Ruhemann's Purple
Definition
The purple-blue condensation product formed when ninhydrin reacts with alpha-amino acids; the visible development indicator used in latent fingerprint examination on paper and cardboard.
- Reaction
- Ninhydrin with alpha-amino acids
- Appearance
- Purple-blue coloured product
- Use
- Visualising latent prints on paper and cardboard
Common questions
Why does ninhydrin work on paper but not on non-porous surfaces?+
Amino acids from sweat are absorbed into porous surfaces like paper, where ninhydrin can react with them over time to form Ruhemann's Purple. Non-porous surfaces do not absorb the residue the same way, so other development methods are used there instead.
Does the colour of Ruhemann's Purple fade or change over time?+
The developed colour can shift and intensify over hours to days after treatment, and examiners sometimes apply a post-treatment such as zinc chloride to shift the colour and enhance fluorescence for photography under specific lighting.
Related terms
- Alpha-Amino Acids
- Molecules bearing an amine group on the carbon adjacent to the carboxyl group; the primary molecular target of ninhydrin, DFO, and indanedione...
- DFO (1,8-Diazafluoren-9-One)
- A fluorescent ninhydrin analogue used for latent fingerprint development on porous surfaces; excite at 450-470 nm (with secondary absorption extending into the...
- HFE (Hydrofluoroether)
- A class of solvents used as carriers for ninhydrin, DFO, and indanedione formulations; lower toxicity and flammability than older acetone or ethanol...
- Indanedione-Zinc
- A fluorescent amino acid reagent (indane-1,3-dione complexed with zinc chloride) that produces an orange-yellow fluorescent product; reported to be more sensitive than...
- Ninhydrin
- A chemical reagent (2,2-dihydroxyindane-1,3-dione) that reacts with alpha-amino acids in latent print residue to produce a purple-blue compound (Ruhemann's Purple); the primary...
- Physical Developer (PD)
- A multi-bath silver-deposition wet-chemistry method that deposits silver metal onto fatty latent print residue on porous surfaces; the only technique that recovers...
- Sequential Processing
- The application of chemical development methods in a fixed, evidence-preserving order that maximises cumulative recovery while preventing earlier treatments from destroying the...
- Treatment Log
- The contemporaneous written record of every chemical method applied to an item of evidence, including reagent batch, examiner identity, date/time, and result;...
- Vacuum Metal Deposition (VMD)
- A technique in which gold and zinc are sequentially evaporated in a vacuum chamber; the gold deposits uniformly while zinc nucleates differentially...