Fingermarks in blood: Mechanical models and the color of ridges.

This article treats fingermarks in blood on non-porous surfaces and addresses the question of "which came first": the fingermark or the blood. Three mechanical models were systematically examined: (1) A blood-contaminated finger pressed against a clean surface; (2) blood contaminates a latent print that had been placed on a clean surface; (3) A clean finger pressed against a blood-contaminated surface. The questions of reliability and limits of all three models were discussed. The relevancy of the approach to "which came first", based solely on the color of ridges was questioned. The first mechanical model most simulated a real situation, when previously cleaned, a blood contaminated finger touched a clean Formica or glass surface with pressure of 100-500g. Concerning the second model, it was observed that in the case of a greasy latent print, placed on an inclined surface and contaminated with appropriate amount of blood, the color of ridges were normally darker than the color of its valleys. As for the third model, it was concluded that it works only in about 25% of cases. While investigating this model, two phenomena were observed: ridge color inversion and valley color inversion. In conclusion the color of ridges can not be the only and ultimate indicator to the question of "which came first", the fingermark or the blood stain.

Acetone facilitated DNA sampling from electrical tapes improves DNA recovery and enables latent fingerprints development.

Electrical tapes (ETs) are a common component of improvised explosive devices (IEDs) used by terrorists or criminal organizations and represent a valuable forensic resource for DNA and latent fingerprints recovery. However, DNA recovery rates are typically low and usually below the minimal amount required for amplification. In addition, most DNA extraction methods are destructive and do not allow further latent fingerprints development. In the present study a cell culture based touch DNA model was used to demonstrate a two-step acetone-water DNA recovery protocol from ETs. This protocol involves only the adhesive side of the ET and increases DNA recovery rates by up to 70%. In addition, we demonstrated partially successful latent fingerprints development from the non-sticky side of the ETs. Taken together, this protocol maximizes the forensic examination of ETs and is recommended for routine casework processing.