Walker Test
Definition
Filter-paper transfer test for nitrites in propellant residue. The cloth is pressed against desensitised filter paper, sprayed with sulphanilic acid and alpha-naphthylamine, and orange-to-pink spots map the propellant pattern.
- Target analyte
- Nitrites in propellant residue
- Method
- Filter-paper transfer test
- Reagents
- Sulphanilic acid and alpha-naphthylamine
- Positive result
- Orange-to-pink spots
Common questions
What does the Walker test actually establish about a garment or target?+
It visualizes the spatial pattern of nitrite-containing gunshot residue deposited on fabric, which helps map the distribution of unburned or partially burned propellant and can support estimates of muzzle-to-target distance when compared against test fires from the same firearm and ammunition.
Why is the Walker test usually paired with other range-of-fire methods?+
The color reaction confirms nitrites are present but does not by itself give a precise distance, so examiners compare the pattern's density and spread to test patterns fired at known ranges, and may combine it with microscopy or elemental analysis of the residue for stronger conclusions.
Related terms
- Bullet Wipe
- Greyish to brownish ring of lubricant, lead and copper rubbed off the bullet onto the margin of an entry hole in clothing....
- Contact Shot
- Muzzle pressed against skin or clothing at the moment of firing. Hallmarks are a stellate (star-shaped) wound from sub-dermal gas tracking, a...
- Modified Griess Test
- A contact-transfer colour chemistry test for nitrite-containing propellant combustion residue. Sulphanilic acid undergoes diazotisation with nitrous acid (from NO2 on the target...
- Powder Tattooing
- Stippling pattern of unburned and partially burned propellant grains embedded in the skin around an entry wound. Cannot be wiped off. Pathognomonic...
- Range of Firing
- The distance from the muzzle of the firearm to the target at the moment of discharge. Estimated from wound geometry, soot pattern,...
- Sodium Rhodizonate Test
- Colour test for lead. The cloth is treated with sodium rhodizonate followed by buffer; lead deposits give a pink-to-red colour, confirmed with...
- Soot (Blackening)
- Submicrometre carbon-rich particles from incomplete propellant combustion, deposited on the garment or skin surface around the entry hole at contact and near-contact...