Nanotechnology for forensic sciences: Analysis of PDMS replica of the case head of spent cartridges by optical microscopy, SEM and AFM for the ballistic identification of individual characteristic features of firearms

A novel application of replica molding to a forensic problem, viz. the accurate reproduction of the case head of gun and rifle cartridges, prior and after been shot, is presented. The fabrication of an arbitrary number of identical copies of the region hit by the firing pin and by the breech face is described. The replicas can be (i) handled without damaging the original evidence, (ii) distributed to different law enforcement agencies for comparison against other evidences found on crime scenes or ballistic tests of seized firearms, (iii) maintained on a file by the laboratories. A detailed analysis of the morphological features of the replicas has been carried out by standard microscopy techniques as well as by advanced microscopy such as scanning probe and scanning electron leading to a quantitative morphological characterization of the case heads down to the nanometer scale. The assignment of the cartridge replicas to the shooting weapon is demonstrated to hold below the micron scale, while it is hindered at the nanometer level both by the manufacturing differences and by eventual modifications occurring on the firing pin.