Trace metal detection test

The trace metal detection test or technique was developed during the Vietnam War in the 1960s to identify individuals who may have been carrying firearms against their skin. A 0.2% solution of 8-hydroxyquinoline in isopropanol is sprayed on the skin. After several minutes, the skin is illuminated with shortwave ultraviolet (UV) light, revealing a pattern and type of metal based on trace amounts of metal transferred to the skin, which are invisible under normal lighting. 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. The presence and persistence of a detectable residue depend 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.

Preservation of the results is achieved with black-and-white and color photography. The use of a shortwave ultraviolet filter over the camera flash can be employed to record the results, although a steady source of UV is preferred. In either case, a darkened area is needed for screening purposes. Another reagent, 0.5 percent 2-nitroso-1-naphthol in acetone, eliminates the need for a UV light but is limited to a four-hour window compared to the forty-eight hours for 8-hydroxyquinoline.