Detection of Gunshot Residues

The ability to determine whether an individual has fired a firearm is of great significance in the investigation of both homicides and suicides. Thus, over the years a number of tests have been developed in an attempt to fill this need. The first such test was the "paraffin test" also known as the "Dermal Nitrate" or "diphenylamine test."1 It wasintroducedinthe UnitedStatesin 1933 by Teodoro Gonzalez of the Criminal IdentificationLaboratory, Mexico City police headquarters. In this test, thehands were coated withalayerof paraffin. After cooling, the casts were removed and treated with an acid solution of diphenylamine, a reagent used todetectnitratesandnitritesthat originate from gunpowder and may bedepositedontheskinafterfiring a weapon. A positive test was indicated by the presence of blue flecks in the paraffin. Although this test may give positive resultsonthehandsof individuals who fired weapons, it also gives positive results on the hands of individuals who have not fired weaponsbecauseof the widespreaddistribu-tion of nitrates and nitrites in our environment. Theparaffin testisinfact nonspecific and is of nousescientifically.

In 1959, Harrison and Gilroy introducedaqualitative colormetric chemical test to detect the presence of barium, antimony, andleadonthehands of individuals who fired firearms. Thesemetals, whichoriginatefromthe primer of a cartridge on discharge of a weapon, are deposited on the back of the firing hand as discrete particulate matter (Figure 12.1). In revolvers these metallic particles come primarily from the cylinder-barrel gap, and in automatic pistols from the ejection port. The technique developed by Harrison and Gilroy was intended as a relatively simple inexpensive test for detection of these residues. In the test a square of white cotton cloth was moistened with hydrochloric acid and then used to swab the hand. The swab was treated with triphenylmethylarsonium iodide for the detection of

Figure 12.1 Gas cloud containing primer residue flowing backward onto of firing hand.

antimony and sodium rhodizonate for the detection of barium and lead. limited sensitivity of this test prevented its widespread adoption.

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