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Research on Persistence of Touch DNA Will Help Investigators Collect More Usable Samples

by Jo Ellen Nott

The National Institute of Justice (“NIJ”) is the research, development, and evaluation agency of the U.S. Department of Justice. Its motto, “strengthen science, advance justice,” informs all its activities. One crucial area of forensic science it has helped strengthen through grants is DNA research and development. Since the late 1980s, law enforcement demands for tools and technologies of DNA testing have continued to exceed what is available in their jurisdictions. 

In 2018, the Forensic Technology Working Group at NIJ asked for studies that would “provide foundational knowledge and practical data” about the persistence of DNA left on surfaces versus DNA collected from individuals via bloodstains or visible fluids at crime scenes or found on victims. The Massachusetts Institute of Technology Lincoln Laboratory received an NIJ grant to quantify how long touch DNA would persist on different surfaces under varying conditions. Scientists at South Dakota State University then took the Lincoln Lab findings and created predictive models of how touch DNA degrades. 

Issues that forensic scientists face when dealing with touch DNA are many: low quantity of useable DNA, high variability in the amount left by one person, high variability in the amount left from person to person, and, most importantly, the degradation of DNA and the many factors that cause it to break down over time. 

The researchers conducted experiments to answer two crucial questions: (1) How do surface type, environmental condition, and exposure time affect DNA touch evidence? and (2) Does the stability of touch DNA samples differ from control DNA samples? 

In their experiments, the scientists put control DNA and touch DNA samples on steel bolts and cotton fabric. They exposed the samples to varying temperatures and humidity conditions and UV light exposure. Exposure time was 14 days for control DNA and seven days for touch DNA.

The scientists fit their observations of the changes in the DNA samples to a linear, mixed effects model and found that: 

· The amount of DNA left by touch varied more than in the control samples.

· DNA samples degraded less on stainless steel than fabric.

· DNA samples degraded more in high temperature and low humidity.

· DNA samples were more stable at low temperatures.

UV light had the biggest effect on DNA degradation on both materials to the extent that samples exposed to UV light were too highly degraded to be useful in a forensic analysis.

The takeaway for forensic labs and law enforcement is that investigators can recover more useable DNA in cool and dry indoor environments than hot and humid outside conditions. They will also recover more useable DNA from stainless steel objects than from fabric.

A challenge for the researchers was the low and variable quantities of touch DNA they were provided to analyze. Because of this, they were not able to evaluate the level of DNA degradation as well as they hoped to. In future experiments, they plan to increase the initial amount of touch DNA to obtain more accurate degradation results.  

The NIJ reports that “these studies provide the most comprehensive information to date on the persistence of touch DNA evidence.”

Source: National Institute of Justice




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