Hair analysis for cocaine: Factors in laboratory contamination studies and their relevance to proficiency sample preparation and hair testing practices

Hair samples were contaminated by rubbing with cocaine (COC) followed by sweat application, multiple shampoo treatments and storage. The samples were then washed with isopropanol for 15 min, followed by sequential aqueous washes totaling 3.5 h. The amount of drug in the last wash was used to calculate a wash criterion to determine whether samples were positive due to use or contamination. Analyses of cocaine and metabolites were done by LC/MS/MS. These procedures were applied to samples produced by a U.S. government-sponsored cooperative study, in which this laboratory participated, and to samples in a parallel in-house study. All contaminated samples in both studies were correctly identified as contaminated by cutoff, benzoylecgonine (BE) presence, BE ratio, and/or the wash criterion. A method for determining hair porosity was applied to samples in both studies, and porosity characteristics of hair are discussed as they relate to experimental and real-world contamination of hair, preparation of proficiency survey samples, and analysis of unknown hair samples.