Light in Forensic Science

Issues and Applications

Giorgia Miolo, Jacqueline L Stair, Mire Zloh, Giorgia Miolo, Jacqueline L Stair, Mire Zloh

• Éditeur: Royal Society of Chemistry
• Année: 2018
• ISBN:
  9781788013987
• Édition: 1
• Sujet: Ciencias físicas
• Sous-sujet: Óptica y luz

The identification and quantification of material present and collected at a crime scene are critical requirements in investigative analyses. Forensic analysts use a variety of tools and techniques to achieve this, many of which use light. Light is not always the forensic analyst's friend however, as light can degrade samples and alter results.

This book details the analysis of a range of molecular systems by light-based techniques relevant to forensic science, as well as the negative effects of light in the degradation of forensic evidence, such as the breakage of DNA linkages during DNA profiling. The introductory chapters explain how chemiluminescence and fluorescence can be used to visualise samples and the advantages and limitations of available technologies. They also discuss the limitations of our knowledge about how light could alter the physical nature of materials, for example by breaking DNA linkages during DNA profiling or by modifying molecular structures of polymers and illicit drugs. The book then explains how to detect, analyse and interpret evidence from materials such as illicit drugs, agents of bioterrorism, and textiles, using light-based techniques from microscopy to surface enhanced Raman spectroscopy.

Edited by active photobiological and forensic scientists, this book will be of interest to students and researchers in the fields of photochemistry, photobiology, toxicology and forensic science.

Chapter 1

Light for Crime Scene Examination

Valerio Causin^*a and Giuseppe Guzzini^b

^a Università di Padova, Dipartimento di Scienze Chimiche, Via Marzolo 1, 35131 Padova, Italy;

^b Sezione Investigazioni Scientifiche CarabinieriVia Salvo d’Acquisto 637122 VeronaItaly

^*E-mail: [email protected]

Table of Contents

1.1 Introduction

1.2 A Brief Summary on the Theory of Light

1.3 Imaging on the Crime Scene: Finding Traces

1.3.1 Observation in the Absorption/Reflection Mode

1.3.2 Fluorescence

1.3.3 Chemiluminescence

1.4 Photographic Techniques: Documenting Traces

1.4.1 Absorption Mode

1.4.2 Diffuse Reflection Mode

1.5 The Lab-on-the-scene Approach

1.6 Conclusion

References

1.1 Introduction

Forensic science is a very powerful investigative tool, irreplaceable in many instances for the elucidation of complex cases and for an objective understanding of the dynamics of criminal acts. Looking back at the history of forensic science, every time a new technique became available for acquiring data on the crime scene, a disruptive step forward was introduced in the ability of police forces to identify and prosecute criminals and eventually to fight crime. This was especially true at the end of the 19th century when the first studies on fingerprints as means for the identification of individuals were published by Faulds and Galton.^1,2 Just a few years had elapsed from these seminal works when, in Argentine in 1892, Juan Vucetich was the first to solve a criminal case using fingerprints for the identification of a felon. This started the era of modern forensic science. An equally revolutionary advancement came with the development of DNA typing, in 1985.³ Since then, more and more sensitive techniques have been devised, decreasing the minimum sample size for obtaining a reliable DNA profile. Less than 30 years later, it is almost impossible to imagine investigation without DNA. Technology and science are shaping and enhancing the ability of forensic science to achieve its purpose, i.e. the study of traces related to crimes.⁴ Traces can be defined as the remnants of an activity and forensic science endeavours to deduce from the traces left on the crime scene as much information as possible on the crime itself. This concept is very well synthesised by the well known Locard’s principle, which is often defined as ‘every contact leaves a trace’ even though Locard himself never formulated such an expression. Locard’s words are very effective in stating this basic concept:

it is impossible for a criminal to act, and especially to act with the intensity that a crime requires, without leaving traces of his presence.⁵ This was later elaborated introducing the notion that traces can be evidence left by the felon on the crime scene, but also, for a reverse action, they can be items collected from the crime scene and transferred to the felon.⁶

Kirk very fittingly formulated the definition of traces as mute witnesses:

wherever he steps, whatever he touches, whatever he leaves, even unconsciously, will serve as a silent witness against him. Not only his fingerprints or his footprints, but his hair, the fibers from his clothes, the glass he breaks, the tool mark he leaves, the paint he scratches, the blood or semen he deposits or collects. All of these and more, bear mute witness against him. This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong, it cannot perjure itself, it cannot be wholly absent. Only human failure to find it, study and understand it, can diminish its value.⁷

From this short historical introduction, it should clearly emerge that traces and the Locard’s principle are foundation stones without which forensic science would not exist. Acknowledging that the purpose of forensic science is interrogating material remnants of a criminal activity provides a theoretical and philosophical framework for implementing science into the administration of justice in the most effective way. Differently to what appears in fiction, the role of the forensic scientist, in fact, is not to determine if the suspect is guilty or not, but it is to reconstruct as precisely as possible the chain of events associated to a crime, giving to the Court, to the investigators or to the lawyers reliable information to properly do their job.

