Imagerie moléculaire d’empreintes digitales par spectrométrie de masse : 
potentiels et applications en science forensique

Mass spectrometry is an analytical chemistry technique that significantly contributes in many areas of research by providing qualitative and quantitative information on molecular composition. The ongoing development of new technologies and new methods to improve the specificity and sensitivity of detection makes this technology a tool that continues to grow in interest. In forensic science, mass spectrometry is used to analyze trace level species as 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, such as explosives, firearm residues, fire debris, illicit drugs, poisons, pharmaceutical compounds, traces of blood, and many more. This area of science is in constant development to counter new stratagems resulting from the ingenuity of criminals. The recovery of latent fingerprints from crime scenes has been one of the most important and common methods in forensic investigation. The modernization of chemical analysis technologies led scientists to explore new possibilities to further analyze fingermarks sampled from a crime scene. Indeed, the detection of chemicals a suspect has been in contact with before or during the crime can provide valuable insights in criminal investigations. This information can positively contribute to forensic investigations, especially when fingermarks found at the crime scene are blurred, dirty or incomplete and do not allow for formal identification. In this regard, imaging mass spectrometry (IMS) has shown to be a powerful tool for the analysis of fingermarks by combining the suspect identification and the detection of numerous endogenous and exogenous compounds. In 2009, the laboratory of Professor Simona Francese developed an IMS method for the analysis of fingermarks based on matrix-assisted laser desorption/ionization (MALDI). This method proved its robustness in conjunction with forensic enhancement techniques (FETs) used by technicians in criminal identification. Since then, their laboratory has demonstrated the capability to detect several endogenous and exogenous substances in fingermarks that can contribute to forensic investigations. Most recently, their method has been implemented by the UK authorities as a complement to forensic standard operational procedures. However, the use of MALDI matrices has several disadvantages for IMS analysis, such as the generation of in-source fragments and clusters that contaminate spectra at low masses and considerably increase background noise. Further, its mode of deposition can lead to specific molecule delocalization in the sample. In order to overcome these issues, new horizons have been explored for the development of alternative IMS methods in contrast to those proposed by Francese et al., such as MALDI matrix sublimation and the use of metals as desorption/ionization agents. The primary objective is to evaluate various IMS laser desorption/ionization (LDI) assisted by matrix or metal methods for the detection and imaging of molecules contained in a fingermark. Due to the fragility of the fingerprint pattern, solvent-free deposition methods were explored. The choice of the best method was evaluated according to the number of compounds observed, the signal intensity, the background noise and other practical advantages for the analysis of fingermarks. In this regard, the deposition of metallic silver by sputtering has led to the detection of several endogenous and exogenous substances. In addition, the silver assisted LDI (AgLDI) method generates molecular images at high spatial resolution (~10 μm) useful for criminal identification purposes. Thanks to the conductive properties of silver, a second research objective was considered, that is to evaluate the capability of AgLDI-TOF IMS to be implemented in complement to FETs used by crime scene technicians. Indeed, the major advantage of this method lies in its ability to render a surface conductive, an essential criterion for MS analysis by LDI-TOF. Through a collaboration with the fingerprint specialist of the Sûreté du Québec, Alexandre Beaudoin, we have been able to demonstrate the possibility of generating molecular images of fingermarks revealed in their laboratory on various nonconductive surfaces, including paper, plasticized cardboard, plastic, as well as 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 FETs on multicolor surfaces. The primary purpose of forensic science in MS is nevertheless to identify substances that can contribute to criminal investigations. In this regard, we have demonstrated the potential of AgLDI IMS for the detection in fingermarks of several commonly found illicit drugs (THC, cocaine and heroin), as well as various cosmetic and personal care products linked to the lifestyle of the suspect. Another highly interesting substance for chronological reconstruction of events at the crime scene is the detection of blood in fingermarks. Through a collaboration with the criminal identification technicians from the Service de Police de la Ville de Montréal (SPVM), we have demonstrated the possibility of generating molecular images of bloody fingermarks first revealed by blood-specific enhancement techniques in their laboratory on several nonconductive surfaces.

Keyphrases: Mass spectrometry imaging, Silver assisted laser desorption/ionization, Forensic science, Fingerprint, Fingermark, Nonconductive surfaces, Illicit drugs, Blood traces, Personal care products and cosmetics