Fingermark age determinations: Legal considerations, review of the literature and practical propositions

A common framework to situate digital and physical traces in time

In this article, three main approaches to situate forensic traces in time were revisited under the prism of the Sydney Declaration and adapted to be applicable to a large range of physical and digital traces. The first approach is based on time tags which are time-based characteristics produced as the result of an activity at a specific time. They can either be directly related to time (i.e., time stamps) or indirectly (i.e., time indicators). While relatively straightforward, time tags require scientific knowledge to be correctly interpreted and to account for the risks of desynchronisation, anomalies and manipulation. The second approach is based on time dynamics and aim at measuring changes that occur as a function of time, such as caesium pulsation (i.e., on which international atomic time is based) or body cooling after death (i.e., from which time since death can be inferred). However, time dynamics phenomena are generally also influenced by other case-specific factors (e.g., environmental factors), and thus more difficult to reliably implement in practice. Finally, the third approach relies on relative sequences, using information unrelated to time, such as relative positions or dynamics of traces at the scene. As each approach has its potential and limitations, a combination of traces from different (both material and digital) sources and approaches is recommended to answer time questions in practice (When? How long? In which succession?) and enhance the reliability of the dating endeavours. It is strongly recommended to consider the principles of the Sydney Declaration when implementing or developing dating methods, as they point at potential issues that are often forgotten in forensic research and practice, such as uncertainties linked to the concept of trace, scene investigation, the asymmetry of time, the importance of context and the multiplicity of purposes. Future research should focus on improving the reliability of these dating approaches by combining and systematising their usage in investigative practice, as well as in broader intelligence processes.

A preliminary study of fingerprint aging using near infrared hyperspectral imaging (NIR-HSI)

Fingerprinting is one of the most commonly used techniques to obtain pieces of evidence for identification of individuals. An estimation of how long a trace has been left at a crime scene could represent an important improvement for criminal investigations. There is no reliable analytical method, however, to estimate the age of a fingerprint, since this is an uncontrolled process and changes are affected by factors such as environmental conditions. This study aims to better understand the aging process of fingerprints and identify the relevant variables and limitations of the fingerprint aging process using near infrared hyperspectral imaging (NIR-HSI). For this purpose, aging of the fingerprints of 13 volunteers was evaluated using partial least squares - discriminant analysis (PLS-DA) as a preliminary exploratory approach. Four different modelling approaches were evaluated. The percentage of correctly classified pixels varied from 20.92% to 66.67%. An analysis of the associated spectra found that during the first days of aging the degradation of fat-soluble components, as well as the elimination/absorption of water, seemed to follow non-uniform trends and vary in degradation rate from donor to donor. Better classification tended to occur over longer aging times.

A comparison of the natural and groomed fingermark lipid composition of different donors using GC/MS

The lipid composition of natural fingermarks was studied and compared with the composition of groomed residue. Approximately 100 specimens were collected from 6 donors over three sessions (in October, December and July) and analysed using gas chromatography / mass spectrometry (GC/MS). The measured lipid content was generally lower and more variable in natural fingermarks than in groomed fingermarks. Some significant variability was noticed. Relative standard deviations were the highest between donors (generally above 100%) but were also relatively high within donor within a session (from 21% to 80%) and between sessions (from 34% to 126%). The fingermarks from one of the donors generally contained higher relative amounts of lipids in both groomed and natural residue compared to the others. All other fingermarks led to very variable amounts and did not allow classifying the other donors as constantly "good" or "poor" donors. Squalene was the major compound in all marks, particularly in groomed specimens. A correlation between squalene, cholesterol, myristic acid, palmitoleic acid, stearyl palmitoleate and pentadecanoic acid was highlighted. Oleic and stearic were also correlated together but generally more in natural than groomed marks. The obtained results may be particularly useful to better understand the detection mechanisms for techniques targeting lipids and to develop artificial fingermark secretions to further support the development of detection techniques.

Evaluation of 3D and 2D chronomorphometrics for latent fingermark aging studies

Latent fingermark age determination is a relatively new subdiscipline of friction ridge pattern analysis that has the potential to ascertain two key pieces of information: the "who" and the "when" from a single evidence item. In this proof-of-concept study, the sensitivity and suitability of a series of 3D and 2D chronomorphometrics, ie, quantitative measurement of form as a function of time, are examined under various temperatures (55, 21, 4, and -20°C) at 6 months. 3D imaging with an optical profiler (OP) in tandem with a conventional 2D fingermark enhancing method, ie powdering with titanium dioxide, is the technique selected. From a chronomorphometric perspective, the 3D-OP detects micrometer variations in surface texture with regard to the heights (Sa and Ra) and volume (Vu) of the ridges, whereas 2D imaging provides information on color contrast (MI and IA), the fingermark visual quality score (QS), and the relative area extension of color-coded maps of ridge clarity (BlueGreen, BG). Statistical analyses have revealed different degrees of sensitivity of the 2D and 3D metrics for detecting the effect of temperature and time, being 3D the most discriminating. In these tested conditions, warmer temperatures (55 and 21°C) have shown the most impact on ridge degradation with the lowest levels observed at -20°C. The concurrent application of 2D and 3D metrics would be considered the best approach to advance the knowledge on fingermark aging processes and possible mathematical models.

