Chemical Analysis for Forensic Evidence

By Arian van Asten

Chemical Analysis for Forensic Evidence provides readers with the fundamental framework of forensic analytical chemistry, describing the entire process, from crime scene investigation to evidence sampling, laboratory analysis, quality aspects, and reporting and testifying in court. In doing so, important principles and aspects are demonstrated through the various forensic expertise areas in which analytical chemistry plays a key role, including illicit drugs, explosives, toxicology, fire debris analysis and microtraces such as gunshot residues, glass and fibers. This book illuminates the underlying practical framework that governs how analytical chemistry is used in practice by forensic experts to solve crime.

Arian van Asten utilizes a hands-on approach with numerous questions, examples, exercises and illustrations to help solidify key concepts and teach them in an engaging way.

• Provides a forensic analytical chemistry framework based on how professionals actually use chemistry to solve crimes
• Introduces leading principles necessary to forensic practice understanding
• Answers key questions with a wealth of illustrations and real-world examples

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 24, 2022
• ISBN: 9780128207215

Arian van Asten

Arian van Asten obtained his Master’s degree with honors in 1991 and his PhD in 1995 (for which he received the Kolthoff award in 1996) at the University of Amsterdam (UvA). After working for more than 10 years in the chemical industry he joined the Netherlands Forensic Institute in 2006. In June 2012 he was appointed professor at the Faculty of Science of UvA on a special chair in Forensic Analytical Chemistry. Since September 2018 he is a full-time professor of Forensic Analytical Chemistry and On-Scene Chemical Analysis at UvA. On the same date Van Asten was also appointed director of the Master’s of Forensic Science program at the Institute for Interdisciplinary Studies at UvA. Together with professor Maurice Aalders he leads the Co van Ledden Hulsebosch Center, the Amsterdam Center for Forensic Science and Medicine (www.clhc.nl). He has authored or co-authored more than 50 peer reviewed scientific publications on (forensic) analytical chemistry.

Book preview

Chapter 1: An introduction to forensic analytical chemistry

Abstract

In this first chapter, the field of forensic analytical chemistry will be introduced. At first glance, this field is nothing more than the application of analytical chemistry methods within a forensic laboratory requiring experts skilled in chemical analysis. However, there is more than meets the eye, by merging concepts, looking at case examples, studying questions of legal interest and introducing forensic principles, it will be shown that forensic analytical chemistry represents a special branch with quite unique ways of working.

Keywords

Analytical chemistry; Criminalistics; Forensic analytical chemistry; Forensic science

1.1 What will you learn?

1.2 Definitions

1.3 Questions of interest to a legal system

1.4 Forensic science principles

Further reading

1.1. What will you learn?

In this first chapter, the field of forensic analytical chemistry will be introduced. At first glance, this field is "nothing more" than the application of analytical chemistry methods within a forensic laboratory requiring experts skilled in chemical analysis. However, there is more than meets the eye, by merging concepts, looking at case examples, studying questions of legal interest, and introducing forensic principles, it will be shown that forensic analytical chemistry represents a special branch with quite unique ways of working.

After studying this chapter, readers are able to

• define analytical chemistry and forensic science and combine these concepts to describe the scope of forensic analytical chemistry

• describe questions of interest to criminal law and understand how these questions govern the chemical analyses conducted by the forensic experts

• list the basic principles of forensic science and provide examples involving chemical analysis of physical evidence

1.2. Definitions

When entering a new field or even when working in it for many years, it can be very useful to occasionally take a step back and think about the underlying reasons and motives of the associated activities. What do analytical chemistry and forensic science entail and what is the added value of chemical analyses in the criminal justice system? An often very useful step to increase your understanding to a higher, more abstract level is to try to write down a concise and yet comprehensive and precise definition of a certain science area or fields of expertise you are active in. This will also allow you to explain to others what you do and why, even if they are laymen. In the case of forensic analytical chemistry, the topic of this book, the additional challenge is that the worlds of analytical chemists and forensic scientists have to be merged. This is literally what happens when trained analytical chemists obtain a job at a forensic laboratory such as the Netherlands Forensic Institute (NFI). They enter a new world in terms of the application of their skills as over time they also become experienced forensic experts. Interestingly, the reverse situation in which a trained forensic scientist acquaints him or herself with analytical chemistry is much less frequently observed in forensic laboratories. Let us therefore start with an attempt to define the work of the analytical chemist:

What would be your definition of analytical chemistry? icon

As analytical chemistry comprises of a very broad range of activities, methods and instruments, many definitions and descriptions of this hybrid, interdisciplinary, and even somewhat elusive branch of chemistry exist. A generic description is given below but this is by no means the only valid description nor can this condensed text be attributed to the author as many very similar variations exist:

