DPA Test & Gunpowder Contamination: Understanding False Positives

The Diphenylamine (DPA) test detects nitrates/nitrites but lacks specificity for gunshot residue (GSR).

Chemistry

• DPA + nitrate (HNO₃) → blue-purple diphenylnitroamine (visible color change).
• Gunpowder contains nitrocellulose (nitrate ester) and often potassium nitrate.
• Organic nitrates (nitroglycerin, PETN) also react.

False Positive Sources

• Gunpowder particles: Unburned or partially burned propellant on hands/clothing → positive DPA.
• Environmental nitrates: Fertilizers, cured meats, certain medications, urine, soil.
• Non-GSR nitrites: Corrosion inhibitors, food preservatives.

Forensic Significance

• DPA is a presumptive screening test only.
• Confirmatory: SEM-EDX (scanning electron microscopy-energy dispersive X-ray) for characteristic GSR particles (Pb, Ba, Sb spheres).
• Modern protocols: DPA largely replaced by modified Griess test (nitrite) + SEM-EDX.

A positive DPA indicates nitrate exposure, not definitive GSR. Context and confirmatory analysis are essential.