The trace metal detection test or technique was developed during the
Vietnam War
The Vietnam War (also known by #Names, other names) was a conflict in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. It was the second of the Indochina Wars and was officially fought between North Vie ...
in the 1960s to identify people who may have been carrying
firearms
A firearm is any type of gun designed to be readily carried and used by an individual. The term is legally defined further in different countries (see Legal definitions).
The first firearms originated in 10th-century China, when bamboo tubes ...
against their skin. A 0.2% solution of
8-hydroxyquinoline
8-Hydroxyquinoline (also known as oxine) is a chelating agent which has been used for the quantitative determination of metal ions.
In aqueous solution 8-hydroxyquinoline has a pKa value of ca. 9.9 It reacts with metal ions, losing the proton a ...
in
isopropanol
Isopropyl alcohol (IUPAC name propan-2-ol and also called isopropanol or 2-propanol) is a colorless, flammable organic compound with a pungent alcoholic odor. As an isopropyl group linked to a hydroxyl group (chemical formula ) it is the simp ...
sprayed on the skin and illuminated with shortwave
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiati ...
(UV) light after several minutes reveals a pattern and type of
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typi ...
based on trace amounts of metal transferred to the skin that are invisible under normal lighting. The technique was later adopted by police in the
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., federal district, five ma ...
to help determine if a person had carried a firearm. In a
California
California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the ...
crime an automatic handgun carried in the waistband of a criminal reportedly produced an impression of the weapon's serial number. 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.
Preservation of the results is done with
black-and-white
Black-and-white (B&W or B/W) images combine black and white in a continuous spectrum, producing a range of shades of grey.
Media
The history of various visual media began with black and white, and as technology improved, altered to color. ...
and
color photography
Color photography is photography that uses media capable of capturing and reproducing colors. By contrast, black-and-white or gray- monochrome photography records only a single channel of luminance (brightness) and uses media capable only of ...
. Use of a shortwave ultraviolet filter over the
camera flash
A flash is a device used in photography that produces a brief burst of light (typically lasting 1/1000 to 1/200 of a second) at a color temperature of about 5500 K to help illuminate a scene. A major purpose of a flash is to illuminate a ...
can be used 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
Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour.
Acetone is miscible wi ...
, eliminated the need for a UV light but is limited to a four-hour-window versus the forty-eight for 8-hydroxyquinoline.
[Study of the Use of 2-Nitroso-1-Naphthol as a Trace Metal Detection Reagent, C W Kokocinski; D J Brundage; J D Nicol; 1980; http://www.ncjrs.gov/app/publications/abstract.aspx?ID=73257]
References
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Forensic techniques