Hair is one of the most defining characteristics of mammals.  Like other body tissues, it contains minerals that are deposited as the hair grows, hence why we can identify so much from hair analysis.

In order to ensure that results are as accurate as possible, taking a proper sample from the appropriate area is important.

The hair sample for a human should be taken from the base of the hair, close to the scalp, cutting as close to the skin as possible and keeping the 1-1.5 inch sample with the fresh growth.  

If the hair is longer than 1.5 inches snip the ends off  that would have been furthest from the skin and discard. 

It is important once the sample has been taken not to wrap in aluminum foil, saran wrap, or put into a plastic bag, these can all contaminate the sample.  Put the fresh cut sample straight into a paper envelope by itself and seal.