Chemistry against Crime


As nighttime descended on the quiet neighborhood, the silence was shattered by the loud barks of an agitated dog; its fur and paws matted with blood. The dog led the Crime Scene Investigation (CSI) team down a dim tree-shaded pathway, where they found the body of a woman lying at the foot of some steps, in a pool of blood. Dressed in a heavy winter coat, no injuries were visible on the woman’s body except for a single gunshot wound on her temple. The contents of the woman’s purse scattered on the stairs around her; but no cell phone or wallet were in sight, indicating the possibility of a mugging. Ten feet from her body lay a man's Timex watch with a broken 8" wristband; the time on its screen marked 9:32. The crime scene was cordoned off and thoroughly inspected by the CSI team. Samples and potential evidence were collected and preserved.

Sounds familiar? The previous scene might be the opening scene of an episode of Law & Order or CSI, but it may also be a scene from real life. Over the past decade, television programs focusing on solving crimes have consistently topped popularity charts all over the world. Viewers seem to be captivated by the crime-solving tools that law enforcement officers have at their disposal and the skills they possess.

Indeed, Crime Scene Investigation has come a long way from the days of Sherlock Holmes to modern times, as Forensic Science joined the scene. Today, investigators have an amazing array of chemicals and devices with which to examine the minutest evidence that no criminal can hope to escape from a crime scene without leaving behind at least some detectable evidence.

One of the most important contributors to the forensic scientist’s investigative arsenal has been the science of chemistry.

The CSI team brought out their chemical kits, and mysterious gaseous fumes were released. Under the light of the UV lamp and just like magic, fingerprints were exposed on the victim’s clothes, on top of the stairs, and all over the crime scene.

Fingerprint Magic

We have all seen these magic fumes on TV, but apparently they are real; not at all a product of magic. In fact, they are a product of chemical labs where various chemicals that are used to reveal invisible fingerprints are created. Over the past half century, a number of improvements in the way fingerprints are analyzed and identified have been made, many resulting from the use of chemical reactions.

In many crime labs, there are four kinds of chemical reagents used to expose invisible, or latent, fingerprints. They are cyanoacrylate, silver nitrate, iodine, and ninhydrin. Perhaps you know cyanoacrylate by its trade name: Super Glue; the same product you purchase at any superstore. When cyanoacrylate is heated or mixed with sodium hydroxide (NaOH), it releases fumes that interact with the amino acids in the fingerprint residues found on an object, thus making a white print.

After exposure to cyanoacrylate, the fingerprints can then be captured on film as is or treated with a fluorescent pigment that sticks to the fingerprint. The fingerprint then fluoresces, or glows, under a laser or ultraviolet light source.

In this method, an object suspected to have latent fingerprints is exposed to the fumes inside a gadget known as a fuming chamber. The end result is that the fumed fingerprints are now hard and stable as one would expect from Super Glue. Instead of setting up a fuming chamber at the scene of a crime, CSI technicians often use a handheld wand-shaped tool that heats up a small cartridge of cyanoacrylate mixed together with fluorescent pigment. This tool then releases gases in close proximity of the latent prints, allowing the technician to fix and dye the fingerprint simultaneously.

When CSI technicians apply silver nitrate—a chemical ingredient found in black-and-white photographic film—to a latent fingerprint, the chloride found in fingerprint residue interacts with it to form another compound called silver chloride. This new compound reveals a black or reddish-brown fingerprint in the presence of ultraviolet light.

The third chemical used to reveal latent fingerprints is iodine. When heated up, crystalline iodine releases iodine fumes into a fuming chamber, where the iodine interacts with the oils found in the latent print, thus producing a brownish colored fingerprint. Unfortunately, this kind of print has a tendency to fade rather quickly. Therefore, it must be captured on film right away or fixed by spraying it with a "fixing solution" made of water and starch. This fixing solution allows the print to last for weeks or even months.

The fourth kind of chemical reagent used to reveal latent fingerprints is ninhydrin. When an object suspected of containing latent fingerprints is sprayed with a solution of ninhydrin, it may take several hours for the fingerprints to show up because ninhydrin reacts very slowly with the oils in the fingerprint. However, heating up the object can reduce the reaction time and the resulting fingerprint will be a purple/blue print.

The collected fingerprints were matched with those of the victim’s and persons of interest in the case, including previous suspects in similar robbery and assault cases. However, fingerprints alone cannot solve the case; they are only one step in the CSI investigation.

Invisible Blood Show Yourself

After collecting the fingerprints, the CSI team put on their sunglasses and started liberally spraying the crime scene area with a special solution, instantly creating an eerie blue glow that shone in the darkness of the night. A trail of glowing shoeprints from the entrance of the property leading towards the body was revealed. Inside the house, the trail continued until it reached the doorway of a room that appeared to be an office or a study of some sort. Once inside, the team sprayed the special solution again; only this time, the blue glow appeared all over the carpet, as well as on the floorboards underneath it.

All of us who are fans of forensic TV shows have seen the CSI hero, in obligatory sunglasses of course, dim the lights and spray the “magical” substance over the carpet, dramatically revealing the blood splatter of poor victims. How do they do that, you may wonder?

