Gunshot residue

The first recorded use of paraffin wax as a lifting medium was done by Dr. Iturrioz in 1914 and was popularized in 1933 by Teodoro Gonzalez of the Mexico City Police Laboratory.

This test consisted of coating a suspect's hands with paraffin wax, allowing it to solidify and peeling it away before adding a diphenylamine/sulfuric acid reagent.

[3] This is no longer used in casework due to the high number of false positives caused by the commonality of nitrates and nitrites in a variety of mundane products such as fertilisers.

[2] In 1971 John Boehm presented some micrographs of gunshot residue particles found during the examination of bullet entrance holes using a scanning electron microscope.

If the scanning electron microscope is equipped with an energy-dispersive X-ray spectroscopy detector, the chemical elements present in such particles, mainly lead, antimony and barium, can be identified.

GSR mostly derives from its propellants and primer cap; which includes an explosive, oxidizer, fuel, lubricants, stabilizers and other additives.

Both publications highlight that certain markers of exclusion and reference to the general population of collected particulate can help the expert in designating GSR-similar particles as firework-sourced.

[5] A positive result using SEM-EDX spectroscopy will generate x-ray spectra characteristic of GSR, likely containing combinations of metals such as Pb-Sb-Ba or Sb-Ba.

[a] Gunshot residue can also be removed from surfaces by washing, wiping, or brushing it off, so a negative result cannot fully rule out a gun was not fired by the tested object or area.

Depending on the type of fire arm and ammunition used, it will typically travel no farther than 3–5 feet (0.9–1.5 meters) from the muzzle of the gun.

Advanced analytical techniques such as ion beam analysis (IBA), carried out after scanning electron microscopy, can support further information allowing one to infer about the source of gunshot residue particles.

Organic residues can come from propellants like nitrocellulose and trinitrotoluene, plasticisers like triacetin, stabilizers like diphenylamine and possible reaction products of said compounds.

[1] Organic gunshot residue can be analyzed using methods such as micellar electrokinetic capillary electrophoresis (MEKC),[2] high-performance liquid chromatography and gas chromatography-mass spectrometry.

[2] It is an extremely sensitive, specific, and efficient method as it can obtain information on the origin of particulate debris, and it can be done on surfaces or objects.

The test involves dampening a cloth with 0.1M hydrochloric acid (HCl), swabbing the item being analysed and allowing that to dry before subjecting it to various reagents.