A cold case — an investigation of a crime, usually, a violent one, where all leads have been exhausted and the trail has gone cold. But in recent years, various technologies have begun heating up many of these cold cases, uncovering new leads for us, and providing justice for victims. One immediate example that should come to your mind is fingerprint detection.
Types of Fingerprints
Before we discuss the techniques, we must explore the main types of fingerprints: patent prints, plastic prints, and latent prints.
Patent prints might be left in blood, ink, or dirt. Plastic prints are somewhat similar; these are three-dimensional fingerprints left on soft surfaces such as wax and wet paint. Both of which can be photographed without the aid of chemicals. Latent prints are left on surfaces due to the natural oils and sweat secreted from the skin, therefore posing a problem, although they remain on the surface, we must utilize chemical techniques to make them visible.
Fingerprint Powders
‘Dusting for prints’ is a well-known practice, it involves the use of powder made of very fine particles which are dusted over a surface lightly using a brush. The particles cling to the moisture and oil in the latent prints, making them visible. We can subsequently photograph these prints or their impressions can be removed using transparent tape.
The powder consists of a pigment and a binder but their composition varies widely. The pigment helps to provide contrast, whereas the binder helps the powder stick to the print. Commonly used pigments are colloidal carbon particles, and flakes of metals including aluminium, zinc, and copper. Common binders are gum arabic, iron powder, and rosin. This method is not without its issues. The fingerprint powder is applied with a soft brush, potentially damaging the prints while also raising the issue of DNA transfer.
Chemical Developers
A large number of chemical developers can be used to visualize latent fingerprints. Some of these cause the print to take on a particular colour, whereas others cause it to fluoresce under particular colours of light.
By far the most commonly used chemical developer is ninhydrin. Ninhydrin was discovered in 1910 by Siegfried Ruhemann, who discovered that when it came into contact with skin secretions, it formed a purple colour- Ruhemann’s purple.
The sweat deposited as part of fingerprints contains amino acids. Ninhydrin, when sprayed onto the surface, reacts with these amino acids to produce the ammonium salt of Ruhemann’s purple, and a clear purple colour. Water is a required reagent for this transformation, so the reaction must be carried out in a high-humidity environment. Additionally, Ruhemann’s purple degrades in the presence of light and oxygen, so the developed prints must either be protected, or photographed for posterity.
Another such compound is 1,8-Diazafluoren-9-one (DFO). This reacts with the amino acids in the fingerprint to produce a pinky-red colour, which also fluoresces (glows) when illuminated by a blue-green light.
Cyanoacrylate Fuming
Another method of visualizing fingerprints was allegedly discovered by accident. In 1982, Japanese scientists working on cyanoacrylates - a type of molecule used in superglue - noticed that the vapours from the cyanoacrylate had made the fingerprints visible on one of the fume hoods they were using. The cyanoacrylates had polymerized on contact with the fingerprint residue, following the line of the ridges.
This technique is particularly suitable for rough, non-porous surfaces. Although the cyanoacrylate polymer is colourless, we can make it visible with powder after fuming. The more recent the fingerprint, the more effective this method is.
Vacuum Metal Deposition (VMD)
Vacuum metal deposition (VMD) is a technique originally used to apply metal coatings to glass to form mirrors. However, it can also be used to help develop fingerprints. The technique involves adding thin layers of metal atoms onto a surface under a vacuum. Later experiments showed that fingerprints developed with a combination of metals produced a more adequate result. Gold and zinc are now the usual combinations of metals utilized in the technique due to their differing characteristics. Gold can be deposited over the entire surface, including the latent print, and on top of the oils which form part of its composition, therefore, diffusing into it. This is important, because when the zinc is deposited, it won’t condense on the oily deposits, and will only deposit on the existing small nuclei of gold.
VMD requires a vacuum chamber, filaments for the evaporation of the metals used, and some manner of the viewfinder to monitor metal deposition. It gives excellent results on non-porous surfaces and is unrivalled when it comes to lifting prints from plastic bags. However, it can be affected by the presence of bodily fluids and is also not particularly good at lifting prints from highly plasticized plastics.
Conclusion
Fingerprint powders, Cyanoacrylate Fuming, Chemical Developers, and Vacuum Metal Deposition (VMD) have gained the most popularity when it comes to forensic chemistry. However, we can't term the most 'suitable' technique as the techniques used are largely dependent on the situation itself - state, age of fingerprint, surface, etc. Hundreds of other techniques can also be used but are not detailed here, although a much smaller subset of these is widely incorporated in crime scenes.
- Navya Gupta
Bibliography
"Fingerprints - Crime Museum". Crime Museum, 2022, https://www.crimemuseum.org/crime-library/forensic-investigation/fingerprints/. Accessed 28 Dec 2022
"BVDA - Fingerprint Powders". Bvda.Com, 2022,
https://www.bvda.com/en/fingerprint-powders. Accessed 28 Dec 2022.
"Fingerprint Analysis: How It’S Done". Forensicsciencesimplified.Org, 2022, https://www.forensicsciencesimplified.org/prints/how.html#:~:text=Chemical%20Developers%3A%20Porous%20surfaces%20such,amino%20acids%20and%20inorganic%20salts. Accessed 28 Dec 2022.
Pccd.Pa.Gov, 2022, https://www.pccd.pa.gov/training/Documents/Conferences%20and%20Training/Vacuum%20Metal%20Deposition%20Brochure.pdf. Accessed 28 Dec 2022.
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