Hydrogen, for example, is found everywhere on earth with various proportions of similar atoms that differ only in their mass - the hydrogen isotopes. The isotopic ratios differ from place to place, as do those of oxygen, carbon and nitrogen isotopes. Traces in the hair and bones give indications of the regional origins of food, water and air taken in by the victim. The forensic biologist runs the analyses through a database to find clues to the victim's identity.