Desensitisation (Griess Protocol)
Definition
The step in the Modified Griess protocol in which background nitrite contamination (from sweat, atmospheric nitrogen, detergent) is removed from the garment before the test paper is applied, preventing false-positive colour reactions.
- Test family
- Modified Griess protocol for gunshot residue
- Target contaminant
- Background nitrites from sweat, air, detergent
- Step order
- Performed before the treated test paper is applied
- Failure mode prevented
- False-positive colour reaction
Common questions
What happens if the desensitisation step is skipped?+
Nitrite residue already on the fabric from sweat, atmospheric exposure, or laundering can react with the Griess reagent the same way nitrite from gunshot residue does, producing an orange-pink colour that looks like a genuine muzzle-to-target distance pattern where none exists.
How is the garment desensitised before testing?+
The suspect area of the garment is pre-treated to strip out pre-existing nitrite before the sulfanilic acid and alpha-naphthol test paper is pressed onto it, so only nitrite deposited at the moment of firing remains to react and produce the orange-pink colour.
Related terms
- Azo Dye
- A class of organic dyes containing one or more azo groups (R-N=N-R'). Formed in the Griess test by coupling of a diazonium...
- Lead-Free Primer
- A primer composition that omits lead styphnate, using DDNP or other compounds, producing no lead from the primer channel during discharge. Examples...
- 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...
- Sodium Rhodizonate
- A chelating agent producing a scarlet-to-red colour with lead ions at acid pH. Applied to entry-wound margins and garment entry holes to...
- 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...
- Stippling
- Abrasion marks on skin or fabric produced by partially burned propellant grains exiting the muzzle with sufficient velocity to mechanically impact the...
- Test-Fired Control Panels
- Fabric panels fired at known, measured muzzle-to-target distances using the same or equivalent weapon and ammunition as in the case, processed with...