The Hidden Detective: How Chemistry Helps Catch Criminals

by Zoe Smallwood, PhD student. Originally published in issue seven of Resonance.

From Sherlock Holmes to the modern criminal sleuths of today, the entertainment world loves a good crime thriller. In recent years, the world of forensic science has been cast into the spotlight in several TV shows. These programmes often depict glamorous people shining UV lights in dark rooms and, inevitably, catching the culprit (usually with a witty pun or catchphrase to go with it). These scenes make for great television, but how accurate are these depictions and how much chemistry is involved?

Figure 1: The chemical structure of luminol.One of the most common scenes from forensics shows is the detection of latent (i.e., invisible to the naked eye) blood that someone may have tried to clean away. First, the area needs to be treated with a visualiser to allow any blood traces to show up. The visualiser reacts with the blood and emits a glow which can then be seen and photographed (although the glow is much shorter-lived than depicted on screen). Where there is no blood the area remains dark. The traditional choice of visualiser is luminol (figure 1) which is activated using an oxidising agent to allow it to glow upon contact with blood. When exposed to the iron in haemoglobin, it acts as a catalyst, enhancing the rate of reaction between luminol and the hydroxide ions used to activate it. This reaction releases energy as photons, resulting in a blue glow.1,2,3

However, blood is not the only thing luminol will glow upon contact with; other candidates include urine, iron metal and horseradish! This means that the results must be interpreted carefully to determine if it is blood that is present. Luminol also glows upon exposure to bleach, which may be a useful clue in itself if a clean-up operation has been performed at a crime scene before the detectives arrive. However, it can also damage DNA evidence before recovery, so only a small area may be treated at first.1,2,3

Figure 2: The chemical structure of ninhydrin.Another key piece of evidence in a forensic trail is fingerprints. Patent (visible) fingerprints are easy to spot and examine as you will know if you have ever touched wet paint or ink. However, latent fingerprints need a bit more work to be visualised and recorded. Our fingers naturally secrete oils, which are transferred to surfaces when we touch them. Depending on the surface they are on, fingerprints can be enhanced or revealed by dusting with fine powders that stick to the oils. Different surfaces require different types of powder, so a wide range exist for the correct scenes. But what if the surface is porous, like paper from a ransom note? This is where powders give way to a compound called ninhydrin (figure 2).

Often dissolved in a volatile solvent such as ethanol or acetone, ninhydrin is sprayed onto a surface and the solvent allowed to evaporate. When the hydroxyl groups in ninhydrin react with the terminal amine groups in the amino acids and proteins in the fingerprint residue, a dimeric compound is formed which is purple-red in colour, clearly showing where the fingerprint lies (figure 3). If there are no proteins or amino acids to react with, the residue remains colourless when the solvent evaporates.4

Dimeric product from the reaction of ninhydrin with protein and amino acids in fingerprint residues.

Before the development of the state-of-the-art analytical instrumentation that we use today, the field of forensic science had to cope with cruder methods. In the 1800’s, arsenic trioxide, As2O3 , was considered an effective and almost undetectable method of poisoning someone. The compound was odourless and poisoning gave symptoms similar to cholera, a common disease at the time. These properties meant the compound became known as ‘inheritance powder’ due to its use to dispose of spouses and family members! This was until a London chemist by the name of John Marsh was asked to investigate a case suspected to involve arsenic poisoning. Marsh used hydrogen sulphide, H2S, to detect the presence of arsenic. Unfortunately, by the time the trial came around the test results had decomposed, resulting in the jury declaring the defendant innocent. Sometime afterwards, the defendant confessed to the killing, which motivated Marsh to improve his test so the same mistakes would not happen again. He constructed a setup which involved reacting a sample of body tissue with zinc and acid.

Equation one, synthesis of arsine gas (AsH3) from As2O3As2O3 + 6H2SO4 + 6Zn → 2AsH3 + 6ZnSO4 + 3H2O

Poisoned tissue would produce arsine gas, AsH3 , which could be ignited to leave behind a stable black residue that would not decompose over time.

Equation two, the combustion of AsH3: 2AsH3 → 3H2 + 2As

Although the improved test came too late to convict the killer, the Marsh Test (as it came to be known) became a common test for arsenic poisoning and arsenic trioxide’s status as a near-perfect poison came to an end.5

One of the most famous forensic scientists of all time is suprisingly Sir Arthur Conan Doyle’s character Sherlock Holmes. Despite being a fictional character in stories published in the 19th century, some of the science and forensic work that Holmes conducts in his cases was not too far from the cutting-edge forensics of the time. It raised awareness of the application of science and chemistry to catching perpetrators and exonerating innocent parties. For example, his first use of fingerprint analysis was in 1890, 21 years before Scotland Yard began using the technique!6 To commemorate his contribution to chemistry, the Royal Society of Chemistry presented him with an honorary fellowship; the first (and currently only) fictional character to receive one. Although Holmes was obviously not able to receive his medal in person, the award was presented by none other than a Northern Irish chemist by the name of Dr John Watson.7

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References:

  1. Luminol, Chemistry World
  2. Crime Scene Chemistry – Luminol, Blood & Horseradish, Compound Interest
  3. Luminol, Wikipedia
  4. Ninhydrin, Wikipedia
  5. Marsh test, Wikipedia
  6. Six methods of detection in Sherlock Holmes, OUPblog
  7. NI chemist honours Sherlock Holmes, BBC News