Measurement uncertainty for the determination of amphetamines in urine by liquid-phase microextraction and gas chromatography-mass spectrometry.

A gas chromatography-mass spectrometry method for the determination of amphetamines in urine samples by means of liquid-phase microextraction was validated, including calculation of measurement uncertainty. After extraction in the three-phase mode, acceptor phase was withdrawn from the fiber and the residue was derivatized with trifluoroacetic anhydride. The method showed to be very simple, rapid and it required a significantly low amount of organic solvent for extraction. The limits of detection were 10 and 20μg/L for amphetamine and methamphetamine, respectively. The calibration curves were linear over the specified range (20μg/L to 1400μg/L; r(2)>0.99). The method showed to be both precise and accurate and a relative combined uncertainty of 2% was calculated. In order of importance, the factors which were more determinant for the calculation of method uncertainty were: analyte concentration, sample volume, trueness and method precision.