Estimation of measurement uncertainty for the quantification of 11-nor-delta 9-tetrahydrocannabinol-9-carboxylic acid and its glucuronide in urine using liquid chromatography-tandem mass spectrometry.
Recently, the estimation of the measurement uncertainty has become a significant issue in the quality control of forensic drug testing. In the present study, the uncertainty of the measurement was calculated for the quantification of 11-nor-delta 9-tetrahydrocannabinol-9-carboxylic acid (THC-COOH) and its glucuronide conjugate (THC-COOH-glu) in urine using liquid chromatography-tandem mass spectrometry. The procedure was based on liquid-liquid extraction of a volume of urine (800 µL) with ethyl acetate. The sources of uncertainty were identified and classified into four major categories as follows standard preparation, calibration curve, method precision and bias. The overall contribution of combined standard uncertainty on THC-COOH increased in the order of standard preparation (0.9%), method precision (10.4%), calibration curve (30.3%) and bias (58.4%) and, while calibration curve (53.0%) and bias (40.4%) gave the bigger contributions to the combined standard uncertainty for THC-COOH-glu than method precision and standard preparation, which accounted for 6.3 and 0.3%, respectively. The reliability of a measurement was expressed by stating the expanded uncertainty of the measurement result at 95% confidence level. The concentrations of THC-COOH and THC-COOH-glu in the urine sample with their expanded uncertainties were 10.20 ± 1.14 ngmL and 25.42 ± 5.01 ngmL, respectively.