Extraction methods for recovering drug from hair prior to MS analysis, as already discussed in previous sections, require extensive validation. One way to determine whether the drug is being completely extracted is to apply the selected solvent in sequential intervals and to observe the completeness of the extraction with time. It is also useful to compare the results of a solvent extraction method with the results obtained by a validated digestion method. Another analytical consideration is the equilibration of internal standard to assure equivalent recovery of internal standard and drug that is bound to the sample matrix. However, if the drug in question binds strongly to melanin and the melanin is not removed from the sample to be extracted, the distribution of the internal standard between melanin and the soluble fraction, and how this will affect quantitation, must be considered. An extraction method may also be evaluated by use of Standard Reference Materials (SRM) hair standards from the National Institute of Standards and Technology (NIST) program (53). These standards are immensely important and should be used to evaluate the reliability of the extraction protocol that is under consideration. Finally, another method of validating a procedure is to participate in a hair proficiency testing program whereby the method may be compared with those of other laboratories.
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