Latent fingerprints are left with trace sweats and oils from unique patterns, providing the first great forensic human identifier about a century ago.
One of the few problems, however: the fingermarks can dehydrate over long periods of time. Cold cases may thus be a challenge.
But a team at the Sûreté du Québec police force in Canada has put together a methodology involving fuming, dyes and lasers which produced a clear fingerprint on a challenging plastic bag surface from a double-homicide scene from the 1980s, as they report in the journal Forensic Science International.
“The current case presents a uniqueness due to the age of the revealed fingermark, and the paired success of cyanoacrylate fuming,” writes the Canadian team. “It would thus be of great interest of future cold case analysis using this technique to identify the factors having made this revelation possible.”
The plastic bag was found at the crime scene, and had been preserved in a paper evidence bag for decades.
Since the plastic bag is non-porous, it also further fostered the dehydration process in the roughly 30 years it was left untreated in storage. Its lack of texture also “eased the deposition process.”
The Sûreté forensic experts decided to try fuming with superglue: namely E-Z Bond Instant Glue (Thin), with cyanoacrylate.
Hanging the bag in a sealed cabinet, a small amount of the glue was poured into an aluminum cup, which was then placed near a heater set to 80 degrees Celsius. A 1500 mL beaker with near-boiling water was set at the bottom of the cabinet—and a tube emitting air was placed into the water to bubble it.
For 12 minutes, the process continually attached cyanoacrylate vapors to the residues on the bag within the enclosed space.
Then came the staining.
The solution of Rhodamine 6G and methanol was mixed with a magnetic stirrer until completely dissolved, creating a bright orange mix.
Under a fume hood, the solution was sprayed over of the superglue patterns, and the excess was flushed away with pure methanol.
The treatment process thus rehydrated the marks left from the fingerprint decades before, and locked them in permanent patterns, the experts write.
The evidence was then left to dry in the fume hood, according to the paper.
Once dry, an Arrowhead 532 nm laser was used to examine the patterns. Through orange-stained goggles, pictures are taken with a Nikon D7000 camera mounted with an orange-curved barrier filter.
A good fingerprint was thus produced for the first time from the two homicides.
No suspect has yet matched the fingerprint from the double-murder scene, the team reports. However, the cold-case technique could crack open it and other cases in the near future, they report.
“It also reiterates the importance opening cold cases in order to treat and reassess their exhibits,” they write. “Despite the age of a fingermark, cyanoacrylate combined with rhodamine 6G and visualized with a laser can provide new evidence … This opens the possibility of making an identification and ultimately change the course of the investigation.”