How about a fluorescent marker?

Would Tetracycline work in hair?

Hilary S_W

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Hilary Le Q. Stuart-Williams Ph.D.
(Climate, behaviour study and ecology using stable isotopes)
Research Officer - Stable Isotope Laboratory
School of Environmental Sciences, University of East Anglia
Norwich NR4 7TJ, England
Telephone 01603 593837 (Work), 01603 507719 (Fax)
 01603 503430 (Home)

More data at : www.uea.ac.uk/~sil/