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A fast and sensitive ultra-performance liquid chromatography tandem mass spectrometry method was developed for the measurement of caffeine in caffeine-laced pants and in urine and skin of a pants user. The substance and its internal standard  (N-ethylnorcotinine) were separated by reversed phase chromatography with 5 mM ammonium formate pH 3.0 and 0.3% formic acid in acetonitrile mobile phase (83:17 v/v) by isocratic elution and detected by tandem mass spectrometry operated in multiple reaction monitoring mode via positive electrospray ionization. Linearity was studied  from 1.4 to100 ng/mL range for urine, from 5 to 100 ng/cotton swab for skin caffeine and from 1.3 to 100 µg/samples for 4 cm2 textile samples. Good determination coefficients  (r2 = 0.99) were found in all cases. At three concentrations spanning the linear dynamic ranges of different samples mean recoveries of caffeine were always higher than 80% and intra-assay and inter-assay imprecision and inaccuracy were always better than 105%.  For the first time, caffeine content in this cosmetotextile was determined together with the measurement of caffeine released on the user skin, the absorbed amount with resulting urinary concentrations

Ultra-Performance Liquid Chromatography Tandem Mass Spectrometry Measurement of Caffeine in Caffeine-Laced Pants and in Urine and Skin of a Pants User