Advanced solvent-free application of ninhydrin for detection of latent fingerprints on thermal paper and other surfaces

This work presents the first known experiments of ninhydrin sublimation in vacuum to detect latent fingerprints on thermal paper. In this method, latent fingerprints become visible in rich detail without the background black staining known from the application of ninhydrin solutions to thermal paper. The method involves hanging the thermal paper samples 15 cm above a heating source with dispersed ninhydrin crystals in a vacuum chamber. The optimized conditions for ninhydrin sublimation are 50 mg ninhydrin, 2 to 5 mbar vacuum, and 150 degrees C heating source temperature for 30 min. The application of this method is also successful on the new euro notes. Latent fingerprints can be developed across the transitions from paper to optical variable device (OVD).