Forensic scientists have made great strides in techniques to recover latent fingerprints that were unobtainable using older technology. There is still a problem with not being able to tell how long ago a print was made.
A new analysis technique may solve that problem.
Two scientists — Shin Muramoto and Edward Sisco — at the National Institute of Standards and Technology (NIST), developed a method to fix the date a fingerprint was made, up to four days in the past.
Measuring Palmitic Acid
The method measures the migration of palmitic acid from the tops of the ridges to the valleys of the print. Palmitic acid is a saturated fatty acid found in the oil our skin exudes, and that makes up the fingerprint itself. When we touch something with bare skin, a small deposit of palmitic acid — mixed with a lot of other stuff — is left behind.
Muramoto and Sisco found that the palmitic acid moves from the ridges to the valley between the ridges of a print at a predictable rate. By measuring how much palmitic acid has migrated, the approximate time the print was left behind is determined.
The technology has a long way to go before it will become part of the crime scene investigator’s toolkit. The prints used to study this phenomenon were left on a clean silicon substrate, something that is not going to be one of the more common materials at most crime scenes. A print left on a different, less ideal surface might not be suitable for dating at all.
The analysis also has to be conducted on the print itself, not on a lift taken from the crime scene and mounted on a card. CSIs might have to take the print-containing surface whole into the lab, or cut it out of whatever it was part of for transport to the lab.
The analysis would have to be done almost immediately, as the limits of the current technology extend only four days after the print is left. The scientists hope to be able to extend that time frame to ten days or more.
Finally, the analysis requires the use of time-of-flight secondary ion imaging mass spectrometry, requiring equipment and training many crime labs won’t have.
If the new technique becomes practical, it could remove one aspect of a common alibi. When a suspect’s prints are found at a crime scene, it’s not uncommon for the suspect to have had limited access to the premises. If he was there lawfully a week before the crime took place, he can say the print was left there then, not when the crime was committed. If investigators could tell when the print was left, that explanation might fall apart.
So, while the current state of the art might not be so useful, it bears promise for the future. Marconi’s first radio worked only so far as the other side of the room it was in, but it’s come a long way since then.
References:
Muramoto, Shin and Sisco, Edward. Strategies for Potential Age Dating of Fingerprints through the Diffusion of Sebum Molecules on a Nonporous Surface Analyzed Using Time-of-Flight Secondary Ion Mass Spectrometry. Analytical Chemistry, 2015, 87 (19) pp. 8035-8038.
