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.^[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] 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.^[3]

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.^[4] 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.^[5]

References