In most cases criminals are concerned about not leaving any traces at the crime scene. Therefore, it is common to burn exhibits or throw them into water (e.g. plastics, bottles, foils etc) with the intention to wash all fingerprints away. But is this really what happens?
Fingerprints consist of different endogenous secretions e.g. amino acids, sebaceous, salts, urea and fatty acids. Those which contain more oily secretions that are hydrophobic, are still preserved after getting wet.
Latent, non-visible, fingerprints can be detected and analysed by using different chemicals. For example, with special powders (eg. fingerprint, magnetic or fluorescent powders) that are applied on the exhibits surface and stick to the ingredients of the trace. However, these powders cannot be used to detect latent prints on wet surfaces because the fine powder would accumulate due to water drops.
As a consequence, it is necessary to find a way to be able to detect and visualize latent fingerprints on wet surfaces. The solution: a special spray called SPR (Small Particle Reagent). SPR makes fingerprints visible by spraying a solution consisting of fine coloured particles (White, Black, UV-active) on the exhibit’s wet surface. These particles stick to the endogenous secretions and make the print detectable. Once a print is evident the surplus spray needs to be washed away with distilled water. Immediately after this step, the print can be taken with a common gel lifter. On more difficult surfaces e.g surfaces with a strong texture the trace needs to be saved photographically.
Nevertheless, this method should not be used for interiors or porous surfaces, as SPR causes heavy soiling. In addition, the spray can also be used for latent fingerprints on difficult surfaces as sticky tape. All in all, SPR- spray is a good method to visualize latent fingerprints on wet exhibits to eliminate the chance that traces are contaminated during the drying process.
Read more about different Methods for visualizing Fingerprints.