I was just reading scientific and forensic journals regarding test method and procedure.

Before testing hair, it is cleaned with methylene chloride (paint stripper, more or less) or better yet, petroleum ether (distilled gasoline from what I can understand).

(Petroleum ether is only available to accredited labs, classrooms, ext. It's extremely flammable. It is required storage in ventilated cabinets.)

The hairs, I learned are washed with these substances to remove thc from the outside of the hair follicle. This is to disprove the possibility of the second hand smoke excuse.

Sounds like paint stripper is the best readily available substance to remove thc, unless you work at a lab. (definitely something to avoid getting in eyes or inhalation)