Fingerprints are more than just patterns; they’re chemical identities
Nov. 2019/3/14 14:17:02 By LEAP Chem
On March 24, 1994, a man walking his dog in Albany, Georgia, found the body of a woman named Angela Sizemore, who had been raped and murdered and left in the front seat of her SUV behind an apartment complex. When they arrived at the scene, crime-scene technicians collected semen from the body and fingerprints from the vehicle in an attempt to identify the killer. At the time, DNA analysis was accurate enough to pinpoint a suspect on the basis of the genetic code within sperm cells, and fingerprint analysis could ID a suspect from the loop-and-whorl patterns left behind by substances on the skin.
The semen analysis identified Georgia resident Marcus Ray Johnson as having had sex with Sizemore sometime before her death. Witnesses also placed someone fitting Johnson’s description near the site where police believed Sizemore was murdered the previous night. Investigators collected a total of 38 fingerprints from the victim’s vehicle; all but one was too smudged to use properly. The one print that was clear went to AFIS, the digital fingerprint database commonly used by law enforcement, but it wasn’t a match for any prints logged there.
Johnson claimed that he’d had consensual sex with Sizemore on the night of the murder after meeting her in a bar and had left her alive. He was convicted of murder and sentenced to death row, where he continued to assert his innocence.
Today, scientists are working to get more from fingerprints than just loops and whorls, developing methods to analyze molecules such as DNA, amino acids, or explosives in fingerprint residues. Years after his conviction, Johnson’s lawyers hoped new techniques, applied to the prints collected in 1994, might uncover information that would exonerate their client.
With these methods, scientists can now work with samples containing as little as 500 nL of material, making the chemical analysis of fingerprints more feasible. For instance, a fingerprint contains much less DNA than the cheek swabs for which most forensic processes have been optimized. It’s only in the past decade that chemical analysis techniques have become sensitive enough to glean a biological profile from such a small sample, says Tracey Dawson Cruz, a forensic molecular biologist at Virginia Commonwealth University. The idea is not necessarily to do away with pattern comparison during fingerprint analysis—and, indeed, other scientists are focusing on ways to improve the clarity of the pattern itself—but instead to bolster the analysis with more information. Smudged prints needn’t be a completely lost cause.
In some older cases, DNA from archived fingerprints may be “the only chance” for biological evidence, Dawson Cruz says. She is among those working to extract more from fingerprints, tapping into the potential treasure trove of chemical information hidden within.
As Johnson neared the end of his permitted appeal period nearly 20 years after the murder, his lawyers, seeking a final opportunity to uncover exculpatory physical evidence, asked the court to allow DNA analysis of the fingerprints collected in 1994. Georgia’s state forensic laboratory had never done such an analysis, but Johnson’s lawyers came to court armed with results from VCU scientists, including Dawson Cruz. These researchers had been exploring the possibility of retrieving usable DNA profiles from fingerprints collected decades earlier.