Analytical chemistry is the study of the separation, identification, and quantification of chemical compounds in natural and man-made materials. icon

Initially, unraveling the composition of materials and establishing the presence of chemical compounds in samples involved the use of so-called wet-chemical methods. These classical approaches are based on a chemical reaction in solution (hence the term wet-chemical) to induce a color formation or a color change in the presence of a given analyte. In this way, the human eye is used as a detector based on our, for mammals rare, ability to see and distinguish colors. In qualitative analysis, the main aim is to demonstrate the presence of a compound of interest. As will be demonstrated in this book, these so-called colorimetric test reactions still play an important role in contemporary forensic analytical chemistry, especially in situations where the presence of certain chemicals needs to be established rapidly and on the scene. When the amount of a certain compound in a material is also an important consideration, a quantitative analysis can be performed through a titration method. A solution of a known concentration of a special reagent, a so-called titrant, is then gradually added to the sample solution. The volume at which a predefined color change (sometimes promoted through the presence of an indicator) is visually registered can then be used to accurately determine the concentration of an analyte.

The onset of many technological, electronical, microengineering advances in the 1950s and later the transistor, IC (integrated circuit) and computer revolution in the 1980s had an immense impact on analytical chemistry. Advanced instrumentation became available for highly specific and detailed chemical analyses based on a wide range of detection principles, exploiting the entire electromagnetic spectrum (including spectroscopic techniques like UV-vis, IR, FTIR, NIR, Raman, AAS, AES, XRD, and XRF) and introducing powerful new techniques such as Nuclear Magnetic Resonance (NMR is in principle also a spectroscopic technique) and Mass Spectrometry (MS, see all full technique names in the Abbreviations section). In addition to these techniques, the introduction of separation or so-called chromatographic techniques greatly enhanced the capabilities of analytical chemists. Initially, this was limited to volatile and semivolatile compounds with gas chromatography (GC) but from the 1980s it also became useful for polymers and other involatile and thermolabile compounds with the introduction of liquid chromatography (LC) and related techniques. The combination of compound separation and subsequent spectroscopic or mass spectrometric characterization completed the instrumental revolution in analytical chemistry. So-called hyphenated systems (e.g., GC-MS, LC-UV, GC-IR, LC-MS, and GC-AES) allow detailed and sensitive compositional analysis of complex mixtures by the temporal separation of the mixture constituents, allowing chemical identification and quantification of the pure compounds as they enter the detector sequentially. The latest additions to the analytical chemical toolbox include powerful chemical imaging systems allowing spatially resolved chemical analysis (e.g., Raman microscopes, MA-XRF) and comprehensive, multidimensional chromatography (GCxGC-MS, LCxLC-MS) providing ultimate resolution and capacity in compound separation. These developments also triggered extensive instrument automation making the chemical analysis and sample pretreatment less laborious and allowed laboratories to greatly increase sample numbers and throughput. Instruments with autosamplers often enable 24/7 chemical analysis providing immediate results when the analyst enters the laboratory in the morning. The consequence of these developments is that the tasks of the laboratory expert has significantly shifted toward data analysis, interpretation and reporting, equipment maintenance, and quality control as the number of analyses and the amount of data per analysis continue to increase. With respect to data analysis, chemometrics has emerged as a new expertise area in analytical chemistry to create information and knowledge from data by using advanced (often multivariate) mathematical computer methods. Correct interpretation and representation of the results also require robust statistical analysis to provide insights with respect to uncertainty and the magnitude of random and systematic errors. Laboratories for which the reliability and robustness of their analytical chemical findings is of the utmost importance (e.g., hospital laboratories, forensic institutes, process analysis teams) make significant investment in quality systems, have dedicated staff working on quality control, and maintain quality certifications and accreditations granted and checked by authorized inspection bodies. In a forensic setting, these quality requirements also include detailed record keeping of the whereabouts and custody of physical evidence, the so-called chain of custody. In this introductory chapter, there is no need to discuss these techniques and aspects in detail; all these topics will naturally emerge in the various chapters as the forensic analytical chemistry framework unfolds. Instead, the basic aspects governing analytical chemistry are put in perspective below.

Analytical chemistry in a nutshell icon

Qualitative analysis (what?)

• Chemical identification of pure compounds

• Determination of mixture constituents

• Detection of trace analytes in samples

Quantitative analysis (how much?)