Preliminary efficiency evaluation of development methods applied to aged sebaceous latent fingermarks

Latent fingermarks are important trace evidence found in crime scenes mainly used for identification purposes. Once deposited, the composition of samples changes over time influencing the efficacy of latent fingermark development methods. In this sense, the aims of this work were to characterize the fatty acid (FA) profile of sebaceous latent fingermarks by GC-FID and to preliminarily evaluate the development efficiency of enhancement methods (powder dusting, iodine fuming and silver nitrate method) in a 30-day period of aging under controlled parameters of temperature, photoperiod and humidity. Results showed that myristic (7.51 ± 0.76% - 13.39 ± 1.26%), palmitic (35.91 ± 1.07% - 40.81 ± 2.52%), stearic (6.67 ± 0.36% - 9.13 ± 0.36%) and oleic (18.08 ± 0.25% - 20.93 ± 0.26%) acid varied significantly (p < 0.05) over the 30-day period of analysis. Regarding development efficiency, fluorescent orange powder and the silver nitrate method also increased their efficacy to develop latent fingermarks over time while the iodine fuming method decreased its efficiency. Silver black powder had constant efficacy in the tested period. Changes in the constitution of sebaceous marks possibly influenced the development efficiency of enhancement techniques. This knowledge is important to better understand the kinetic of aging and its influence on the development method.

Advances in fingermark age determination techniques

Fingermarks have long been recognized as one of the most reliable and valuable evidence for personal identification. In practice, fingerprint analysis primarily concentrates on latent fingerprint visualization. However, fingerprint visualization techniques do not always enable individualization when fingermarks collected in crime scenes are fragmentary, ambiguous, or deformed. Age determination techniques based on physical and chemical composition changes in fingerprints over time have attracted researchers' attention in recent years. Nevertheless, the components of fingerprints are liable to factors including donor features, deposition conditions, substrate properties, environmental conditions and revealing methods. All the influences mainly contribute to unreliable outcomes of age estimation. Recent developments in fingermark age determination have moved forward to more precise approaches. The advanced methods can be classified into two categories including techniques based on the modifications of physical characteristics and chemical composition characteristics. Herein, the review includes the five types of variables that influence the aging process. The methodologies are subsequently highlighted along with their advantages and disadvantages. Furthermore, photography, optical, microscopy and electrochemical methods, and vibrational spectroscopy and mass spectrometry (MS) techniques are summarized in detail, with an emphasis on their utilization.

Latent Fingermark Aging Patterns (Part IV): Ridge Width as One Indicator of Degradation

This fourth article of the series is taking an in-depth analysis at the visible aging of latent fingermarks regarding changes in ridge widths over time. The objective is to quantify and statistically describe significant ridge size variations under controlled indoor conditions. The effect of three environmental variables are examined: type of secretion (sebaceous- and eccrine-rich) and type of substrate (glass and polystyrene) when aged in three light conditions (direct natural light, shade, and dark). Prior to width measurements, fresh and aged fingermarks were powdered with titanium dioxide (TiO₂ ) and sequentially photographed at predetermined times over 6 months. Three independent observers measured the ridges from thirty predetermined locations using strategically placed intersecting lines on the print. Results indicate that fingermarks deposited on glass are more resilient to degradation compared with those deposited on plastic. The presence of direct natural light plays a negligible role on degradation compared to secretion and substrate types.

Proteomics as a new tool to study fingermark ageing in forensics

Fingermarks are trace evidence of great forensic importance, and their omnipresence makes them pivotal in crime investigation. Police and law enforcement authorities have exploited fingermarks primarily for personal identification, but crucial knowledge on when fingermarks were deposited is often lacking, thereby hindering crime reconstruction. Biomolecular constituents of fingermark residue, such as amino acids, lipids and proteins, may provide excellent means for fingermark age determination, however robust methodologies or detailed knowledge on molecular mechanisms in time are currently not available. Here, we address fingermark age assessment by: (i) drafting a first protein map of fingermark residue, (ii) differential studies of fresh and aged fingermarks and (iii), to mimic real-world scenarios, estimating the effects of donor contact with bodily fluids on the identification of potential age biomarkers. Using a high-resolution mass spectrometry-based proteomics approach, we drafted a characteristic fingermark proteome, of which five proteins were identified as promising candidates for fingermark age estimation. This study additionally demonstrates successful identification of both endogenous and contaminant proteins from donors that have been in contact with various bodily fluids. In summary, we introduce state-of-the-art proteomics as a sensitive tool to monitor fingermark aging on the protein level with sufficient selectivity to differentiate potential age markers from body fluid contaminants.

