Evaluation of Five Methods to Develop Latent Prints on Thermal Paper

Forensic latent fingerprint laboratories determine the proper techniques for fingerprint visualization based on the substrates upon which they are deposited. Typical forensic analysis of thermal paper evidence involves the application of ninhydrin and/or 1,2-indanedione dissolved in a polar solvent. However, polar solvents create an undesirable reaction with the thermal paper’s internal properties and often lead to discoloration of the evidence. When this occurs, not only are the fingerprints less likely to be visible due to the loss of contrast, but the evidentiary print on the receipt may be lost entirely. This research sought to compare five development methods to determine their effectiveness at developing latent fingerprints on thermal paper. This experiment had two main components including analysis of fingerprints experimentally deposited on receipts and fingerprints naturally occurring on receipts. In the first part of the experiment, 10 receipts from Kroger and 10 receipts from Costco Wholesale were subjected to each of five development methods: sequential application of 1,2-Indanedione and ninhydrin, heat methods (oven, hot water immersion, and hairdryer), and p-Dimethylaminocinnamaldehyde (PDMAC) paper. The receipts were photographed following these treatments and rated by three individual examiners on a five-point quality scale, which categorizes the visualized prints according to their utility for individual identification. Statistical analyses suggested PDMAC paper as an effective substitute for current methods since, unlike the effects of ninhydrin, there was no background darkening of the receipt paper. The process itself was simple, although the receipts remained sandwiched between the impregnated paper for three hours; this time requirement, while significant, was comparable to the current 1,2-Indanedione and ninhydrin time commitment. The resulting fingerprints were also comparable in quality to those visualized with 1,2-Indanedione. In contrast, fingerprints developed from the other methods were of poor quality, if visualized at all. Based on these preliminary results, fingerprints naturally occurring on receipts from various businesses were developed with PDMAC paper sandwiching to determine how long after processing photographs should be taken. Photographs were taken at 40 mins, 1 hr, 3 hrs, 5 hrs, 27 hrs, 1 wk, and 2 wks. Photographs taken between 27 hrs and 2 wks after processing were found to have the highest fluorescence. Future research is needed to determine what length of time receipts should be sandwiched between PDMAC paper in order to best visualize the fingerprints. Once confirmed, this protocol could improve the overall ease and effectiveness of developing latent prints on thermal paper, thus leading to more accurate comparisons