All of the following methods are useful for detecting heavy metals, except:

Paraffin test (dermal nitrate test): Is used to determine if a suspect has discharged a firearm and not used for detecting heavy metals. In this test, the blowback powder residues on the back of the palm and web of the alleged firers hand are collected on paraffin cast and tested for nitrate using diphenylamine reagent. A blue colour develops when residue is present. This test is now obsolete because of false positive results. Ref: Essentials of Forensic Medicine and Toxicology By Dr K S Narayan Reddy, 27th Edition, Pages 207-8