The nail biomarker has the following potential limitations in clinical study: 1) the response of the nail as an elemental biomarker is not as well characterized for some elements (e.g., Al, magnesium [Mg], copper [Cu], and Zn) as for others (e.g., Se, As, Cr, Hg, Cd). However, while this is a limitation, previous studies indicate that some useful information can be obtained for these elements; 2) in the study populations of developed countries, nails are generally environmentally sheltered and frequently washed as a result of the prevalent hygiene practices. However, nails can still become contaminated through the use of some medications and nail polishes. Elemental contamination can also be imparted to nail samples by the cutters used to produce the clippings. These limitations (polish and cutters) can largely be overcome by employing ultrasonic cleaning procedures with polar and non-polar solvents. Previous study indicates that the cleaning procedures are effective in removing or substantially reducing contamination from polishes and cutters;⁴⁹ 3) nail specimens collected in large cohort studies may include some small nail mass samples, for instance, with nail masses of 20 milligrams or less. In these small samples, some of the elements may be below the detection limits. Nevertheless, the small sample may be able to be analyzed by serial assessments of the same sample to obtain or generate the robust database for hypothesis tests.

source - http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2998551/