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  • Sarah Foo
    • May 18
    • 5 min read

The Science of Forensic Chemistry: Analyzing Evidence in Criminal Investigations

“Every contact leaves a trace” is the basis for Edmund Locard’s Exchange Principle and what drives forensic science. Locard was a pioneer in forensic science and believed that the perpetrator of a crime would bring something to the crime scene and leave it there, giving forensic scientists a way to find them. These traces can be in the form of bodily fluids, fibers from clothes, residues of chemical compounds, and many others. However, just finding these traces isn’t enough to catch the criminal; what is essential is being able to accurately and precisely link the evidence to the particular suspect to leave no room for reasonable doubt.


Forensic chemistry definition

Fingerprinting


A well-known method is fingerprinting. We each have unique fingerprints, owing to our finger ridges. If you dip your finger into paint and press it onto a surface, a visible print will be left behind. However, considering most criminals aren’t taking up finger painting when they commit a crime, the prints left behind are more likely to go unseen to the naked eye, known as latent prints. Still, these prints will be left behind as the many glands in our finger ridges secrete sweat that would be deposited on the surface in the pattern of our prints. Hence, forensicists have ways to reveal these prints. One way is powder dusting, where fine, distinctively colored powders are applied to a surface. These powders would stick to the sweat left behind, revealing the fingerprint. Another way is through chemical reagents such as ninhydrin, commonly used on porous surfaces. Ninhydrin reacts with amines to give a distinctive dark purple color, making it great for detecting the amine group in amino acids in the latent fingerprints. Another method is cyanoacrylate fuming, a seemingly fancy technique that simply uses super glue. Polymerized cyanoacrylate ester from the superglue is deposited on the sweat, leaving behind a white pattern. This discovery was a fortunate accident, resulting from a Japanese scientist who observed it after using superglue for an unrelated purpose. 


Traces of blood


Blood is a typical trace left behind at violent crime scenes. Although it can be wiped and cleaned away, traces of blood not visible to the human eye can remain at the scene. A reagent that used to be used to detect blood is benzidine, although it is no longer used due to its carcinogenic properties. Safer methods include the Kastle-Meyer test, which uses phenolphthalein, hydrogen peroxide, and Luminol. Although the reagents are different, they all work due to the peroxidase property of hemoglobin in our blood. Our blood acts as a catalyst for the oxidation of the reagents, resulting in their distinct color changes. However, this is not a definitive blood test since many other substances can also have similar effects. 


Infrared spectroscopy


Forensic techniques also involve determining the chemical composition of evidence found. This can be done through infrared spectroscopy. Each compound has its unique absorption spectrum, which can be measured when infrared radiation passes through a sample at different frequencies. The absorption spectrum results from the different chemical bonds present in a compound and the different frequencies of energy they absorb, hence giving varying absorption bands. The region that allows for a compound to be differentiated from others is known as the “fingerprint region”. By comparing the absorption spectrum of the unknown sample with a known compound, scientists can determine the identity of the sample or match it to another. This is useful when testing for drugs or showing that a unique compound found on a suspect matches the one found at a crime scene.


Mass spectroscopy 


Another method is using mass spectroscopy. This involves breaking down the molecules in the sample into different ions and putting them through a magnetic field. When ions travel through a magnetic field, their paths will deflect depending on their charge and mass, with lighter and more charged ions having a higher degree of deflection. This allows for the charge-to-mass ratio of the cations to be determined. Furthermore, since most of the produced cations have only a charge of +1, this allows for the molecular weight of the molecules in the sample to be found. However, more than that information is needed since different compounds can have the same molecular weight, so the recorded fragmentation pattern is needed. Fragmentation of the bonds depends on the different functional groups and their positioning within the molecule. This results in different bonds being broken and different types and numbers of ions being produced, allowing different molecules with the same weight to be distinguished. This allows for identifying specific compounds, hence having a similar purpose to infrared spectroscopy. Unfortunately, this method would use up the sample, thus preventing further tests from being done on it.


SEM-EDX


The last method to be covered is using scanning electron microscopy and energy-dispersive x-ray spectroscopy, known as SEM-EDX. The scanning electron microscopy utilizes electrons instead of light, allowing for the observation of even smaller particles than the ones that can be seen through a regular light microscope. Energy-dispersive X-ray spectroscopy involves shooting the sample with an electron beam and knocking out an electron from a low-energy electron shell. This causes an electron from a higher energy electron shell to take its place, resulting in energy loss through a characteristic X-ray that differs from element to element. This is particularly useful when analyzing gunshot residue, which is the remnant of the primer used in the gun that was fired. It can be found on anything near the gun at the time of firing, particularly the hands of the shooter. It usually consists of a specific mix of lead, antimony, and barium elements. Hence, the use of SEM-EDX can allow forensic scientists to examine suspects for gunshot residue as it is not easily washed off. 


Using chemistry to solve cases


An example of how chemistry has helped in solving cases is a case from 1912 solved by the man himself, Edmund Locard. When Marie Latelle was found murdered through strangulation, her boyfriend, Emile Gourbin, was the prime suspect. However, he seemingly had an airtight alibi for when the crime took place, having reportedly been playing cards with his friends. Locard took samples from the underneath of Gourbin’s nails and examined them. He found cells that could be Latelle’s, but this was long before DNA testing entered the scene, and there was not enough solid evidence. However, he also found pink powder, and after testing it, he discovered that it was rice starch with bismuth, magnesium stearate, zinc oxide, and a reddish iron oxide pigment, Venetian red. Although make-up was mass-produced then, this combination was created especially for Latelle. Faced with this new evidence, Gourbin confessed to the murder and shared how he had adjusted the clocks so that his friends would not be aware of the true time he was with them. 


There are many other methods used in forensic science that allow investigators to track down criminals and obtain justice for victims. As more research takes place and new inventions are introduced to the field, forensic scientists are able to gather evidence more effectively and accurately to solve these mysteries. 



Reference List


Cipriano, Andrea. “Locard’s Exchange Principle: “a Silent Witness.”” Uncovered, 16 Aug. 2023, uncovered.com/locards-exchange-principle/.


Fuller, John. “How Locard’s Exchange Principle Works.” HowStuffWorks, 17 June 2008, science.howstuffworks.com/locards-exchange-principle2.htm.


“Geoforensics Case Histories - J. David Rogers.” Mst.edu, 2020, web.mst.edu/~rogersda/forensic_geology/Geoforensics%20Case%20Histories.htm.



Lennard, C. “FORENSIC SCIENCES | Fingerprint Techniques.” Www.sciencedirect.com, 1 Jan. 2005, pp. 414–423, www.sciencedirect.com/science/article/pii/B0123693977002004, https://doi.org/10.1016/B0-12-369397-7/00200-4.



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