The two papers cited by Ram provide good insight into methods of hydrolyzing protein but both deal with preparation of samples for amino acid analysis. Accurate determination of trace metals in biologic samples depends heavily of proper sample preparation. This means the use of metal-free reagents and labware. Looking at the history of the accepted ambient concentrations of various trace metals in the marine environment you can see a gradual decrease in concentration. This is not an indication that the environment is getting cleaner but rather that improvements in sample handling are resulting in less contamination of the sample. See http://www.spectronus.com/uploadcache/Spectroscopy-Hot_Block.pdf, http://www.epa.gov/glnpo/lmmb/methods/icmsmeth.pdf, or http://www.elsevier.com/wps/find/bookdescription.cws_home/680840/description for additional information on methods.