Detrimental effect of metal contaminants on forensic DNA analysis and strategies for mitigation

Metal contamination in biological samples poses a significant challenge in forensic DNA workflows, leading to low DNA yield, degradation of DNA, overestimation of DNA quantity, generation of a partial profile, or a null profile. Metals like Fe and Ca are inherent components of various biological samples, including blood, bone, teeth and muscles. Metals are also accumulated in the biological samples either from environmental exposure or from occupational hazards. Metal surgical implants, substance use disorder, tattoo inks, cosmetic products, and low-cost jewellery can also contribute to metal contaminants in the biological samples. Samples collected from metallic surfaces and/or the use of metallic collection devices interact with each other to leach metals into the samples. The current review focuses on the various sources of metal contamination in forensic biological samples, their detrimental effect on DNA damage, and steps of forensic DNA profiling such as extraction, quantification, amplification, and capillary electrophoresis. Metal ions inhibit DNA polymerase by competitive binding with Mg2+, fluorescence quenching during quantification, and modification of electroosmotic flow during separation. New generation multiplex kits have been developed for forensic applications. Such kits use high-fidelity DNA polymerase and an appropriate buffering system to overcome the problems associated with metal contaminants. However, proper attribution of the metal contaminant and selection of a suitable strategy to demineralize the samples, either by using a chelating agent or size exclusion chromatography, along with the new generation multiplex kit, might significantly improve the quality of a DNA profile.