Determination of methamphetamine and methylphenidate in urine by liquid chromatography/time-of-flight mass spectrometry coupled with solid-phase dispersive extraction, and its pharmacokinetic application

Abstract

A simple and rapid analytical method for the determination of methamphetamine (MA) and methylphenidate (MPD) in urine was developed, which involves liquid chromatography/time-of-flight mass spectrometry (LC/TOF-MS) and solid-phase dispersive extraction (SPDE). The developed method could be used in pharmacokinetic studies. Oasis® WCX gel was used in SPDE. LC separation was carried out on a Poroshell 120 EC-C18 column (100 mm × 4.6 mm I.D., 2.7 µm) using a mixture (30:70) of methanol/10 mM ammonium formate aqueous solution (pH 3.0). The calibration curves showed good linearity in the range of 2 to 100 ng/mL for MA and 0.5 to 200 ng/mL for MPD. The limits of detection were 0.5 ng/mL and 0.1 ng/mL, and the limits of quantification were 2 ng/mL and 0.5 ng/mL for MA and MPD, respectively. The average recoveries of MA and MPD from urine samples spiked with MA at 5 and 50 ng/mL and MPD at 1 and 100 ng/mL were 93.9–94.8% (RSD: 1.6–2.1%) and 91.5–106.5% (RSD: 1.3–3.0%), respectively. Periodic analyses of urine samples collected from a volunteer receiving clinical doses of MPD revealed the presence of ritalinic acid (RA), an inactive major metabolite of MPD. Furthermore, when the RA/MPD ratios corrected by the urinary creatinine level were plotted against time, the regression curve showed linearity after 4 h. Pharmacokinetic study revealed that the extrapolated line of the regression line reached the origin in the graph, suggesting that the regression line could be used to calculate the dosage and timing of administration.