Ambient-air ozonolysis of triglycerides in aged fingerprint residues

In forensic science, reconstructing the timing of events occurring during a criminal offense is of great importance. In some cases, the time when particular evidence was left on a crime scene is a critical matter. The ability to estimate the fingerprint age would raise the evidentiary value of fingerprints tremendously. For this purpose the most promising approach is the analysis of changes in the chemical compositions of fingerprint residues in the course of aging. The focus of our study is the identification of human specific compounds in fingerprint residues, characterized by a significant aging behavior that could analytically be used for the age determination of fingerprints in future. The first challenge is the sensitive detection of trace amounts of relevant human specific fingerprint compounds. Highly sensitive LC-MS methods were developed for the reliable structure identification of unsaturated triglycerides and their natural degradation products in order to proof the aging mechanism that takes place in fingerprint residues. Thus our results build the fundamental basis for further forensic method development and potential application in forensic investigation. Ozonolysis was found to be one of the major lipid degradation pathways in fingerprint residues in ambient air. High-resolution tandem mass spectrometry (HRMS²) was carried out to identify the ozonolysis products (TG48:0-monoozonide) formed under exposure to the highly reactive ozone in atmospheric air. The obtained products were confirmed by matrix assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI). Despite several challenges and limitations in the age estimation of fingerprints, the identification of individual degradation products of specific unsaturated lipids in aged fingerprint samples represents a significant analytical progress, resulting in a strong increase in the validity of chemical analysis of fingerprints.

Ink dating part II: Interpretation of results in a legal perspective

The development of an ink dating method requires an important investment of resources in order to step from the monitoring of ink ageing on paper to the determination of the actual age of a questioned ink entry. This article aimed at developing and evaluating the potential of three interpretation models to date ink entries in a legal perspective: (1) the threshold model comparing analytical results to tabulated values in order to determine the maximal possible age of an ink entry, (2) the trend tests that focusing on the "ageing status" of an ink entry, and (3) the likelihood ratio calculation comparing the probabilities to observe the results under at least two alternative hypotheses. This is the first report showing ink dating interpretation results on a ballpoint be ink reference population. In the first part of this paper three ageing parameters were selected as promising from the population of 25 ink entries aged during 4 to 304days: the quantity of phenoxyethanol (PE), the difference between the PE quantities contained in a naturally aged sample and an artificially aged sample (R_NORM) and the solvent loss ratio (R%). In the current part, each model was tested using the three selected ageing parameters. Results showed that threshold definition remains a simple model easily applicable in practice, but that the risk of false positive cannot be completely avoided without reducing significantly the feasibility of the ink dating approaches. The trend tests from the literature showed unreliable results and an alternative had to be developed yielding encouraging results. The likelihood ratio calculation introduced a degree of certainty to the ink dating conclusion in comparison to the threshold approach. The proposed model remains quite simple to apply in practice, but should be further developed in order to yield reliable results in practice.

Ink dating, part I: Statistical distribution of selected ageing parameters in a ballpoint inks reference population

The development of ink dating methods requires an important amount of work in order to be reliably applicable in practice. Major tasks include the definition of ageing parameters to monitor ink ageing. An adequate parameter should ideally fulfil the following criteria: it should evolve as a function of time in a monotonic way, be measurable in a majority of ink entries, be as accurate and reproducible as possible, and finally it should not be influenced too much by transfer and storage conditions. This work aimed at evaluating the potential of seven ageing parameters for ink dating purposes: the phenoxyethanol quantity, relative peak areas (RPA), three solvent loss ratios (R%, R%*, NR%) and two solvent loss parameters (R_NORM, NR_NORM). These were calculated over approximately one year for 25 inks selected from a large database to represent different ageing behaviours. Ink entries were analysed using liquid extraction followed by GC/MS analysis. Results showed that natural ageing parameters (NR% and NR_NORM) were not suitable ageing parameters for ink entries older than a few weeks. RPA used other compounds present in ink formulations in combination to PE in order to normalise the results. However, it presented particular difficulties as they could not be defined for all inks and were thus applicable only for 64% of the studied inks. Finally, the PE quantity, R% and R_NORM allowed to follow the ageing of the selected inks over the whole time frame and were identified as the most promising. These were thus selected to test three different interpretation models in the second part of this article. The possibilities and limitations of ink dating methods will be discussed in a legal perspective.

