Evidence collected at a crime scene is not necessarily readily visible to the naked eye. The most important clues can sometimes be miniscule, or even dissolved into an organism. These clues may only be discovered by highly skilled forensic scientists. Of course, they only become significant if they can be matched to something that implicates a perpetrator.
The scientists who make those connections are forensic chemists: toxicologists, biologists, fingerprint experts, who are all skilled in proper science as well as police procedure, following and protecting clues to find where the evidence leads.
The proof was in the pudding: the cook was hanged for it.
Chemical analysis has historical context in determining accidental death or murder. Mary Blandy was tried in February of 1752 for the poisoning of her father, who disapproved of her relationship with her married lover. Unfortunately for her, Dr. Anthony Addington had created a rudimentary test for arsenic, considered undetectable, as it is odorless and easily dissolves in food and drink. The Doctor was called to testify, proved that the victim had involuntarily ingested arsenic, and Mary was hanged for her crime a few months after she had faked her grief at her father’s funeral.
The invisible witness: a test for the ages.
It wasn’t until 1836 that a test proved the presence of arsenic scientifically. Until then, using only the symptoms, a doctor might falsely diagnose arsenic poisoning to the effects of cholera. James Marsh, a practical chemist appointed to the Royal Military Academy was an inventor devoted to metallurgical interests. Since arsenic contains certain impurities such as iron pyrite and sulfuric acid, Marsh developed a reliable method of measuring these elements to determine if arsenic had been a factor in the death of a person.
Once scientific testing became accepted by courts, chemists began applying many other processes to methods of identifying evidence and linking it to a suspect. There are now tens of thousands of specific tests that authenticate proof.
The four horseman of absolute proof.
Once evidence is brought to the lab for identification and testing, one or more of four different teams of trained investigators will begin analysis: these include forensic serologists, who examine body fluids, forensic pathologists (medical examiners) who examine human remains, firearms and weapons technicians, who classify, test and match firearms, explosives and projectile evidence, and forensic chemists who examine materials, fluids and elements to identify composition and source.
Separating simple scientists from forensic experts: police procedure.
Because collected evidence requires absolute purity, forensic scientists follow proper police procedures. A chain of custody must be maintained for everything found at a crime scene, including bodies, photographs and objects. A tiny hair follicle, a footprint in blood or a shell casing — everything collected is sealed in special containers to prevent contamination and degradation, is then catalogued, documented and preserved. Evidence is kept under lock and key including when it is checked out for analysis. Whenever a piece of evidence is worked on, the procedure and result is meticulously described in the log. Even the crime scene itself must be preserved and guarded until all evidence is captured and all theories have been scrutinized.
The four tops: specialists in concert.
Today, forensic chemists can be specialists in four broad areas: biochemistry, nuclear magnetic resonance, toxicology and polymer engineering. In each category, procedures may vary from simply testing a compound against another to generate a colour change (like in a pregnancy test), to using a mass spectrometer to reveal characteristics of an unseen molecule.
The forensic chemist will examine organic and inorganic material to determine the composition and make a match. Sometimes impurities in a sample can be additional identifiers that pinpoint a specific object, such as very specific paint on an old vehicle. The fact that most substances are not found in a pure condition, but contaminated with dirt and either related or unrelated elements make the job challenging, but not impossible.
If the expected proof is in the mix, it will be found and documented. In arson cases, for example, volatile chemicals such as gasoline or gunpowder leave chemical and visual physiognomies, which can not only be found in burn debris, but matched to a source.
Nuclear spectrophotometry makes use of the fact that nuclei of some molecules absorb radiation at characteristic frequencies in strong magnetic fields. These modalities include X-ray analysis as well as DNA mapping and can provide bulletproof evidence.
Toxicologists do more than process blood alcohol samples from drunken driving traps. Semen from sexual assaults, tiny samples of bodily secretions found in insects attached to a corpse — all of these proffer proof.
Forensic polymer engineers study structures and mechanisms. The remnants of the steel superstructure from the Twin Towers of the World Trade Center in NY City after September 11, 2001 were warped by intense heat. Analysis proved that aviation fuel spilled from full tanks when the jetliners impacted the buildings burned so hot that the girders melted, causing the both buildings to collapse, and killing nearly 3,000 people. Sadly, the perpetrators were already executed by the inferno they caused.
Your Turn: Do you know any other uses of forensic chemistry, which we might have missed here? Let us know in the comments — we’d love to hear from you.