Forensic Chemistry – Using laboratory chemicals to reveal fingerprints
You’ve seen it on your loved one CSI TV program when a police scientist uses gas fumes to reveal fingerprints found at a crime scene. Have you ever wondered what these fumes are? Have you ever heard of terms like “Super Glue” mentioned by the characters CSI?
In this article, I will discuss the different types of laboratory chemicals that are used to reveal invisible fingerprints.
In many crime labs, there are four types of chemical reagents used to detect invisible or latent fingerprints. They are cyanoacrylate, silver nitrate, iodine and ninhydrin.
What is cyanoacrylate? You may know this item by its trade name Super Glue. This is the same Super Glue you can buy at any Home Depot or Walmart. Cyanoacrylate is the active ingredient that makes up 98% of Super Glue. This item has become a very practical and innovative forensic tool. When cyanoacrylate is heated or mixed with sodium hydroxide (NaOH), it gives off fumes that react with the amino acids found in fingerprint residue found on an object, thus making a white print.
After exposure to cyanoacrylate, fingerprints can be captured on film as they are, or treated with a fluorescent pigment that sticks to the fingerprint. The fingerprint then fluoresces, or glows, under a laser or ultraviolet light source.
An object suspected of having latent fingerprints in this method is many times exposed to the fumes inside a device called a smoke chamber. The end result is that the vaporized fingerprints are now hard and stable as you would expect from Super Glue. Instead of placing a smoke chamber at a crime scene, CSI technicians often use a wand-shaped hand tool that heats a small cartridge of cyanoacrylate mixed together with a fluorescent pigment. This tool then releases gases in close proximity to the latent prints, allowing the technician to fix and paint the fingerprint at the same time.
Silver nitrate is a chemical ingredient found in black and white photographic film. When CSI technicians apply silver nitrate to a latent fingerprint, the chloride found in the fingerprint residue reacts with the silver nitrate 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 called iodine. When heated, the crystalline iodine releases iodine fumes in a fume chamber where the iodine reacts with the oils found in the latent print, thus creating a brownish fingerprint. Unfortunately, this type of print tends to fade quite quickly. It must therefore be immediately captured on film or fixed by spraying with a “fixing solution” made of water and starch. This fixation solution allows the print to last for weeks or even months instead of a few minutes.
The fourth type of chemical reagent used to reveal latent fingerprints is Ninhydrin. Ninhydrin, also known as triketohydrinde hydrate, has been used for many years by CSI technicians to reveal latent fingerprints. An object suspected of containing latent fingerprints is sprayed with a solution of ninhydrin. It may take several hours for fingerprints to show due to the fact that ninhydrin reacts very slowly with the oils found in the fingerprint. However, heating the object to a temperature of 80 to 100 degrees Fahrenheit can reduce the response time. The resulting fingerprint will be a purple/blue print.
Hopefully, you have been educated about the types of chemicals used on your pet CSI show. The next time you hear one of these chemical tools being used on TV, you’ll know exactly what they’re talking about.
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