Incorporating organic gunshot residue into the forensic workflow: A study of preservation and stability of the pGSR and OGSR

In recent years, trace evidence examiners started to assess the value of incorporating two complementing analytical measurements: primer residue (pGSR) and organic gunshot residue (OGSR) data, aiming to strengthen firearms-related investigations. Still, there is a need to understand the cost-benefit of any method's modification for collecting, analyzing, and interpreting the combined information. This study aims to answer practical questions concerning optimal storage conditions for OGSR and pGSR, the feasibility of sequential analysis on the same sample, and whether pGSR or OGSR should be analyzed first. Samples were collected from shooters ( n = 128) using a standard carbon adhesive stub. The stability of OGSR and pGSR was evaluated at the time elapsed from collection at the scene until analysis at the laboratory (1, 2 days, 1, 2 weeks, and 1, 2, 6 months) and storage conditions (room temperature or freezer). The experimental design includes the effect of the analysis sequence by either liquid chromatography-mass spectrometry (LC-MS/MS) and scanning electron microscopy energy dispersive X-ray spectrometry (SEM-EDS) on the detection rates. The results indicate a multi-testing approach does not compromise the integrity of pGSR or OGSR evidence when following appropriate protocols. The detection of OGSR and pGSR is not significantly different at the studied storage conditions or times when samples are preserved in sealed SEM stubs. These findings may alleviate some concerns regarding the feasibility of conducting an OGSR examination on samples that cannot be submitted to the laboratory immediately after collection and assist agencies in establishing preservation, storage, and acceptance criteria for future adoption. • Regardless of pGSR/OGSR differences, a universal preservation protocol is feasible. • Freezer storage is preferred but not required when using sealed SEM stubs. • OGSR is transferred not only as vaporous deposits but also as microscopic flakes. • Sequential testing by LC-MS/MS and SEM-EDS is effective for pGSR and OGSR detection. • Gentle microextraction protocols remove OGSR with no substantial modification to pGSR.