Forensic Chemistry – Using Laboratory Chemicals to Reveal Fingerprints

You have seen it on your favorite CSI TV program when the police scientist uses gaseous fumes to expose fingerprints found at a crime scene. Have you ever wondered what those fumes are? Have you ever heard of terms like “Super Glue” mentioned by the characters on CSI?

In this article, I will discuss the various kinds of laboratory chemicals that are used to reveal invisible fingerprints.

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.

What is cyanoacrylate? Perhaps you know this item by its trade name, Super Glue. It is this same Super Glue that you can purchase 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 releases fumes that interact with the amino acids that are found 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.

An object that is suspected to have latent fingerprints in this method many times is exposed to the fumes inside a gadget called a fuming chamber. The end result is that the fumed fingerprints are now hard and stable as one would expect from Super Glue. In place of setting up a fuming chamber at the scene of a crime, CSI technicians often times use a hand held 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.

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 fingerprint residue interacts with the silver nitrate and forms 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 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 in lieu of a few minutes.

The fourth kind of chemical reagent used to reveal latent fingerprints is ninhydrin. Ninhydrin, also known as triketohydrindene 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 the fingerprints to show up due to the fact that ninhydrin reacts very slowly with the oils found in the fingerprint. However, heating up the object to a temperature of 80 to 100 degrees Fahrenheit can reduce the reaction time. The resulting fingerprint will be a purple/blue print.

Hopefully, you have been educated on the kinds of chemicals used on your favorite CSI show. The next time you hear any of these chemical tools used on TV, you will know exactly what they are talking about.