External contamination of hair by cocaine: an issue in forensic interpretation

This paper explores the variables by which hair samples may become contaminated with cocaine and thereby generate false positives during analysis of hair samples. A novel method for following the incorporation and removal of cocaine from hair was developed. This method allowed a large number of specimens to be analysed under a variety of conditions with high precision. The quantity of cocaine was carefully followed in each step of a published procedure. Regardless of washing technique, a substantial amount of cocaine could still be found in the final hair digest. Very few of our externally contaminated samples could be identified as externally contaminated by previously published criteria and washing procedures. Attempts to further decontaminate this hair were without success. Our data strongly suggest that external contamination of hair by drugs of abuse may make the interpretation of forensic results problematical.