Scientists show that fingerprints are more than just identifiers – they can link a suspect directly to a chemical with minimal sample preparation

Source for feature photos: “Finger Print on Enter Key of Keyboard” by Pixabay

In the forensics world, finding fingerprints is a huge boon; their presence and subsequent match to a known standard can quickly link a suspect to a crime scene. But what if we could get more than just identity from fingerprints – say whether the donor touched drugs, psychoactive plants or explosive compounds beforehand?

While this is not a new idea, previous collection methods were only able to obtain either the chemical information (the things they touched) or the fingerprint itself, but not both. Technicians would swab the fingerprint and smudge the distinctive whorls, loops and arches beyond usability in an effort to identify the drugs or explosive compounds present. If the CSI techs wanted the identification, they abandoned analyzing for any illicit substances to preserve the fingerprint pattern.

Dr. Rabi Musah and her student Kristen Fowble at SUNY Albany took a different approach. Musah used a technique called mass spectrometry imaging (MSI) – specifically Laser Ablation-Direct analysis in real time- Imaging Mass Spectrometry (LADI-MS) – to simultaneously obtain the chemical information and the fingermark pattern without destroying the sample. The technique is relatively new (less than three years old) and uses the location, or spatial distribution, of chemicals in the fingerprint to obtain a picture, much like how in a digital photo, pixels make up each point of the photo image.

Musah’s method used a laser to ablate or remove pieces of the fingerprint. These portions contain a bunch of molecules that are either endogenous or exogenous. Endogenous, or normally present compounds originate from or are excreted onto the skin, including proteins, lipids (like cholesterol), salts and organic molecules. Exogenous molecules are those not normally present, such as the heroin or acetylsalicylic acid from an aspirin pill that the fingerprint donor touched.

As the laser moves across the fingerprint and the microscopic portions are ablated from the collection device, such as a piece of tape, the mass spec collects information on how much and where all these molecules are located (Fig 2). The quantity of the molecule at each location is plotted to the picture using a heat map, where a higher concentration corresponds to an yellow color, and lower concentrations correspond to a deep red color. Places with no compound present are black. The stark difference between the presence and absence of chemicals create a nice contrast between the fingerprint and the background.

Using an endogenous molecule like cholesterol, researchers recreated a picture of the fingermark, with the ridges and patterns intact. At the same time, they collected information on an exogenous molecule like cocaine. Overlaying the two pictures of chemical spatial distribution showed that the fingermark is the origin of the exogenous molecule of interest. In addition, Fowble & Musah were able to identify cocaine in a cocaine-laden fingerprint four days after the initial deposition.

Though the technique sounds highly technical, LADI-MS has several advantages that decrease sample manipulation compared with other MSI methods. Because the technique is performed in an ambient environment (i.e. room temperature and pressure), there is no special step necessary for sample introduction, such as HPLC-MS which has a separation step before introduction to the mass spec. Furthermore, unlike other techniques that require the sample to be dissolved first, LADI-MS analyzes the collected fingerprint directly. Less manipulation means lower chances of contamination from outside sources or losing the miniscule sample.

Additionally, though the current study focused on only three exogenous compounds (cocaine, active ingredients in psychoactive plants, and pseudophedrine, used to make methamphetamine in illicit meth labs), this technique has great potential to identify compounds in complex mixtures. Further investigation could determine if the technique can identify drugs or chemical metabolites in the blood from a bloody fingerprint, thereby demonstrating if the donor had drugs in their system at the time of deposition.

Fowble and Musah’s novel technique maximizes the amount of information obtained from a simple fingerprint while decreasing the technical manipulation needed for analysis. 

Title	Simultaneous imaging of latent fingermarks and detection of analytes of forensic relevance by laser ablation direct analysis in real time imaging-mass spectrometry (LADI-MS)
Authors	Kristin L. Fowble, Rabi A. Musah
Journal	Forensic Chemistry
Publisher	Elsevier
Year	2019
Link	https://www.sciencedirect.com/science/article/pii/S2468170919300700