The use of fingerprints dates back several millennia. Over time, fingermarks, the trace left by fingerprints, have been found in many parts of the world, such as on Paleolithic cave walls, Babylonian clay tablets, Egyptian tomb walls and Greek pottery. Some of these were inadvertently affixed as a natural result of trade, while others were voluntarily affixed, either as a decoration or as a signature to protect the products from counterfeiting. It was not until the 19th century that fingermarks were introduced to forensic science as a means of criminal identification. This advent swiftly replaced the Bertillon system of judicial anthropometry, which is based on the recording of eleven anatomical measures of the head and body, as well as special signs of the criminal (weight, eye color, scars), all accompanied by photographs of the face.
For over one hundred years, fingerprints reigned as one of the most trusted identification methods used in forensic investigations. Fingermark recovery is carried out by a crime scene technician using several optical, physical and chemical methods. Initially, a white judicial or laser light source is used to visually inspect the crime scene for fingermarks. Subsequently, one or more physical or chemical revelation techniques are used, followed by photography using various light sources as necessary. In all cases, the ideal forensic enhancement technique must be chosen by the expert in digital trace recovery to avoid destroying the evidence. For this purpose, a better understanding of the chemical composition of a digital trace, namely the fingermark, also referred to as latent fingerprint, is needed.
The fingermark results from a transfer of naturally produced substances present on the fingertip or from other body parts such as the face touched by an individual. These substances present on the fingertip originating from skin secretion, including sweat and sebum (endogenous content), are mixed with contaminants from the external environments and microbial communities (exogenous content). Consequently, examining the chemistry of a fingertip can provide a glimpse into the lifestyle and personal habits of the owner, including their general hygiene, diet, medication, personal care products used and locations visited. When applied to forensic investigations, the profiling of a suspect’s lifestyle can lead to a better understanding of the facts of a case, especially when fingermarks found at the crime scene are blurred, dirty or incomplete and do not allow for formal identification.
In this regard, the modernization of chemical analysis technologies led scientists to explore new possibilities to further analyze fingermarks sampled from a crime scene. Mass spectrometry (MS) is an analytical chemistry technique that significantly contributes in many areas of research by providing qualitative and quantitative information on molecular composition. In forensic science, mass spectrometry is used to analyze traces for evidence in a police investigation. The scientific interpretation of results contributes to an essential input of information to support police investigations and judicial processes. Since its introduction in forensic science, mass spectrometry has identified various substances found at the crime scene, including explosives, firearm residues, illicit drugs, poisons, pharmaceutical compounds and traces of blood. This area of science is in constant development to counter new stratagems resulting from the ingenuity of some outlaws.
Imaging mass spectrometry (IMS) is a molecular imaging technique used to visualize the spatial distribution of molecules across a surface, such as biomarkers in tissue sections for medical research. More recently, this technique has shown to be a powerful tool for the analysis of fingermarks, since it combines the detection of numerous endogenous and exogenous substances present at the fingertip’s suspect while creating molecular images of the fingermark, therefore of the fingerprint, allowing suspect identification. After a decade of research in this field, the laboratory of Pr. Simona Francese, at Sheffield Hallam University (United Kingdon), was the first to implement an IMS method into the current fingerprinting workflow of the police authorities. To use this technology here in the province of Quebec, an alternative method was developed in our laboratory for its application on currently available IMS instruments in the city of Montreal.
Silver-assisted laser-desorption ionization (AgLDI) IMS was initially developed in our laboratory to profile and image low molecular weight olefin containing molecules from thinly cut tissue sections and is applied here for the chemical analysis of fingermarks. After the deposition of a thin layer of metallic silver (~14 nm) on the fingermarks, we were able to detect numerous endogenous substances, including fatty acids, wax esters, squalene, cholesterol and triglycerides, as well as many exogenous substances, such as a variety polymers and quaternary ammonium compounds found in common personal care products. The silver deposition is carried out using a metal sputtering system in order to provide a homogenous coating across the fingermark for the generation of high spatial resolution molecular images.
Thanks to the conductive properties of silver, this method allows for the analysis of fingermarks left on several nonconductive porous and nonporous surfaces. Indeed, sample conductivity is an essential parameter for IMS analysis using time-of-flight mass analyzers, which are commonly found in the Montreal area. However, another important aspect to consider when developing a method for the analysis of fingermarks is their capacity to complement the forensic enhancement techniques already in use by crime scene technicians. Through a collaboration with the fingerprint specialist of the Sûreté du Québec (SQ), we have been able to demonstrate the possibility of generating high quality molecular images of fingermarks first revealed in the SQ laboratory on various nonconductive surfaces, including paper, plasticized cardboard, plastic, as well as fingermarks collected using forensic lifting tape. In this context, the analysis of fingermarks by AgLDI IMS also makes it possible to counter several contrast issues frequently encountered when using optical revealing techniques on multicolor surfaces, such as cigarette cardboards. Looking at the distribution of individual molecules enables us to generate multiple chemical images of the fingerprint pattern of the suspect independently of the color of the background. Using the chemical specificity of individuals, IMS can also be used to differentiate overlapping fingermarks from two individuals. It can also be used for the detection of overlapping inks in falsified documents.
The primary purpose of MS in forensic science is nevertheless to identify substances that can contribute to criminal investigations. In this regard, we also evaluated the potential of AgLDI IMS to detect and map various exogenous substances present in fingermarks, such as those contained in cosmetics, personal care products, illicit drugs (THC, cocaine and heroin) as well as traces of blood. Blood is a highly interesting substance to detect because it can be very useful for the chronological reconstruction of events at the crime scene. Therefore, blood-specific enhancement techniques are used at the crime scene when blood is suspected to be present. Through a collaboration with criminal identification technicians from the Service de Police de la Ville de Montréal (SPVM), we have demonstrated that blood-specific molecular images can be generated from bloody fingermarks deposited on various conductive and nonconductive surfaces first revealed by common blood forensic enhancement techniques.
Beyond the scientific perspectives that can be explored in the laboratory, the continuation of this research is essentially based on the interest of the police forces in implementing molecular imaging in the processes of evidence analysis. We and others have demonstrated that IMS methodology has reached a level of maturity capable of generating high-quality molecular images from fingermarks, and we foresee that it will be successfully used in forensic investigations to provide additional information to criminal investigations.
Nidia Lauzon is a Ph.D. candidate at the Chemistry Department of the Université de Montréal in the laboratory of Pr. Pierre Chaurand.
Pr. Pierre Chaurand (Ph.D. 1994, Université Paris Sud, Orsay, France) is Professor of Chemistry at the Université de Montréal (2009 – present). His expertise’s are in fundamental and analytical mass spectrometry. He is one of the pioneers of the imaging mass spectrometry (IMS) technology. His research interests are focused on the development of new strategies and methods to improve the specificity and sensitivity of IMS with applications in forensic science and clinical biology.
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