Chemical discrimination of multilayered paint cross sections for potential forensic applications using time‐of‐flight secondary ion mass spectrometry

Abstract

Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) equipped with a bismuth imaging source and an argon gas cluster ion beam (GCIB) was used to image polished cross‐sections of four automotive multilayer paint samples. Secondary ion mass spectrometry chemical imaging of the individual layers was possible after a GCIB sputter ion dose of (7 × 1015) ions/cm2 was applied for the removal of polishing residue, at which point the chemical composition of the individual clear coats could be distinguished using principal components analysis. For the differentiation of the four clear coat chemistries, only four secondary ion peaks were necessary; C2H5O+ (m/z 45.04), C9H9NO2+ (m/z 163.09), and C10H11NO2+ (m/z 177.10) that appeared to be fragments of the carbamate‐based clear coat, and C7H11+ (m/z 95.09) that was strongly associated with the polyurethane‐based clear coat. Clear identification of the four paint samples based on this short peak list highlights the strength of the SIMS technique as a potential forensic approach to discriminate automotive paints and suggests that many more variables could be included in the multivariate and statistical analysis to differentiate a wider range of clear coat chemistries.

Surface Analysis Techniques in Forensic Science: Successes, Challenges, and Opportunities for Operational Deployment

“…ToF-SIMS has been proposed as a method of mapping organic and inorganic components in the cross section of multilayer paint chips. To ensure that the signal was representative of the paint, and not simply contamination from the resin, it was necessary to first use the gas cluster ion beam in the SIMS system to clean the surface (137). Although this approach should not be expected to compete with existing techniques for throughput, it does offer the potential advantage of detecting organic and inorganic constituents simultaneously.…”