The “central” chemical in this drama is “luminol” (C8H7O3N3), a powdery compound made up of nitrogen, hydrogen, oxygen and carbon. The investigator mixes the luminol powder with a liquid containing hydrogen peroxide (H2O2), and other chemicals, and pours the liquid into a simple spray bottle. The hydrogen peroxide and the luminol are actually the principal players in the chemical reaction, but in order to produce a strong glow, they need a catalyst, in this case the iron in the victim’s blood.

To perform a luminol test, the CSI team simply sprays the mixture wherever they think blood might be. If hemoglobin and the luminol mixture come in contact, the iron in the hemoglobin accelerates a reaction between the hydrogen peroxide and the luminol. The luminol loses nitrogen and hydrogen atoms and gains oxygen atoms, resulting in a compound known as 3-aminophthalate. The reaction leaves the 3-aminophthalate in an energized state. The excited electrons quickly fall back to a lower energy level, emitting the extra energy as a light photon. With iron accelerating the process, the light is bright enough to see in a dark room.

If luminol reveals apparent blood traces, investigators will photograph the crime scene to record the pattern.

The team detected no signs of forced entry of the property, neither were there any signs of a struggle inside. The pattern of blood discovered by luminol indicated that the murder took place inside the office where the victim was shot and immediately collapsed on the carpet; she was then carried outside by a suspect, most likely a man who wears size 10 shoes, signified by the trail of bloody shoeprints uncovered by luminol. The perpetrator tried to wash all traces of blood from the house, but luminol managed to detect the minutest amount of blood that remained. Upon these revelations, the detectives deduced that the murder was not likely the result of a simple mugging as primarily suspected. They construed that the murderer engaged in foul play and tampering with the evidence, in order to give the crime the appearance of an assaulted robbery, when in fact it was not. Based on fingerprints, shoeprints, the broken watch and other circumstantial evidence, the detectives now had a prime suspect: the husband. They requested a warrant for his arrest.

Gunshot Residue Stuck on You

The investigators used a filtered vacuum device to collect trace evidence from the carpet in the office and from the victim’s clothes and purse. A few items, including the recovered Timex watch, were collected and tested for gunshot residue.

We often hear about “Gunshot Residue (GSR)” sticking to the hands of the killer and ultimately exposing him; but what exactly is it and how is it detected?

The answer is through simple chemistry. When a firearm is discharged, an assortment of vapors and particulate material are expelled in the area around the firearm. These products of firearm discharge can be collectively referred to as Gunshot Residues (GSR) and are used to estimate firing distances, identify bullet holes, and most importantly, to determine whether or not a person has discharged a firearm. The residue may settle on the hands, sleeves, face and other part of the shooter, as well as any other object or person within the residue fallout radius.

Through various techniques, this residue may be detected and used as powerful evidence. Laboratory examination of GSR under a Scanning Electron Microscope (SEM) is considered the most reliable method. Adhesive tapes are applied to a person’s hands and then placed under an SEM, which the technician uses to locate and identify specific residue particles and their composition.

Another method for GSR detection is through chemical analysis methods that detect byproducts of the burning of primer and gun powder. These byproducts include the metals: lead, antimony, and barium. CSIs obtain GSR residue by swabbing the suspect's hands, arms, and clothing with a moist Q-tip or filter paper. The Q-tip or filter paper is then treated with a solution of “diphenylamine”, a chemical that interacts with these metals by producing a color change. The test is positive if the color blue is produced.

The tests performed concluded the presence of GSR on the Timex watch that the investigators believed to be owned by the husband. Upon this evidence, an arrest warrant was immediately issued and following his arrest, the CSI team swabbed the suspect’s hands, face and clothes for trace evidence. 

And Chemistry Solves the Case

Thorough scientific testing led the CSI team to conclude that the husband has recently fired a shotgun, and although the clothes he was wearing had no trace of blood, his shoes did. The shoeprints also matched those detected by the CSI team in the scene of the crime, and although he denied that the broken watch was his, his fingerprints were on its wristband. Faced by the compelling evidence against him, the man confessed to murdering his wife. He claimed she refused to help him through his money troubles even though she was very rich; he was facing bankruptcy and jail and she still refused to loan him the money. He saw no way out but to murder her to inherit her money; he thought he could get away with it by staging the murder to appear like a mugging that occurred outside their house, when in fact he had called his wife into his office upon her arrival and shot her in cold blood. It did not help that he lost his watch on the scene, but in any case he did not stand a chance, not with the CSI’s weapon: chemistry against crime.


David E. Newton; 2007. The New Chemistry, Forensic Chemistry.
Wayne W. Bennett, Kären M. Hess, Christine M. H. Orthmann; 2006. Criminal Investigation 8th Edition

About Us

SCIplanet is a bilingual edutainment science magazine published by the Bibliotheca Alexandrina Planetarium Science Center and developed by the Cultural Outreach Publications Unit ...
Continue reading

Contact Us

P.O. Box 138, Chatby 21526, Alexandria, EGYPT
Tel.: +(203) 4839999
Ext.: 1737–1781

Become a member

© 2023 | Bibliotheca Alexandrina