The trace metal detection test or technique was developed during the Vietnam War in the 1960s to identify people who may have been carrying firearms against their skin. A 0.2% solution of 8-hydroxyquinoline in isopropanol sprayed on the skin and illuminated with shortwave ultraviolet (UV) light after several minutes reveals a pattern and type of metal based on trace amounts of metal transferred to the skin that are invisible under normal lighting.^[1] The technique was later adopted by police in the United States to help determine if a person had carried a firearm. In a California crime an automatic handgun carried in the waistband of a criminal reportedly produced an impression of the weapon's serial number.^[2] Presence and persistence of a detectable residue depends primarily on the amount of perspiration, length of contact, and time since exposure. A few minutes of exposure can leave detectable residue and the result can remain for up to forty-eight hours.^[3]

Preservation of the results is done with black-and-white and color photography. Use of a shortwave ultraviolet filter over the camera flash can be used to record the results, although a steady source of UV is preferred.^[4] In either case a darkened area is needed for screening purposes. Another reagent, 0.5 percent 2-nitroso-1-naphthol in acetone, eliminated the need for a UV light but is limited to a four-hour-window versus the forty-eight for 8-hydroxyquinoline.^[5]

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