Visualize fingerprints on brass after removal of secretions

The use of fingerprints left at the scene of a crime to identify an offender was first suggested in the nineteenth century,1, 2 and this branch of forensics continues to play an integral role in a wide range of criminal investigations.3 Sweat secretions deposited onto a surface leave an impression of the papillary ridge pattern of the finger, referred to as a latent fingerprint. Latent prints are not easily visible to the naked eye, and various techniques have been developed to enhance andmake them visible.4 In general, because these methods all require physical or chemical interaction between the enhancing reagent and fingerprint deposit, secretions must still be present. To tackle the problem of enhancing fingerprints on brass after removal of secretions, we have developed a technique that exploits a natural chemical reaction occurring between fingerprints and metal. Recent research has shown that this reaction, which involves corrosion, can change both the chemical and physical characteristics of the metal surface.5, 6 Unlike conventional treatments, our technique requires no enhancement of the fingerprint deposit prior to visualization. For our research, we selected brass because it is used to manufacture gun cartridge cases. Fingerprint enhancement on spent cases has attracted considerable attention, not least due to the problems it presents in terms of heat damage to the secretions during the firing process.7 We recently showed how fingerprints on brass cartridge cases that we left out for several days in open air at room temperature can still produce corrosion sufficient for visualization, even after they have been washed in warm water and detergent to remove the residue.8 Our technique required application of a potential to the brass (∼1,500V) followed by introduction of a conducting carbon powder (grain size ∼10μm). This process was facilitated by the use of a cascade developer (Foster and Freeman, Evesham, UK), an electrostatic detection device usually employed to enhance indented writing on paper, that coats tiny (∼400μm) spherical beads with the conducting powder. Figure 1. Fingerprint deposited on brass, left for five days, washed and then photographed (a) before and (b) after the application of a potential and a conducting powder.