idocaine to be measured in plasma of healthy subjects topically treated wit h lidocaine (5% ointment) on normal skin over a 32-h period after dosing. ( C) 1999 Elsevier Science B.V. All rights reserved.