Automated particle analysis of D.B. Cooper's tie

Automated particle analysis (APA) using scanning electron microscopy (SEM) and energy-dispersive x-ray spectrometry (EDS) is a well-known method in forensics, especially as it is applied to gunshot residue analysis. Here we describe the analysis of particles lifted from D.B. Cooper's tie, which was left on the hijacked plane in 1971. A Tescan MIRA4 SEM was used to locate, image, and analyze ~180,000 particles on a single 12-mm SEM stub used to sample the Cooper tie. Initial analysis of the data for unusual or distinctive elemental compositions found many interesting particles, including titanium-antimony, silver-mercury, gold-palladium, and uranium. Due to the short acquisition times in APA, elemental detection is limited to major and some minor elements. Therefore, it is crucial to relocate particles of interest for deeper analysis by SEM-EDS. Particle identifications can be confirmed or revised after consideration of longer count time EDS spectra and more detailed SEM imaging. The types and quantities of particles found on the Cooper tie suggest specialty metals (titanium, gold-palladium, mercury-silver, tungsten-cobalt) and industrial facilities (industrial aluminosilicate minerals) as possible associations. Although some previously identified materials of interest were not supported by the 2024 reanalysis results, other new targets of interest were identified. In particular, the mineral particles on the tie may offer a fresh look into Cooper's geographic locality and activities. This work also highlights the forensic value of a tie as an unwashed particle reservoir.