|MadSci Network: Chemistry|
Dear Anna, the method of choice might well be Atomic Absorption Spectroscopy, which (generally) allows determination of as little as 0.02 ng/mL (reference: Applied Spectroscopy volume 37 1983 page 411). By creating a standard calibration curve, you should be able to determine concentration in shampoo samples. You can find the basics of analytical atomic absorption calibration curve/unknown concentration determination in any Instrumental Analysis text (I can recommend a few). A little research in such texts at a University library may well uncover a procedure. Feel free to email me at firstname.lastname@example.org if you have further questions. Good Luck Mike
Try the links in the MadSci Library for more information on Chemistry.