• Determination of analyte content or compound purity

• Establishing mixture composition

• Measuring trace levels of contaminants

Classical, wet-chemical methods

• Colorimetric test reactions

• Titrations

Instrumental methods

• Spectroscopy, NMR, mass spectrometry

• Chromatography (GC, LC, CE, SEC, FFF)

• Electrochemistry (potentiometry, voltammetry)

• Gravimetry (including TGA)

• Calorimetry (including DSC)

Sampling

• Head space sampling and trapping

• Liquid–liquid and solid phase extraction

• Sample dissolution and destruction

• Sample clean-up and preconcentration

• Representative, noninvasive, sterile …

Automation

• Automated sample preparation

• Automated analysis and autosamplers

• Automated data analysis and reporting

Data analysis: chemometrics and statistics

• Calibration

• Error analysis

• Data processing and (multivariate) analysis

• Databases

Quality

• Sample management (chain of custody)

• Method validation

• Quality control

• Certification and accreditation

Now an intrinsic characteristic of analytical chemistry is its applied and serving nature, this field only exists by the virtue of the information-need of others. There is no use in chemically analyzing and characterizing a sample if it is not preceded by a relevant question from a colleague, client, company, institute, industry, or even society in general. The starting point of the analytical chemist is this question as he/she starts to develop methods and use instrumentation to find the answers through chemically characterization. Analytical chemistry thus aims to provide useful chemical information that allows professionals to take correct decisions and undertake the right actions. The analytical chemist thereby increases and ensures the quality of the work of these professionals. Now in the criminal justice system objective, evidence-based information is provided by the forensic scientist. So, the next step is to take a closer look to the definition of forensic science. Again, many descriptions exist as also this field is very diverse and interdisciplinary by nature. It is by no means the goal of the author to provide a comprehensive overview of existing definitions and associated scientific discussions. Instead, having students coming up with their own definitions is much more fun and interesting:

What would be your definition of forensic science? icon

If we again opt for a definition that is both concise and yet comprehensive we could arrive at the following statement:

Forensic science is the application of a broad spectrum of sciences to answer questions of interest to a legal system. icon

Now interestingly, if both definitions are merged suddenly a very elegant (short, accurate, and comprehensive) description of forensic analytical chemistry emerges:

Forensic analytical chemistry entails the separation, identification, and quantification of chemical compounds in natural and man-made materials to answer questions of interest to a legal system. icon

However, having a fancy definition for one's field of expertise is only a first step in a complex journey. The description above provides a generic frame that still does not explain why and how forensic analytical chemists do what they do. The analytical chemistry part has already been discussed allowing the reader to form a rough picture what it takes to chemically characterize compounds in complex samples. What needs to be discussed next are the questions of interest to a legal system. What are these questions in the criminal justice system, which professionals are asking them and what answers do they seek? This will be addressed in the next paragraph.

1.3. Questions of interest to a legal system

In civilized countries, the rule of law ensures a safe and just society. Laws describe the social rules of engagement and uphold and protect fundamental rights of citizens. Disputes are settled in court to break the vicious cycle of vigilantism, retaliation, and violence. In democratic systems adhering to the trias politica principle (separation of powers), the judiciary or judicial branch interprets and applies the laws defined by the legislative branch. In criminal law, the public prosecution office represents society as it indicts and prosecutes suspects of crimes. During the criminal investigation and court proceedings, suspects receive legal counsel as defense lawyers try to demonstrate that the indictment has not been proven beyond a reasonable doubt or is legally flawed. In the inquisitorial system as applied in the Netherlands, the verdict in a case is given by the judges of the magistrate office. In the adversarial system, a society representation in the form of a jury delivers the ultimate decision as the judge guards the court proceeding and decides on the admissibility of the evidence presented by the prosecution and defense council. Within the criminal justice system, the role of the police is to enforce the law. In this role, they also conduct criminal (tactical) and crime scene (technical) investigations under the supervision and leadership of the public prosecution office. The police also have the authority to arrest, detain, and interrogate suspects albeit under strict legal regulations. Special police reports of the findings serve as evidence in court and are used to incriminate or exonerate suspects.

On an individual level, a criminal court case has a severe impact on suspects, victims, and their relatives hence establishing the truth is of the utmost importance. On a generic level, correct verdicts based on convincing and legally sound evidence are essential for a just and safe society. Wrongful convictions or exonerations greatly undermine the rule of law and sense of justice. This is the rationale behind forensic science services and forensic case work as part of criminal investigations. By studying the physical evidence, the so-called silent witnesses at the scene of crime, objective information can be obtained that can be of crucial importance for an accurate reconstruction of the events. This assists the police and legal professionals and raises the quality of their actions and decisions.

