Scientect

Imagine your fingerprint as a rubber stamp, and it is preconditioned with colourless (transparent) ink, which gets imprinted on any object you touch. These imprints are not visible to our naked eyes under normal light and hence are referred to as Latent Fingerprints. Some common sources of colourless inks that get easily conditioned on our fingertips include:

• Perspiration (sweat). This comes out of the pores of our palms and fingers. The sweat comprises water, salts, lipids, and amino acids.
• Sebum (on the face). We often rub or pass our fingers on our face, collecting fatty secretions from our face onto our fingertips.
• Hair/scalp. It is a common practice to rub our hands on our scalp, which also collects sebum onto our fingertips. Sometimes, our hair also contains creams which accumulate on our fingertips.
• Eye secretion, ear wax, and pubic regions. Even though not common, these are potential sources of fingerprint deposits as we go about our daily routines.
• The object. Some objects/surfaces we touch might have some colourless coatings, for example, a transparent greasy layer on a metallic object, which may be further transferred onto a different surface in the form of a latent fingerprint.

Latent Fingerprints Formation

Whenever we touch an object, our fingerprints with one or more combinations of the colourless inks from various sources mentioned above leave a stamp on that object. We are not able to see the stamp (impression) even though they are present because they are colourless and to make them visible, we have to give these colourless prints colour through a process known as Latent Fingerprint Enhancement.

Latent Fingerprints Enhancement

Latent fingerprint enhancement is seldom referred to as latent fingerprint development. In the simplest terms, latent fingerprint enhancement means making a colourless fingerprint deposit colourful.

Principle of enhancement: First of all, view the latent print and the object it is deposited on as two separate entities, but they appear to be one because the fingerprint has blended with the colour of the object it is deposited on. In all the techniques of latent print enhancement, the underlying goal is to make the fingerprint come out of hiding using contrast. This is achieved by exposing the latent fingerprint to conditions that will make it have a different colour from its background (the object it is deposited on). Below are some techniques used to give an invisible fingerprint colour.

Enhancing using optical methods (light): The angling of light falling on a colourless fingerprint deposit could give it colour and make it appear different from the object it is found on. For example, on the screen of your mobile phone, there are so many fingerprints on it that you hardly notice but when you tilt and view it at a different angle you now tend to see the fingerprint impressions. Again, it is possible to make visible a latent fingerprint using Alternate Light Source (ALS), a laser light with varying wavelengths. By flashing this specialized light on the fingerprint, they get illuminated and appear visible (colourful) when viewed with the appropriate goggles or filters.

Enhancing using powders: Latent fingerprints can appear colourful and thus visible using fingerprint powders. There are various fingerprint powders chosen for colouring (enhancing) invisible fingerprints depending on certain conditions, for example, the colour and nature of the object containing the invisible fingerprint. In a simple scenario, if the object is white, one will likely use a black powder to enhance the fingerprints and vice versa.

Enhancing using chemical methods. A latent fingerprint under certain conditions needs to be exposed to chemical processes to produce a colourful and observable fingerprint. Some of these methods involve, ninhydrin, iodine, silver nitrate, etc ¹. Using the ninhydrin method as an example, it usually reacts with the amino acids found in sweat secretions in the latent fingerprint to produce a purple-coloured fingerprint (Ruhemans purple) ².

Advanced techniques: Over the years, advanced tools and methods for enhancing latent fingers have been developed to tackle challenging latent fingerprint situations. For instance, the Mass Spectrometry Imaging (MSI) technique can be used to separate overlapping fingerprints ³. Moreover, some of these advancements look beyond enhancing the fingerprint pattern and can even provide information on the diet, medication, lifestyle, and objects the fingerprint donor touched ^4,5.

Preserving Enhanced Latent Fingerprints

After developing an invisible fingerprint into a beautifully coloured print with visible crisp fingerprint ridges. It is usually a requirement to preserve adequately the developed fingerprint which has the potential to serve as a crucial piece of forensic evidence. The conditions and circumstances would determine how the fingerprint will be preserved. The routinely used methods of preservation are:

Tape lifts: The fingerprints are lifted from the surface using adhesive tape and placed on an appropriate latent fingerprint card.

Photographs: Not all surfaces are ideal for tape lifting, hence high-quality photography comes into play to fill this gap. In this case, a detailed and high-quality image of the developed fingerprint is captured using a standard digital camera.

Examination and Matching of Latent Fingerprints

Recovered latent fingerprints from the crime scene may be run through an Automated Fingerprint Identification System (AFIS) for a potential suspect match. Any match from the AFIS requires a further manual examination by a fingerprint expert to confirm either a match or not.

Also, if there is a suspect, the suspect’s fingerprints and latent print(s) found at the crime scene are collected and examined. All ten fingerprints of the suspect are collected on a professional and well-labelled fingerprint card using ink (commonly black). The latent fingerprint is thoroughly examined and compared to see if it came from any of the suspect’s fingers.

Forensic Significance of Latent Fingerprints

Just like most forms of forensic evidence, latent fingerprints can be helpful in the following ways:

• Provide investigative leads.
• Aid in the apprehension and prosecution of an offender.
• Help exonerate an innocent person.
• Link a suspect to one or multiple crime scenes. In addition, it can help establish the relationship between two or more crime scenes.

Conclusions

From now on, know that as you go about your daily activities, you keep leaving invisible stamps on objects you touch with your fingers. Just because you do not see these latent fingerprints (stamps) does not mean they are not there, they are there, creeping steadily like a chameleon waiting to be discovered with the right tools and enhancement techniques.

Latent fingerprint recovery is a crucial component of fingerprint analysis and forensic science; therefore, a lot of the needed resources and expertise should be made available and deployed to ensure that invisible fingerprints are located, enhanced, and safely preserved for proper examination if and when required. A bad latent fingerprint recovery process will affect the examination and eventually the overall results.

References

1.        Pounds CA. Developments in Fingerprint Visualisation.

2.        Bottom CB, Hanna SS, Siehr DJ. Mechanism of the ninhydrin reaction. Biochem Educ. 1978;6(1):4-5. doi:10.1016/0307-4412(78)90153-X

3.        Bradshaw R, Rao W, Wolstenholme R, Clench MR, Bleay S, Francese S. Separation of overlapping fingermarks by Matrix Assisted Laser Desorption Ionisation Mass Spectrometry Imaging. Forensic Sci Int. 2012;222(1-3):318-326. doi:10.1016/j.forsciint.2012.07.009

4.        Hinners P, O’Neill KC, Lee YJ. Revealing Individual Lifestyles through Mass Spectrometry Imaging of Chemical Compounds in Fingerprints. Sci Rep. 2018;8(1). doi:10.1038/s41598-018-23544-7

5.        Groeneveld G, De Puit M, Bleay S, Bradshaw R, Francese S. Detection and mapping of illicit drugs and their metabolites in fingermarks by MALDI MS and compatibility with forensic techniques. Sci Rep. 2015;5. doi:10.1038/srep11716