Latent fingermarks are an important form of crime-scene trace evidence and their usefulness may be increased by a greater understanding of the effect of chemical distribution within fingermarks on the sensitivity and robustness of fingermark detection methods. Specifically, the relative abundance and micro-distribution of sebaceous (lipophilic) and eccrine (hydrophilic) material in fingermarks have long been debated in the field, yet direct visualisation of relative abundance and micro-distribution was rarely achieved. Such a visualisation is nonetheless essential to provide explanations for the variation in reproducibility or robustness of latent fingermark detection with existing methods, and to identify new strategies to increase detection capabilities. In this investigation, we have used SR-ATR-FTIR and confocal Raman microscopy to probe the spatial micro-distribution of the sebaceous and eccrine chemical components within latent fingermarks, deposited on non-porous surfaces. It was determined that fingermarks exhibit a complex spatial distribution, influenced by the ratio of lipophilic to aqueous components, and to a first approximation resemble a water-in-oil or oil-in-water emulsion. Detection of a substantial lipid component in "eccrine enriched fingermarks" (wherein hands are washed to remove lipids) is noteworthy, as it provides a potential explanation for several scenarios of unexpected fingermark detection using methods previously thought unsuitable for "eccrine deposits". Furthermore, the pronounced distribution of lipids observed in natural fingermark deposits was intriguing and agrees with recent discussion in this research field that natural fingermarks contain a much higher lipid content than previously thought.

No abstract

Fingermarks are an important form of crime-scene trace evidence; however, their usefulness may be hampered by a variation in response or a lack of robustness in detection methods. Understanding the chemical composition and distribution within fingermarks may help explain variation in latent fingermark detection with existing methods and identify new strategies to increase detection capabilities. The majority of research in the literature describes investigation of organic components of fingermark residue, leaving the elemental distribution less well understood. The relative scarcity of information regarding the elemental distribution within fingermarks is in part due to previous unavailability of direct, micron resolution elemental mapping techniques. This capability is now provided at third generation synchrotron light sources, where X-ray Fluorescence Microscopy (XFM) provides micron or sub-micron spatial resolution and direct detection with sub-μM detection limits. XFM has been applied in this study to reveal the distribution of inorganic components within fingermark residue, including endogenous trace metals (Fe, Cu, Zn), diffusible ions (Cl-, K+, Ca2+), and exogeneous metals (Ni, Ti, Bi). This study incorporated a multi-modal approach using XFM and Infrared Microspectroscopy (IRM) analyses to demonstrate co-localisation of endogenous metals within the hydrophilic organic components of fingermark residue. Additional experiments were then undertaken to investigate how sources of exogenous metals (e.g. coins and cosmetics) may be transferred to, and distributed within latent fingermarks. Lastly, this study reports a preliminary assessment of how environmental factors such as exposure to aqueous environments may effect elemental distribution within fingermarks. Taken together, the results of this study advance our current understanding of fingermark composition and its spatial distribution of chemical components, and may help explain detection variation observed during detection of fingermarks using standard forensic protocols. File list (3) download file view on ChemRxiv XFM_Mapping_Fingermarks_V1.pdf (4.82 MiB) download file view on ChemRxiv XFM_Mapping_Fingermarks_TOCentry.png (1.22 MiB) download file view on ChemRxiv XFM_Mapping_Fingermarks_SupportingInfo_V1.pdf (7.11 MiB)

No abstract

The quantitative variation in latent fingermark deposits sampled from the same donor (intra-donor) poses considerable challenges to studies into the chemical composition of latent fingermarks. The work presented here investigates approaches to the sampling of latent fingermark residues within this context. The amount of squalene in fingermarks deposited on non-porous surfaces, determined by GC-MS, was used as an indicator of the amount of non-polar material present. It was found that the percentage difference of squalene between deposits from two hands at a given time, without controlling the deposition force, was in the range of 4-100%. This was reduced to 0-44% in alternative sampling approaches where deposition force was controlled. These results demonstrate the significant influence of sampling on subsequent chemical analysis of fingermark residues, and offer possible sampling strategies to overcome issues associated with intra-donor variation.