Chemical Testing

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Following the preliminary examination of an item thought to bear gunshot residues, a sequence of chromophoric (color-producing) tests should be performed. These tests may assist the visualization of residues that would otherwise be invisible and assist in determining a muzzle-to-target distance. These tests may also assist in shooting reconstruction.

Note:

The origins of a number of chemical tests are credited to the chemist Fritz Fiegl (Spot Tests in Inorganic Analysis and Spot Tests in Organic Analysis). These standardized tests originated as industrial spot tests for various elements and compounds; they do not need to be performed by a chemist.

It is important to note that in the courtroom, issues may arise regarding

whether or not a standard protocol was followed by the nonchemist firearm examiner,
the basis for the interpretation of the results as recorded in the examiner’s bench notes.

Up to three tests may be performed; where more than one test is performed, it is important that they are carried out in the following sequence:

Modified Griess Test for nitrite compounds, a product of burning a nitrate, such as smokeless powder, or Reverse Modified Griess Test
Dithiooxamide Test (rubeanic acid test) for cuprous (copper-bearing) materials
Sodium Rhodizonate Test for all forms of lead residues or Bashinski Transfer Technique

The Modified Griess Test does not chemically interfere with the Dithiooxamide Test or the Sodium Rhodizonate Test. However, the Modified Griess Test must be performed first because sodium rhodizonate has the potential to interfere with nitrite testing.

Click here to view a flowchart on GSR testing
Courtesy of Jack Dillon

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