Fingerprints left at crime scenes are one of the tools used to incriminate or eliminate suspects. However, they’re not always visible, and investigators will often rely on chemical techniques in order to visualise them. There are a large number of techniques that can be used to do this; here, we take a look at four of the most common.
Types of Fingerprint
It’s worth clarifying, before we discuss specific techniques, how fingerprints found at crime scenes can be classified. There are three classifications: patent prints, plastic prints, and latent prints.
Patent prints are those that are already visible to the naked eye. These might be prints left in blood, or by blood-covered fingers, or in ink or dirt. Plastic prints are somewhat similar; these are also visible to the naked eye, and are those prints that are left in a pliable substance such as clay or wax. Both patent and plastic prints can often be recorded simply with the aid of photography, and no further work is needed (though sometimes techniques may be used to enhance their visibility).
Latent prints pose more of a problem. These are prints left on surfaces due to the natural oils and sweat secreted from the skin. Though these remain on the surface, they can’t be seen unless something is done to make them visible. This is where some of our chemical techniques come in.
‘Dusting for prints’ is a well-known practice, and references to it date back to 1891. It involves the use of powder made of very fine particles which is dusted over a surface lightly using a brush. The particles adhere to the moisture and oil in the latent prints, making them visible. The prints can subsequently be either photographed or their impressions removed using transparent tape.
The compositions of fingerprint powders vary widely, particularly depending on the surface they are being used on. Commonly, they consist of a pigment and a binder. The pigment helps to provide contrast, whereas the binder helps the powder adhere to the print. Commonly used pigments are colloidal carbon particles, or flakes of metals including aluminium, zinc, and copper. Common binders are gum arabic, iron powder, and rosin. As well as the standard black powders used, fluorescent powders, which fluoresce in the presence of certain colours of light, can also be utilised.
This method is not without its issues. The fingerprint powder is usually applied with a soft brush; though care is taken, this can still sometimes cause damage to the prints. Another issue that use of the brush poses is the potential transfer of DNA, and so crime scene investigators will often resort to using other techniques.
Another method for visualising fingerprints was purportedly discovered by accident. In 1982, Japanese scientists working on cyanoacrylates, the types of molecules used in superglue, found that the fingerprints on one of the fume hoods they were using had been made visible by the fumes of the cyanoacrylate. It seemed that the cyanoacrylates had polymerised on contact with the residue of the fingerprints, and therefore followed the line of the ridges.
This techniques is of particular use for rough, non-porous surfaces. The time it takes for the cyanoacrylate to condense on to the print is fast, particularly when it is heated to accelerate its vaporisation. Durability of the prints produced is also a positive, and though the cyanoacrylate polymer is colourless, it can be visualised more easily using powder after fuming has been carried out. The more recently the fingerprint has been left, the more effective this method is.
A large number of chemical developers can be used to visualise 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 or skin secretions it turned a purple colour. The compound causing this purple colour is now named after him: Ruhemann’s purple. Despite its potential to help develop latent fingerprints, it didn’t see proper application to this area until 1954.
The sweat that is deposited as part of fingerprints contains amino acids (around 250 nanograms per fingerprint). As they do not move significantly from where they are deposited, if they can be visualised, they allow the person’s fingerprint to be seen. 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 from these, or photographed for posterity.
Ninhydrin is not the only compound that reacts with the amino acids in fingerprints. Another such compound is 1,8-Diazafluoren-9-one (DFO). This reacts with the amino acids in the fingerprint to produce a compound similar to Ruhemann’s purple, which is a pinky red colour, but which also fluoresces (glows) when illuminated by blue-green light. Another compound capable of producing fluorescent fingerprints is 1,2-indanedione.
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 vacuum. This was first attempted in 1968 using a mixture of zinc, antimony, and copper powders.
Later experiments showed that fingerprints developed with single metals were not as good as those developed with a combination. Typically, combinations of two metals were used; gold or silver were deposited first, followed by cadmium or zinc. As silver is more susceptible to degradation, and cadmium has issues with toxicity, gold and zinc is now the usual combination of metals utilised in the technique.
VMD requires a vacuum chamber, filaments for the evaporation of the metals used, and some manner of 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 plasticised plastics.
The reason it works is due to the differing characteristics of the metals used. Gold can be deposited over the entire surface, including the latent print, and on top of the oils which form part of its composition. The gold will then diffuse into these oils, meaning no gold atoms are left at the surface. This is important, because when the zinc is then deposited, it won’t condense on the oily deposits, and will only deposit on the existing small nuclei of gold.
The above is, at least, the theory. However, in practice, sometimes the opposite has been seen – the zinc is deposited on the fatty ridges instead of in the valleys between them. Though several theories have been advanced as to why this might be, none of them have yet proven conclusive.
There are many techniques that can also be used that have not been detailed here; with this post, we’ve simply focused on the most common. Hundreds of possible techniques are described in research papers, though a much smaller subset of these are currently used at crime scenes. There’s more detail on a range of these via the references below.
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References & Further Reading