Analysis of 11-nor-9-carboxy-Δ⁹-tetrahydrocannabinol in urine samples by hollow fiber-liquid phase microextraction and gas chromatography–mass spectrometry in consideration of measurement uncertainty

Marijuana abuse can be detected by means of toxicological analysis of the most important metabolite 11-nor-9-carboxy-Δ⁹-tetrahydrocannabinol (THC-COOH) in urine samples. The aim of this study is the establishment of the detailed procedure for analysis of THC-COOH in urine by combination of hollow fiber-liquid phase microextraction (HF-LPME) and gas chromatography–mass spectrometry (GC–MS). The conditions of hydrolysis and extraction were optimized. The method was shown to be very simple and rapid, and a low amount of organic solvent was necessary for extraction. The limit of detection was 1.5 ng/ml. The calibration curves were linear over the specified range (2.0–170 ng/ml; r ²  > 0.99). The main sources of uncertainty were found to be analyte concentration, accuracy, method precision and sample volume. The effect of the analyte concentration on the overall combined uncertainty was most significant. The developed method was successfully applied to a human urine standard reference material at two levels of concentration. The obtained relative combined uncertainty was 8 %, which can be considered acceptable according to international guidelines. The present method seems very useful in clinical and forensic toxicology, because of its simplicity, rapidness and inexpensiveness.