Influence of Different Washing Hair Procedures in the Quantitative Determination of THC and THCCOOH

Environmental exposure can affect hair analysis, making decontamination a critical preanalytical step. No standardized washing protocol exists, and laboratories use different solvent combinations. This study evaluated the impact of three common hair decontamination procedures on THC and its primary metabolite, THCCOOH, in cannabinoid-positive hair samples. Hair samples were longitudinally divided into three aliquots and subjected to three different washing protocols. After alkaline hydrolysis, THC and THCCOOH were extracted by liquid-liquid extraction and analyzed by GC-MS and GC-MS/MS (NCI), respectively. In selected cases, washing solvents were also analyzed to assess analyte loss during decontamination. THC concentrations ranged from 0.03 to 3.9 ng/mg and were significantly influenced by the washing procedure. Compared with Protocol A, mean THC losses of 18.5% and 54.2% were observed after Protocols B and C, respectively, with reductions exceeding 80.0% in several samples following Protocol C. In some cases, this decrease was sufficient to shift samples from positive to negative relative to the 0.05 ng/mg cut-off. Analysis of washing solvents confirmed substantial extraction of THC during Protocol C, particularly in the dichloromethane fraction following the aqueous wash. In contrast, THCCOOH concentrations showed minimal variability across all washing procedures, with mean losses below 10.0%. The choice of washing protocol markedly affects THC quantification in hair, potentially leading to underestimation and misclassification, whereas THCCOOH remains largely unaffected. These findings highlight the need for standardized decontamination procedures to ensure reliable and comparable results in forensic and clinical hair analysis.