We thank Keiron – la Casa dei Penalisti for this article.
Contribution of Chiara Biglieri
Fingerprints have been used as a means of identification and as evidence in criminal investigation for more than 100 years. They are formed by sweat released from pores present on friction ridge skin of hands or feet’s plantar side.
They have been considered as one of the most essential and useful kinds of physical evidence that can be collected on a scene. The reason behind this can be observed in their two main peculiarities: individuality and permanence. The former means that no two fingerprints have been found to be ultimately the same, because each fingermark is unique to each individual. The latter implies that prints remain the same for the whole life of a person since the third month of gestation when they start their development.
Not all the impressions are the same: there are indeed patent, plastic and latent fingerprints. The patent type is the one that can be recognized on a crime scene in the fastest way: that is a visible fingertip left on a surface, created by blood or other contaminates (as for greasy marks). The plastic fingerprints are impressions left on soft surface such as wax. Conversely, a latent fingerprint is a fingerprint left on a surface by deposits of oils and/or sweat from the finger and it is not usually visible to the naked eye.
It is obviously more difficult to detect latent fingerprints, even if there are different method through which it is possible. Indeed, there are numerous different ways to process pieces of evidence found at the crime scene. Each kind of evidence has a peculiar way to be processed in order to preserve its qualities and not to destroy it. The technique used is also dependent on the surface where evidence is found.
The method this article will analyze is the most common one: the cyanoacrylate fuming method. This is a very useful chemical process in order to detect latent fingerprints on non-porous surfaces, which is any surface that doesn’t absorb moisture, as plastic, metals, ceramics, etc.
The cyanoacrylate fuming method, also known as the super glue fuming, is a well-known technique for processing latent fingermarks. It was developed in the late 1970s by the Japanese. Shortly thereafter, it was brought to the United States by the United States Army Criminal Investigation and Bureau of Alcohol, Tobacco, and Firearms Laboratories.
What is the Cyanoacrylate Glue?
The essential element in order to use this method is the cyanoacrylate glue. Unlike traditional water-based glue, this one is composed of an acrylic resin. In fact, it is an acrylic monomer that transforms into a plastic state after curing. The difference between this specific type of glue and the normal one can be found in its special bonding conditions. Moisture is essential in order to have cyanoacrylate glue works: perfectly dry surfaces aren’t suitable for this specific kind of glue.
How does this Method Work?
Latent fingerprints are composed of several chemicals. It is possible to highlight two main components: the primary one is ordinary sweat, while the other one is primarily solid and can remain on a surface for much longer than sweat which can be considered mostly as water. The second component includes organic compounds like amino acids, glucose, lactic acid, peptides, ammonia, riboflavin, and isoagglutinogens as well as inorganic chemicals like potassium, sodium, carbon trioxide, and chlorine.
This method is considered as a fingerprints’ developing chemical technique which consists in applying a substance that will chemically react with one of the constituent chemicals of latent fingerprints to the area suspected of containing them. The following reaction will make the latent fingerprints visible in order to give the crime scene specialist the opportunity to photograph and analyze them.
At the beginning, this technique was considered as a lab process because of all the chemical components that are necessary to allow visualization and photography of the fingermarks. Shortly thereafter, it became a standard on-scene process and, due to the possibility to stabilize latent fingerprints, one of the most used methods. By comparing the superglue fuming to merely powdering, the former is able to better preserve this fragile evidence: in fact, cyanoacrylate fuming locks the latent print to the surface of evidence and preserve them until their further examination.
There are different methods for processing latent fingermarks using this specific technique starting from homemade chambers until we arrive to commercial ones, which are better for controlling temperature and proper vapor circulation. The general and common idea consists in isolating the item where latent fingerprint is supposed to be and to place it into an airtight cabinet (or another suitable container). So that the fumes can reach every surface of the article, the item is suspended into the chamber.
There are different ways of placing superglue into the chamber. An example involves commercial superglue packs which are simply opened and placed into the cabinet. These packs reduce the problem of spilling liquid glue. Another example consists in placing few drops of superglue into an open and tiny metallic container which is located at the bottom of the cabinet.
In order to speed the fuming process, the tank is carefully sealed and a heat source (e.g. a work light or a hot plate or a coffee-cup warmer) is employed. In this second case, the cyanoacrylate’s container is usually heated to around 176℉ to 212℉ in order to have enough vapor.
Besides, also humidity plays an important role: in fact, a water container is usually placed in the chamber to provide sufficient moisture. If there isn’t sufficient humidity, fingermarks will appear with a poor contrast. It is fundamental to remember that this development process should be constantly inspected to avoid over-development.
It has been pointed out that relative humidity and long-term storage of latent fingermarks before processing had a negative effect on the quality of cyanoacrylate developed fingerprints on polypropylene foils, while relative humidity while fuming has positive effects over the quality of developed prints.
As regards as how much time is needed for this process, the development of latent fingermarks may take between several hours and several days. In order to reduce this amount of time, it is possible to heat cyanoacrylate, even if it is better not to overheat it because of the production of toxic hydrogen cyanide.
Post Treatment Procedures
The superglue fuming isn’t always the last thing to be done in order to process fingerprints: in order to print them out or enhance them, there are different procedures which can be used. It is possible to dust them with regular or magnetic fingerprint powder.
Actually, the two dye stains used most often are Basic Yellow 40 or Rhodamine 6G. The former consists in submersing the already stabilized print in the solution (2-g to 1000-ml of alcohol) for a period in between 10 sec to 1 min. After this, it is removed and rinsed with tap water. The last things to be done are letting the surface dry and examining with an ALS set at 385-485 nm. The latent print will fluoresce bright yellow or light green.
The latter has two possibilities: either the stabilized latent print is submersed, or the solution is flowed over the surface. Then, the latent surface should be hung and allowed to dry. When dry, it is examined using an ALS set at 450-nm with an orange barrier filter.
Negative Aspects of Superglue Fuming
Even if this technique is a well-established method, it has got a main shortcoming that is lack of contrast due to the white cyanoacrylate polymer, particularly on light-colored background. In order to obviate this problem, a second treatment is often required, as we have already described in the previous paragraph. Some studies suggested that it is possible to use fluorescent dyes incorporated into superglue prior to fuming, as Sudan Black-superglue or Rhodamine B-superglue. Through this “incorporation”, it is possible to have better results in a smaller amount of time.
Another negative aspect of this method is over-fuming. Because of that, the quality of ridge details could be reduced. Polymers will adhere in between the latent fingermarks’ ridges.
An additional drawback can be seen in processing aged latent fingermarks which have been exposed to extreme environments. To improve their quality, it is better to fume at lower temperatures because this improves the rate of polymerization.
Cyanoacrylate fuming is a chemical method for development of latent fingermarks on non-porous surfaces. It is widely considered as efficient and non-deconstructive. Even if it takes time to get everything done correctly and even if there are some drawbacks, its qualities towards developing latent fingermarks on wide range of non-porous surfaces are evident. It develops stable white colored fingerprints. It is possible, after the use of this technique, to use subsequent powdering and/or staining procedures in order to get even better fingermarks.