A soft-ionization mass-spectrometry technique enables label-free imaging of fingerprints based on their chemical composition
Fingerprints are commonly used as a method for identifying individuals, whether taken from the scene of a crime, or on official documents, such as the US green card. The distinctive pattern formed by the skin ridges on any given finger — and corresponding fingerprint — enables comparisons to be made between known and unknown samples. Moreover, analysing the chemical composition of latent fingerprints left behind on a surface can reveal other forensic information about the subject, such as whether they have had contact with explosives or illicit drugs.
Now, Graham Cooks and co-workers from Purdue University in the USA have developed a mass spectrometry (MS)-based method1 that can be used to chemically image surfaces directly, without the need for additional chemical labels or special sample preparation. The technique is based on desorption electrospray ionization (DESI), in which a solvent spray hitting a surface produces secondary droplets that contain any compounds that may be present in a latent fingerprint. The droplets are analysed by MS at different points on the sample, resulting in not only a spatially defined image of the print, but also a map of particular chemicals that may be present.
Fingerprints were prepared on glass, paper and plastic surfaces from individuals whose fingers had been exposed to chemicals such as cocaine and the high-energy explosive trinitrohexahydro-1,3,5-triazine (RDX). The images produced with the DESI-MS technique were of high quality and could be used for physical identification. Obtaining the chemical information, however, has the additional advantage of making it easier to distinguish overlapping prints and could also provide vital forensic clues.
Ifa, D. R., Manicke, N. E., Dill, A. L. & Cooks, R. G. Latent fingerprint chemical imaging by mass spectrometry. Science 321, 805 (2008). 10.1126/science.1157199
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Cantrill, S. Weighing up the evidence. Nature Chem (2008). https://doi.org/10.1038/nchem.53