Establishment of the measurement uncertainty of 11-nor-D9-tetrahydrocannabinol-9-carboxylic acid in hair

The quantitative analysis of 11-nor-D9-tetrahydrocannabinol-9-carboxylic acid (THCCOOH) in hair requires a sensitive method to detect a low-pg level. Before applying the method to real hair samples, the method was validated; in this study, we examined the uncertainty obtained from around the cut-off level of THCCOOH in hair. We calculated the measurement uncertainty (MU) of THCCOOH in hair as follows: specification of the measurand, identification of parameters using “cause and effect” diagrams, quantification of the uncertainty contributions using three factors, the uncertainty of weighing the hair sample, the uncertainty from calibrators and the calibration curve, and the uncertainty of the method precision. Finally, we calculated the degrees of freedom and the expanded uncertainty (EU). The concentration of THCCOOH in the hair sample with its EU was (0.60 ± 0.1) × 10−4 ng/mg. The relative uncertainty percent for the measurand 0.60 × 10−4 ng was 9.13%.In this study, we also selected different concentrations of THCCOOH in real hair samples and then calculated the EU, the relative standard uncertainty (RSU) of the concentration of THCCOOH in the test sample [ur(c0)], the relative uncertainty percent, and the effective degree of freedom (veffveff). When the concentrations of THCCOOH approached the cut-off level, ur(c0) and the relative uncertainty percent increased but absolute EU and veffveff decreased.