The following chapter describes a method to measure argatroban in plasma samples using ultra-performance liquid chromatography combined with electrospray positive ionization tandem mass spectrometry (UPLC-ESI-MS/MS). Samples are pre-treated using methanol containing the internal standard (IS) diclofenac. The plasma extracts are dried under a stream of nitrogen. The residue is reconstituted in ammonium acetate–formic acid–water. The reconstituted plasma is injected into the UPLC-ESI-MS/MS. Argatroban and diclofenac show similar retention times in plasma. Quantification of argatroban in the samples is made by multiple reaction monitoring using the hydrogen adduct mass transitions, from a seven-point calibration curve.