However, forensic case work cannot simply be initiated by the forensic expert on the basis of the evidence alone. The evidence has been collected by the crime scene officers of the police for a reason in a given context of a case and on the basis of available tactical information. Hence, the physical evidence is typically accompanied by a formal request to the forensic institute with a question or a set of questions. Such requests typically originate from the team of police officers and public prosecutors investigating the case although the forensic investigation can also be requested by the defense council or magistrate office. At the initial stages after the discovery of a (potential) crime, the investigation is typically coordinated by the police. As valuable leads can quickly disappear over time, the police officers want to reconstruct the events as quickly and accurately as possible as this increases their chances of solving the case. Forensic investigations can assist in the attempt to answer the so-called 5+1 key questions in a given case:

Questions of interest to a legal system (1) icon

How?

(With What?)

Why?

What?

Where?

When?

Who?

Addressing these basic questions is essential for a precise reconstruction of the events. It should be noted that although for some incidents it is obvious that a crime has been committed (e.g., when various witnesses have observed a lethal incident), the investigation of the police can also result in the conclusion that no crime was committed (e.g., when somebody has committed suicide). Now consider the following two questions to better understand the reconstruction framework:

1. Why is the Where question of importance as the police is conducting an investigation on a given crime scene? icon

2. What is the reason that the Why question is usually not addressed by forensic experts studying the physical evidence and forensic trace material?

1. When a body is found, this does not necessarily mean that the victim was killed on this location! Quite often bodies are transported by perpetrators to hidden locations or put into clandestine graves to prevent discovery. An important aspect of the reconstruction is then to determine where the victim was killed. icon

2. Motive is a very important part of the police reconstruction and the indictment by the public prosecutor. However, physical evidence can typically not provide insight in the state of mind of a perpetrator. This is the realm of forensic psychiatry. Forensic toxicology and digital forensics experts can sometimes provide information that is important for the Why question. For instance with respect to the use of certain medication or drugs of abuse (insanity plea) or the use of certain Google search terms prior to the crime (manslaughter vs. premeditated murder).

When the police have completed the reconstruction including tactical information, witness and suspect statements, the crime scene investigation and the evidence reported by the forensic experts, the criminal investigation enters a new phase. If the dossier is unequivocally incriminating a suspect, the public prosecutor will officially indict the suspect and initiate a court case. The indictment specifies the crimes for which the suspect will be prosecuted. Now a very fundamental legal principle is that someone can only commit a crime when the action is clearly described as such in the law and that the associated law articles were in effect (ratified) prior to actions of the suspect. The indictment will therefore not only contain a summary of the facts and findings but will also relate these to relevant penal code articles. The main task of the judge in the inquisitorial legal system is to establish whether the indictment is lawful and thus fulfills all the legal requirements. Additionally, the indictment needs to be proved beyond a reasonable doubt indicating that on the basis of the facts, findings and evidence the judge is convinced that the suspect committed the crimes as described in the indictment. If these two requirements are met, the judge will convict and will arrive at a sentence based on the legal articles and the context of the case. The judge will formulate these deliberations in a final verdict. It is important to grasp that these legal aspects of a criminal investigation are of crucial importance to the work of the forensic expert and the questions they address:

Questions of interest to a legal system (2)

Is the indictment lawful?

Is the indictment proven beyond a reasonable doubt?

Therefore, forensic questions are dictated by the law!  icon

The fact that also the work of the forensic analytical chemist is fully dictated by criminal law and the associated articles in the penal code is best demonstrated through practical examples. Throughout this book such practical examples will involve case descriptions that are fictive but are based on the daily work of forensic institutes such as the NFI and as such are highly realistic.

A case example

Two police officers apprehend two men who are fighting in front of a cafe and bring them over to the police station. In the jacket of one of the men, the officers find a plastic bag containing roughly 10g of a white powder. The man refuses to explain the origin of the material. The officers suspect the white powder to be a synthetic drug and send the evidence for analysis to the forensic laboratory.