Towards Fingermark Dating: A Raman Spectroscopy Proof-of-Concept Study

Fingermarks have, for a long time, been vital in the forensic community for the identification of individuals, and a possibility to non‐destructively date the fingermarks would of course be beneficial. Raman spectroscopy is, herein, evaluated for the purpose of estimating the age of fingermarks deposits. Well‐resolved spectra were non‐destructively acquired to reveal spectral uniqueness, resembling those of epidermis, and several molecular markers were identified that showed different decay kinetics: carotenoids > squalene > unsaturated fatty acids > proteins. The degradation rates were accelerated, less pronounced for proteins, when samples were stored under ambient light conditions, likely owing to photo‐oxidation. It is hypothesized that fibrous proteins are present and that oxidation of amino acid side chains can be observed both through Raman and fluorescence spectroscopy. Clearly, Raman spectroscopy is a useful technique to non‐destructively study the aging processes of fingermarks.

Effect of Aging and Surface Interactions on the Diffusion of Endogenous Compounds in Latent Fingerprints Studied by Mass Spectrometry Imaging

The ability to determine the age of fingerprints would be immeasurably beneficial in criminal investigations. We explore the possibility of determining the age of fingerprints by analyzing various compounds as they diffuse from the ridges to the valleys of fingerprints using matrix-assisted laser desorption/ionization mass spectrometry imaging. The diffusion of two classes of endogenous fingerprint compounds, fatty acids and triacylglycerols (TGs), was studied in fresh and aged fingerprints on four surfaces. We expected higher molecular weight TGs would diffuse slower than fatty acids and allow us to determine the age of older fingerprints. However, we found interactions between endogenous compounds and the surface have a much stronger impact on diffusion than molecular weight. For example, diffusion of TGs is faster on hydrophilic plain glass or partially hydrophilic stainless steel surfaces, than on a hydrophobic Rain-x treated surface. This result further complicates utilizing a diffusion model to age fingerprints.

Electrochemical impedance spectroscopy: A deeper and quantitative insight into the fingermarks physical modifications over time

The present work is focused on a novel approach for the study and quantification of some of the physical changes to which a fingermark deposited on non-porous substrates is subjected as its ageing proceeds. Particularly, electrochemical impedance spectroscopy (EIS) technique has been applied for the first time in order to monitor the electrochemical behaviour of the system constituted by the fingermark residue and the underlying substrate. The impedance spectra proved to be significantly affected by the presence of the mark residue as well as by its ageing process. Opportune fitting operations performed on the experimental data allowed obtaining quantitative electrochemical parameters used to reach useful information on the fingermarks ageing mechanism as well as to calculate the fingermark ageing curves from which fundamental information could be potentially extrapolated.

Fatty Acid Structure and Degradation Analysis in Fingerprint Residues

GC-MS investigations were carried out to elucidate the aging behavior of unsaturated fatty acids in fingerprint residues and to identify their degradation products in aged samples. For this purpose, a new sample preparation technique for fingerprint residues was developed that allows producing N-methyl-N-trimethylsilyl-trifluoroacetamide (MSTFA) derivatives of the analyzed unsaturated fatty acids and their degradation products. MSTFA derivatization catalyzed by iodotrimethylsilane enables the reliable identification of aldehydes and oxoacids as characteristic MSTFA derivatives in GCMS. The obtained results elucidate the degradation pathway of unsaturated fatty acids. Our study of aged fingerprint residues reveals that decanal is the main degradation product of the observed unsaturated fatty acids. Furthermore, oxoacids with different chain lengths are detected as specific degradation products of the unsaturated fatty acids. The detection of the degradation products and their chain length is a simple and effective method to determine the double bond position in unsaturated compounds. We can show that the hexadecenoic and octadecenoic acids found in fingerprint residues are not the pervasive fatty acids Δ9-hexadecenoic (palmitoleic acid) and Δ9-octadecenoic (oleic acid) acid but Δ6-hexadecenoic acid (sapienic acid) and Δ8-octadecenoic acid. The present study focuses on the structure identification of human sebum-specific unsaturated fatty acids in fingerprint residues based on the identification of their degradation products. These results are discussed for further investigations and method developments for age determination of fingerprints, which is still a tremendous challenge because of several factors affecting the aging behavior of individual compounds in fingerprints. Graphical Abstract ᅟ.