Persistence of Touch DNA Evidence for Analysis

Persistence of Touch DNA Evidence for Analysis

Since the first use of DNA evidence in a criminal case in 1986, forensic scientists have considered biological material (such as hair, skin, and bodily fluids) to be relatively reliable physical evidence.

While early technology required a substantial amount of biological material to extract enough DNA to build an individual profile for analysis, researchers have since discovered that they can obtain reliable DNA from more than just bloodstains or visible fluids; they can also obtain it from “touch DNA” that is left behind on surfaces or objects such as doorknobs, window latches, or steering wheels. Although touch DNA can be essential for forensic casework, it also comes with its share of issues, including those related to:

  • Low quantity of useable DNA.
  • High variability in the amount of DNA left by touch; that is, high variability in the amount that one person leaves, and high variability in the amount left from person to person.
  • DNA degradation,including the many factors that can cause DNA to break down over time.

The results from rigorous analysis of these complicated factors have important implications for how touch DNA is collected, analyzed, and interpreted.

In 2018, the Forensic Technology Working Group at NIJ called for “comprehensive, systematic, well controlled studies that provide foundational knowledge and practical data about ‘touch evidence’ persistence in the real world.” That same year, Dr. Meghan Ramsey’s group at the Massachusetts Institute of Technology (MIT) Lincoln Laboratory began quantifying how long touch DNA would persist on certain surfaces under specific conditions. Building on that knowledge, and in collaboration with Dr. Ramsey, scientists at South Dakota State University created predictive models of how DNA degrades on different surfaces under a range of environmental conditions.

Testing for Persistence: Dry and Hot Degrades DNA

The researchers addressed two central questions:

  • How do surface type, environmental condition, and exposure time affect the stability of touch DNA evidence?
  • Does the stability of touch DNA samples differ from control DNA samples?

To address these questions, scientists deposited control DNA and touch DNA samples onto steel bolts and cotton fabric swatches. Then, they examined the DNA residue over time, across varying temperature and humidity combinations, and under UV light exposure.

You can read the full article at the National Institute of Justice.