Do we perform a qualitative analysis only or do we include a quantitative assessment? icon

Interestingly and maybe to some readers somewhat surprisingly, the answer to this question is not found in the laboratory but rather in the law library! If the Dutch Illicit Drug act is taken as an example and when we translate the corresponding article, the course of action becomes obvious:

Dutch illicit drug act—Article 3

"It is forbidden for any of the substances listed as illicit drug to

1. transport these substances to or from the Netherlands

2. grow, manufacture, modify, process, deliver, provide, or transport these substances

3. possess these substances

4. to produce these substances"

In principle a qualitative analysis will be sufficient!  icon

In two separate and dynamic lists, the Dutch government indicates which substances are considered to be illicit drugs (The two lists differentiate between so-called soft and hard drugs and this has an effect on the severity of the punishment). However, for the question at hand this is irrelevant. The point is that the law forbids the possession of any of these listed substances without providing any quantitative limit. This means that any possession, no matter how low the amount or concentration, is punishable by law. Of course, there are many cases especially related to the production and trade of illicit drugs where amounts do matter and are of great importance to establish a reasonable sentence. However, if the indictment is related to article 3, there is no legal incentive to determine the amount of forbidden substance in the sample. Just demonstrating that an illicit substance is present is sufficient. The way this is typically done in the forensic illicit drugs laboratory is through the application of an indicative colorimetric test (e.g., the cobalt thiocyanate test or so-called Scott's test for cocaine) in combination with a qualitative screening with GC-MS (gas chromatography with mass spectrometric detection). As such laboratories typically handle thousands of samples and requests on an annual basis, performing a legally unnecessary quantitative analysis would increase the lead times of the investigation and would reduce the total number of cases that could be processed. The aspects of the chemical identification of drugs of abuse will be discussed in more detail in Chapter 4 .

An interesting hypothetical situation arises when analysis would reveal that the sample only contains a trace amount of a forbidden substance such as cocaine. In principle, any amount constitutes a criminal offense but from a practical perspective it is clear that a suspect had no intent to use or sell drugs when he or she is in the possession of let's say 1μg or a sample containing 1ppm (part-per-million). Possibly, the defendant in that case was not even aware of the possession (although such traces could of course indicate that the suspect has handled forbidden substances). When such details are not described in the law then a practical common standard will have to emerge from jurisprudence and legal debate, for example, through rulings from the courts of appeal or the supreme court.

Now that we have seen a clear example of a law-directed qualitative chemical analysis, it is interesting to try to come up with an example where quantitation is required:

A case example

Can you come up with an example where a quantitative analysis of a compound is needed to determine whether a suspect has committed an offense? icon

Dutch traffic act—article 8 – section 2

"It is forbidden to drive a vehicle after consuming alcoholic beverages such that

1. the alcohol content in breath exceeds 220μg ethanol per liter exhaled air

2. the alcohol content in blood exceeds 0.5mg/mL" icon

Many countries have similar DUI (driving under the influence) laws and there is international accordance between experts at what levels of alcohol consumption the driving capabilities are significantly impaired and can lead to dangerous traffic situations. These levels have consequently been entered in a legal framework and thus necessitate an accurate quantitative analysis of ethanol in human breath and blood. The process usually starts with a roadside breathalyzer test when a driver is stopped by the traffic police either during a random check or as a result of dangerous car maneuvers. These breathalyzer tests provide an indicative quantitative result based on the electrochemical oxidation of ethanol to acetic acid using oxygen in the air. If a positive test result is obtained, that is, a level exceeding the legal limit is found, the next step usually involves taking a blood sample from the suspect for an accurate assessment of the alcohol content. The blood alcohol level can for instance be established using head space sampling in combination with a gas chromatographic analysis.

An interesting situation arises when the accurate blood alcohol analysis would yield a result that is very close to the legal limit. For example, a duplicate analysis giving an outcome of 0.499mg/mL—not guilty!—and 0.501mg/mL—guilty!, respectively. The challenge the forensic expert is facing here is that statistics and scientific laws of probability do not sit well with the absolute decision that must be taken by the judge. However, as measurement uncertainty will always exist and random and systematic errors are part of any quantitative analysis, somehow these worlds must meet to arrive at a justifiable approach. This legal limit dilemma is discussed in more detail in Chapter 5 .

These two examples clearly illustrate how criminal law essentially directs the questions for the forensic experts and the forensic analytical chemical methods applied. These examples have in common that the compounds of interest are directly related to the crime as described in the law. This means that the possession or use of such compounds is inherently part of the criminal activity. As a result, these compounds can be found in the law, either in the form of addendums (e.g., lists of illicit drugs) or directly in the penal law code (e.g., DUI of alcohol). However, forensic chemical analysis has much more to offer and can provide crucial information to solve a case. Almost any material can be chemically characterized for forensic attribution and reconstruction purposes. This includes physical evidence that has been used to commit a crime but is in itself not directly crime related and physical evidence that has no direct relation to the crime but has been created, transported or left as a result of the criminal activities. In the final example of this paragraph, it will be illustrated how the chemical analysis of evidence can assist in the reconstruction of a crime and ultimately the identification of the