Internal quality control samples for hair testing.

For quality assurance, soaked quality hair samples can serve as good substitutes for the detection of chronic drug use and single-dose intake in the hair testing of multi-analytes when authentic hair samples containing the relevant substances are not available as reference material. In this study, we investigated the soaking technique for 29 common pharmaceuticals and drugs of abuse by exposing drug-free hair to reference standard solutions for a few hours. The incorporated amount had a range of 0.27-4.5 ng/mg (unwashed) following 1 h of exposure in an aqueous standard solution. Following the general washing procedures, 27 %-70 % of the incorporated amount remained in the hair, indicating that the drugs penetrated into the hair cells to some extent and were not only deposited on the surface of the hair. Thus, swelling agents such as water and methanol could allow drugs to diffuse from the solution into the hair cells, similar to the incorporation of drugs from the sebum and sweat coating the growing hair. Following the routine analysis of soaked quality samples at two levels for a one-year period, the samples were proved to be homogenous within a batch, with method imprecision less than 20 % (mean: 12 %) at a low level and less than 17 % (mean: 11 %) at a high level. Furthermore, the monitoring of aliquots of the soaked quality samples in control charts showed that the soaked samples were stable within a year except for the unstable analytes chlordiazepoxide and zopiclone, in which a decline of up to 20 % was observed.
Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(Copyright © 2020 Elsevier B.V. All rights reserved.)