Fired cartridge casings (FCCs) remain one of the most challenging substrates encountered in forensic DNA laboratories due to low initial cellular deposition, environmental conditions, and the detrimental effects of casing metals on DNA integrity. Specifically, interactions with the copper ions in brass casings can cause significant DNA degradation. This study aims to evaluate evidence-handling strategies to mitigate DNA degradation under both storage conditions prior to collection and of the resulting DNA extracts over time.

A longitudinal study was designed to assess the impact of differing FCC storage conditions on DNA yield and quality at multiple time points, ranging from extraction immediately after DNA deposition to three months of storage. This research comparatively evaluates two distinct DNA collection and extraction protocols commonly employed by forensic laboratories: a double-swab technique with a rinsing step and a modified lysis buffer, and an immersion/soaking technique with a modified lysis buffer. A known quantity of HEPM cells was deposited onto casings and then packaged under two conditions: 1) sealed in standard manila envelopes, and 2) sealed in manila envelopes that are then further enclosed within a sealable plastic bag containing a desiccant pack. 

Additionally, post-extraction stability of the extract stored at 4 °C was investigated by varying the time interval between the extraction phase and the subsequent quantification and STR typing steps. This phase examined whether residual, co-extracted copper ions continue to negatively interact with and degrade DNA in solution over time. 

By evaluating DNA recovery and degradation across storage conditions, recovery workflows, and extract storage times, this research aims to provide empirical data to support optimal storage conditions for FCCs prior to DNA collection. Additionally, it will examine the quality of profiles between separate extraction methods and whether immediate downstream processing of FCCs post-extraction is a critical consideration for laboratories.  This work will ultimately assist forensic laboratories by establishing best practices to maximize DNA quantity and STR typing success from challenging fire cartridge casing evidence.

Authors

• Lauren Mullen (National Institute of Standards and Technology, United States of America)
• Erica Romsos (National Institute of Standards and Technology, United States of America)
• Peter Vallone (National Institute of Standards and Technology, United States of America)