Imaging mass spectrometry of fingermarks on brass bullet casings using sample rotation

Abstract

A rotation stage was developed to allow the surface of bullet casings to be imaged under ultra-high vacuum (UHV) conditions using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Experiments were performed over a period of seven months to determine how fingermarks deposited on the surface of Webley MkII revolver rounds change over time. Stitching software written in Python was used to combine image strips that were collected by performing ToF-SIMS analysis along the length of the revolver rounds. The ToF-SIMS analysis was performed by analysing a thin strip along the length of the casings, before rotating them through a few degrees and analysing a new strip. This process was repeated until the entire casing had been imaged. The resulting secondary ion images of the fingermarks were compared to optical images obtained from the same and similar rounds that had been exposed to cyanoacrylate fumes and subsequently stained using Basic Yellow 40 (BY40) dye. ToF-SIMS images were shown to display evidence of ridge and sweat pore level detail on samples that displayed no evidence of fingermarks when developed with cyanoacrylate and BY40. The effects of the curvature of the round casings on the morphology of fingermarks were also assessed. ToF SIMS images were compared to marks that had been deposited onto flat paper surfaces using ink. The distortions caused by differences in surface curvature and the deposition methods were found to be within acceptable limits.