Forensic Chemistry

Sex Determination Through the Chemical Analysis of Fingerprints
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Sex Determination Through the Chemical Analysis of Fingerprints

Fingerprints have been a staple of forensic science for decades, providing a reliable (though not perfect) means of identifying suspects and placing people at crime scenes. In recent years, scientists have turned their attention towards exploring the additional information that could be extracted from fingerprints, in particular, chemical information for police intelligence. Fingerprints are composed of a mixture of chemical compounds both excreted through our skin and picked up from the environment.

Over the past decade, analytical chemists have shown that there may be sufficient chemical differences in the fingerprints of male and female donors to differentiate between the sexes. By studying these chemical differences, it may be possible to build a model capable of predicting the sex of a fingerprint donor based on the chemical compounds within the fingerprint.

In a recent study published in Forensic Chemistry, a method was developed to predict the sex of a donor based on the presence of peptides and proteins in fingermarks. The study used MALDI-MS (Matrix-assisted laser desorption/ionization mass spectrometry), a type of mass spectrometry that enables both the measurement and identification of the chemical constituents in a material, in addition to the imaging of samples. MALDI has already been demonstrated to be a powerful technique in the chemical analysis of fingermarks, including imaging fingermarks and detecting the presence of blood and drugs in the mark. Much of the research in the area has been led by Professor Simona Francese of Sheffield Hallam University in the UK, senior author on this recent paper.

In the study, 199 participants donated multiple fingermarks, culminating in hundreds of samples for analysis. The fingermarks used for the analysis were natural, that is to say, deposited with no fingertip preparation beforehand. Many studies in the chemical analysis of fingerprints use “groomed” marks, in which the fingers are rubbed on the face or forehead in order to load the fingers with skin secretions prior to deposition. Although this provides rich samples for analysis, they are not applicable to fingermarks encountered in the real world.

After fingermark collection, MALDI-MS was then used to analyse the samples, focusing on the measurement of peptides and proteins in the deposited marks. To further mimic realistic scenarios, fingermarks were analysed both undeveloped and enhanced by common fingerprint visualisation techniques (white powder and vacuum metal deposition). Based on the chemical profiles produced, a predictive model was then constructed for the purpose of predicting the sex of the donor of unknown fingermarks, such as those that may be discovered at crime scenes.

The technique had a predictive power of up to 86%, demonstrating the potential to differentiate between male and female donors to a degree. There were, however, challenges in this study. Polyethylene glycol (PEG)-based contaminants, routinely used in cosmetics and personal care products, were commonly encountered, interfering with the detection of the actual targets of the analysis. Furthermore, the application of fingermark development techniques also caused interference, with many mass spectra being dominated by the gold nanoparticles used in the vacuum metal deposition method. This suggests the technique may only be truly applicable in the case of undeveloped fingermarks.

Although the technique has a high predictive power, it was not able to determine the sex of the donor in all cases, rendering it unsuitable for conclusively excluding suspects from an investigation based on their sex. However the method could be used to triage fingermarks, allowing investigators to establish which marks are of the greatest importance and which should be prioritised for further study, potentially speeding up forensic investigations.

 

Heaton & Bury et al. Investigating sex determination through MALDI MS analysis of peptides and proteins in natural fingermarks through comprehensive statistical modelling. 2020, Forensic Chemistry, DOI: 10.1016/j.forc.2020.100271

Speeding Up Sexual Assault Investigations with Chemistry
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Speeding Up Sexual Assault Investigations with Chemistry

A recent study has demonstrated a new technique for the rapid detection of semen and the chemical differentiation of condoms, offering a new potential tool to assist sexual assault investigations.

Hundreds of thousands of cases of sexual assault and rape are reported each year, though this is undoubtedly a fraction of the actual number. The successful identification and prosecution of offenders in sexual assault cases often hinges on the ability to detect and collect biological fluids such as semen, essential for supporting victim statements and identifying the offender. In the past, the identification of semen has relied on destructive, non-specific presumptive tests, often based on a colour change reaction in the presence of a specific chemical in semen. In recent years, there has been a push for the development of preparation-free analytical techniques that could be used for the analysis of sexual assault evidence at crime scenes or in hospitals.

In a recent study published in Forensic Chemistry, a method using ambient ionisation mass spectrometry has been developed for the analysis of sexual assault evidence, specifically semen and condoms. Ambient ionisation MS refers to a type of mass spectrometry which allows the rapid, direct analysis of a material, eliminating the need for the time-consuming and destructive sample preparation steps that limit traditional techniques.

This particular technique, called sheath-flow probe electrospray ionisation MS, uses a small, cheap-to-construct probe that is simply touched to the surface of the sample. The probe is then placed in front of a mass spectrometer inlet and a voltage applied to produce an instant unique chemical profile. In all, a sample can be analysed in a matter of seconds.

In this study, semen was successfully detected on various materials, such as fabric and condoms, mimicking the kind of environments the body fluid could be encountered during a sexual assault investigation and, crucially, showing the ability of the technique to work with different surfaces. Whereas presumptive tests for semen are focused on the presence of one chemical, making them prone to “false positives”, this new technique harnesses a suite of chemicals, allowing a more confident identification of semen. It was also shown that semen could still be detected after 40 days of ageing. This is important as during a criminal investigation it may be days, weeks or even longer before evidence is seized and analysed. After 40 days, the chemical profile was remarkably unchanged, indicating that even older semen could potentially be identified.

The study then took this technique one step further, applying it to the analysis of condoms. As criminals become more knowledgeable about forensic evidence such as DNA, there has been an increase in the use of condoms by criminals to protect their identity. By analysing the surface of different brands and types of condom, it was demonstrated that unique chemical profiles were associated with the different condoms, with notable chemicals relating to the condom’s material or flavouring being detected. This has two major implications. Firstly, the ability to detect lubricants and traces from condoms could prove beneficial in confirming condom use, particularly important when biological evidence is lacking. Furthermore, the unique chemical profiles could even open up the possibility of indicating what type of condom was used by the offender, offering more information to the investigation.

Direct analysis techniques such as this have great potential to speed up forensic investigations, but it will no doubt be years before such technology is considered for adoption by police forces.

 

Rankin-Turner et al. Using mass spectrometry to transform the assessment of sexual assault evidence. Forensic Chemistry, 2020, DOI: 10.1016/j.forc.2020.100262

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