In such a context, if contact is always accompanied by the transfer of some material, then the analysis and characterisation of such material can allow the forensic scientist to describe, prove or confirm the contact that originated it. Of course, such a logical path will have a successful outcome depending on a number of non-negligible factors. Transfer, persistence and recovery are the three main processes that, if successful, allow the trace, and especially the information associated with it, to reach the laboratory and eventually the Courtroom. In other words, traces must be transferred onto the crime scene or to the felon, they must remain on the crime scene or on the felon, and they must be found and retrieved from the crime scene or from the felon. This latter step is a considerable bottleneck in the process. Transfer and persistence depend on the dynamics of the crime, they are not related to the training or ability of investigators or scientists. However, if suitable procedures are not applied in the search and recovery of the items from the crime scene, there is a severe risk that some important information is lost or that contamination is introduced. In both cases, the work of investigators would be hindered rather than aided by forensic science. The fragility, the lability, the latency and the corruptibility of traces calls for highly qualified personnel operating on the crime scene, because any mistake made in this phase will jeopardise all the subsequent analyses and interpretation.

Crime scene investigation started as the set of procedures aimed at crystallising the crime scene and at describing it. Ottolenghi, a pioneer in forensic sciences in Italy, in the early 1900’s, extended the concept of Bertillon’s portrait parlé, which was used for giving an objective description of individuals, to the crime scene.⁸ Just as a detailed description of the physical features of a person can bring to a non-ambiguous identification, a careful depiction of the crime scene can give investigators and all those involved in the judicial process a solid foundation on which the verification of crimes and the search for the perpetrators can be developed.

If on one hand crime scene examination started as a mere descriptive activity, the modern implementation of such a critical step of forensic science includes the proactive search for items, the screening of the traces and the application of field tests.

Even though the number of texts dedicated to crime scene examination is much lower than forensic science books, most police forces and supernational bodies, such as European Network of Forensic Science Institutes, publish guidelines or best practice manuals, sometimes available on their websites.^9–11 It is not the purpose of this chapter to describe the technicalities of crime scene investigation, such as how to approach the crime scene, how to move around it and how to collect and store evidence. For such details, the interested reader is referred to the specialised literature.^12,13 and to the aforementioned guidelines and best practice manuals.

As anticipated above, crime scene examination is a set of analytical activities aimed at searching, collecting and preserving all the elements which, either per se or, even more importantly, due to their spatial location, can be considered evidence useful for the reconstruction for the dynamics of a crime and for identification of the perpetrators. The contextualisation of the items is therefore a fundamental element for attributing evidential value to a trace. The operator is not a mere gatherer of items, but is rather a specialist with a strong forensic background which can guide him towards an educated evaluation of traces and of their interrelationship. This is especially relevant in equivocal cases of death, in crimes perpetrated in a domestic context, or in cases of staged or simulated crime scenes, in which the significance of each trace is not due to the nature of the trace itself, but on its coherence with the possible hypotheses which can be set forth on the dynamics of the event.

Differently from what TV shows and fictional literature suggests, crime scene investigators are humans, and as such rely on their senses for searching and examining traces. Our eyes, though, have a limited sensitivity and much information would be lost both because it is too small to be detected in the chaos of a crime scene and because it is latent and invisible to the naked eye. The purpose of this chapter is to review the technical approaches which can be followed for widening the human senses and thus make the search for traces on a crime scene more effective and more productive.

1.2 A Brief Summary on the Theory of Light

As will emerge more clearly later in this chapter, exploiting the interactions between light and matter is a very effective method to detect latent traces and to find information on a crime scene.

Light is electromagnetic radiation, i.e. it is radiant energy which propagates as a wave. The features which define a wave are the wavelength, λ, i.e. the distance between adjacent crests or troughs, or the frequency, ν, i.e. the number of cycles passing by a fixed point per unit time (Figure 1.1).

Figure 1.1 Describing the features of a wave. Polymers on the crime scene, 2015, V. Causin, © Springer International Publishing Switzerland 2015. With permission of Springer.

Frequency is also a very important parameter b...