One of the most common types of evidence collected at crime scenes is latent fingerprints, which are typically lifted using transparent adhesive tape and then placed on a paper card. Now, with support from an NIJ grant, forensic scientists at Virginia Commonwealth University have explored whether lifted and archived fingerprints retain forensically useful amounts of DNA. If archived latent fingerprint samples contain probative DNA, then the chances of solving older cases may be significantly improved.
An individual’s fingerprints are formed before birth and consist of microscopic ridges arranged in rows that divide, join, and end in ways that form characteristic patterns. Those patterns have a natural coating of oils and other biological materials that cover human skin; when pressed on a surface, the patterns leave behind impressions of themselves. These prints include dead skin cells and fine lines of oil and other molecules that can be visualized by coating them with dark powder. Once coated with powder, the patterns can be lifted from the surface using adhesive tape, sealed onto a paper card, and visualized through the clear tape. This “sandwich” of dark powder between tape and paper can be labeled and stored indefinitely.
Along with the lifted powder comes much of the biological material that formed the print impression. Is enough DNA typically included in this material, and is it adequately preserved inside this tape-and-paper sandwich? These are the questions researcher Tracey Dawson Cruz and her team sought to answer by collecting latent prints from volunteers in the traditional tape-and-paper fashion. They stored the prints for four weeks, then lifted the tape and sampled the prints for DNA. To optimize their methods, they used several types of powder for visualizing the prints and employed various techniques for extracting DNA and generating profiles.
DNA profiles commonly used by forensic investigators are constructed by identifying each parental copy (allele) a person has at 20 locations (“loci”) in their genome. These alleles can combine in so many different ways that, except for identical twins, each person’s combination is practically unique. Investigators can also use partial profiles, which result from identifying the alleles at some, but not all, of the 20 loci. These partial profiles are not as conclusive for identification as full profiles, but they can still be valuable in either supporting or excluding an individual as the source of the DNA.
The researchers found that archived latent prints indeed contained DNA and, using optimized methods, they were able to recover at least a partial DNA profile 90% of the time. One sample even produced a full profile. The optimal methods involved visualizing the prints with magnetic powder, which led to the recovery of more than twice the number of alleles than when using no powder at all, and more than eight times the number of alleles than when using nonmagnetic black powder. In addition, extracting DNA from samples cut directly from the tape and cards recovered nearly three times more DNA than swabbing the adhesive tape and paper backing with a cotton swab, whether just once or twice (wet and dry).
The researchers hypothesized that the magnetic powder more readily adhered to the biological material in the latent print and protected it from being inadvertently brushed away. Directly sampling these prints rather than using a cotton swab as an intermediary also helped reduce DNA loss, especially because cotton fibers bind strongly to DNA.
Further insights were gained into the best practices for obtaining DNA profiles from such samples. The concentration step was especially critical. One of the processes studied yielded no DNA profiles, probably due to its high-temperature (and DNA-degrading) incubation step. Additionally, including a purification step after the profile loci were amplified did not improve either the chances of obtaining a DNA profile or the quality of the sample.
Because the extraction of DNA from archived latent prints is destructive and removes the prints from future visual analysis, Dawson Cruz’s team also investigated whether DNA could be recovered from surfaces where latent prints had already been lifted. If so, then both DNA analysts and latent print examiners could collect data from the same touched surfaces. What they found was that these surfaces do have measureable amounts of high-quality DNA. Although they were not able to generate any profiles from this DNA, they feel confident that laboratories that specialize in the collection and processing of low-concentration DNA samples could be successful. Thus, they recommend that, in the future, DNA samples be collected from the same surfaces from which fingerprints are lifted.
These researchers say they have opened the door to a new source of DNA evidence for solving future cases as well as cold cases where leads have been exhausted. Further optimization of their protocols and the use of specialized techniques for testing extremely dilute samples, could lead to improved success in obtaining DNA profiles from touched surfaces and archived latent prints.
About This Article
The research described in this article was funded through NIJ grant 2014-DN-BX-K013 awarded to the Virginia Commonwealth University. This article is based on the grantee report, “Methods for Obtaining STR-Quality Touch DNA From Archived Fingerprints,” (pdf, 23 pages) by Tracey Dawson Cruz, principal investigator, Department of Forensic Science, Virginia Commonwealth University.