Any fan of police procedurals knows that fingerprints can be key evidence in solving a crime. But there’s a problem: There’s no reliable way of knowing whether a fingerprint was left at the time of the crime or days—even weeks—earlier. Inspired by forensic methods for estimating the age of bloodstains, a research team in the Netherlands has come up with a way to do the same for fingerprints (Angew. Chem. Int. Ed. 2014, DOI: 10.1002/anie.201402740). When crime scene analysts assess the age of blood, they measure levels of hemoglobin oxidation products. Saskia A. G. Lambrechts of Academic Medical Center at the University of Amsterdam and coworkers reasoned they could use fluorescence spectroscopy to monitor the levels of oxidation in the lipids and proteins found in fingerprints over time and thereby determine their age. Using this technique, the researchers were able to estimate the age of 55% of men’s fingerprints that were up to three weeks old with an uncertainty of 1.9 days. The technique didn’t work for fingerprints left by women because of their lower excretion levels of the relevant skin biomolecules. To solve that dilemma, the researchers say they’ll need more efficient techniques for studying fingerprint lipids and proteins.