Visualization of latent fingermarks on thermal paper: A new method based on nitrogen dioxide treatment

Abstract

A new chemical method for visualization of latent fingermarks on thermal paper, based on a treatment with nitrogen dioxide (NO2) gas, is presented in this work. The gas is generated by a reaction between zinc and diluted nitric acid in a closed chamber. This newly proposed method does not require fingermark’s fixation reagent after the treatment with NO2 i.e. the visualized fingermark remains permanent for more than one week and without any changes in its quality. The general visualization mechanism is based on providing acidic conditions in order to induce tautomeric transformation of the leuco dye’s molecules in the thermal layer, accompanied by a color change of the papillary lines throughout the whole fingermark. The NO2 method provides satisfactory contrast between the visualized fingermarks and the background surface i.e. thermal layer. The visualized fingermarks are qualified with high clarity and continuity of the friction ridges, and clarity of the 2nd level characteristics. The proposed method was evaluated by dactyloscopic comparison of the number of 2nd level characteristics and according to the results it can be exemplified with high identification capacity. The proposed method is simple, safe, cheap, non-destructive, non-time consuming, applicable for visualization of aged fingermarks, and potentially applicable under terrain (field) conditions in real forensic casework.