The use of fingerprints to identify who was at a crime-scene is standard forensic practice, and well known in the general public. (What you may not know is that two Bengalis, Azizul Haque and Hem Chandra Bose developed the fingerprint classification system that forms the basis of modern fingerprint identification).
Fingerprints are useful in telling us who was present, but since they are a form of physical evidence, they can also be used to determine when they were created on a surface. There’s a cool paper in Analytical Chemistry (Hinners, Thomas, and Lee; January, 2020) that uses chemistry to determine the time when latent fingerprints were deposited on a surface. The team used mass spectrometry to determine how fast specific chemical compounds (known as triacylglycerols) which are found in deposited fingerprints degraded over time.
This is not the first time dating fingerprints has been attempted. Muramoto and Sisco, two researchers at the National Institute of Standards & Technology noticed that some of the chemicals in fingerprints tend to move from the peaks of fingerprints to the valleys over time. Observing the migration of fatty acids and using mass spectrometry, they were able predict the age of fingerprints over time (Analytical Chemistry, 2015). However, the results were difficult to use for forensic purposes due to the issues of in-field reproducibility.
The new process relying on the degradation of triacylglycerols seems to be reproducible in a proof-of-concept study, though the rate of degradation of these compounds does vary a bit from person to person. It will be interesting to see how this is applied in the field, but it’s a positive step. Soon we will be able to positively identify “when” in addition to “who”. Criminals beware.