Plasma esomeprazole assay

The plasma samples for esomeprazole concentration measurement were analyzed using a high-performance liquid chromatography assay, coupled with triple quadrupole mass spectrometer (API 4000, AB SCIEX, USA) equipped with electrospray ionization in positive ion mode. A 50 μL plasma sample was prepared by protein precipitation by mixing with 10 μL of esomeprazole-d3 (500 ng/mL) and 600 μL of acetonitrile. After centrifugation at 13,500 rpm for 10 mins, 2 μL supernatant was injected into the column (Hypersil GOLD, 150*2.1 mm, 5 μm) in the mobile phase, which consisted of 10 mM ammonium formate:acetonitrile with 0.1% formic acid (50:50, v/v). The lower limit of quantification was 1 ng/mL, and precision and accuracy were less than 15% coefficient of variation. The calibration curves had adequate linearity as r >0.9959 in the ranges of 1 to 3,000 ng/mL of sample concentration.