The purpose of 'fingerprinting', in the field of forensics throughout the world, is to assist with forensic identification. During the 20th century, the technique of fingerprinting was accepted as solid evidence, because there were no testimonies going against it. Recently, however, the scientific validity and reliability of the technique, used for forensic fingerprint identification, has been challenged (Abraham et al., 2013).
According to Jackson et al. (2011), contact with the area of friction ridge skin on a surface creates fingerprints, which as a result, leave a unique impression. The layer of thickened skin that appears on the surface of the palms and soles of toes are called friction ridge skin. These ridges form complex and unique print patterns on the skin. A record set of prints of the thumbs and fingers can be reproduced by using ink or other substances. Houck (2007) explained that, in the womb, friction ridges are developed to which they remain the same in a lifetime. The arrangement of the friction ridges is located deep in the layer of skin acting as a template.
Types of Fingerprints
There are three different types of fingerprints that can be made from a friction ridge print. Based on Houck (2007), a friction ridge can be depicted as a pattern in a medium. The three different types of friction ridge prints are patent or visible, latent and plastic. Patent prints can be seen by the naked eye, due to the deposition of transferable materials (liquid blood, paint or dust) which create a 'memory' in the impression such as, clay or wax.
However, from the research of Jackson et al. (2011), latent fingerprints cannot be seen. These prints consist of perspiration from the sweat pores and compose about 95% of water with the remaining from salt and amino acids. Before seeking for latent fingerprints, visualisation is required with the appropriate use of development techniques. Plastic fingerprints, conversely, are formed when a negative ridge impression is reproduced as a three-dimensional object from a soft material such as, clay, soap, candle wax etc.
Fingerprint Technology and Techniques
The main techniques used to identify or enhance latent prints from the research of Jackson et al. (2011) are: acid black 1/acid violet 17/acid yellow 7, fluorescence examination using light sources, iodine fuming, ninhydrin, DFO application, physical developer, powders, small particle reagent and superglue fuming.
Acid black 1, acid violet 17 and acid yellow 7:
These reagents specialise in developing fingerprints contaminated with blood. With acid black 1 and acid violet 17, these reagents work on any type of surface whereas, acid yellow 7 is only for enhancing fingerprints lightly contaminated with blood on non-porous surfaces (surfaces that do not absorb). Additionally, the blood reagent acid violet 17 can only be used after the application of acid yellow 7 on non-porous surfaces. Acid black 1, acid violet 17 and acid yellow 7 create a blue-black, vivid violet and yellow fluorescent colour respectively, when there is a presence of proteins from blood or other fluids.
Fluorescence examination using light sources:
High-intensity fluorescent lighting and lasers are used for observing latent fingerprints with appropriate filters. Due to the presence of contaminants such as, grease, in the sweat, latent prints are visible.
Iodine fuming is an old technique and can be applied to non-porous or porous surfaces; however, it is rarely of use today. The process involves iodine crystals undergoing sublimation where it changes from a solid to a gaseous state, without forming a liquid when heated. The prints are exposed to the vapour and react between the fumes and the components of the latent print, producing a yellowish-brown colour.
Ninhydrin (trikeohydrindene hydrate) is widely used as a reagent for detecting latent prints. It is effective on porous surfaces such as, paper, cardboard and for developing bloody fingerprints. Ninhydrin consists of dissolved ninhydrin crystals in an appropriate solvent and is applied as a fine spray. When reacted with amino acids, from the perspiration constituent of the latent print, it produces a bluish-purple colour or 'Ruhemann's purple' which takes quite a while to develop. However, with the use of heat this can accelerate the reaction from several days to approximately two minutes.
DFO (1, 8-diazafluoren-9-one) is another reagent that reacts with amino acid, besides ninhydrin. The red-coloured fluorescent which DFO creates requires laser or high-intensity light sources for the prints to be viewed. DFO can also be applied on to surfaces similar the treatment of ninhydrin, however it is recommended that DFO is used first in a sequential process.
Physical developer (PD):
The technique of physical developer involves a series of aqueous solutions. These are used to visualise the latent prints on porous surfaces such as, wet paper. The evidential object is first prewashed in a solution of maleic acid, then a submersion in the PD working solution. The solution contains a mixture of a redox (ammonium ferrous sulphate, ferric nitrate and citric acid), surfactant and silver nitrate solution. The object is then rinsed properly in water, dried and recorded as photographs. The PD is effective in obtaining results than other visualisation techniques.
There are many different types of powders that can be used for latent fingerprint detection such as, grey aluminium powder, black or white powder, magnetic powders and fluorescent powders. Applying these powders will require a synthetic fibre brush with natural bristles. Fluorescent powders or luminescent powders are often used for developing visualisation of the prints, and is exposed to laser or UV light for further enhancement.
Small particle reagent:
Small particle reagent consists of molybdenum disulphide particles which are suspended in a detergent solution and can be applied by spraying or from a dish. The particle in the solution sticks to the fatty components of any latent prints, and forms a grey deposit on waxy or polystyrene surfaces in wet outdoor conditions.
Superglue fuming (Ethyl cyanoacrylate):
Superglue fuming is a recent technique suitable for non-porous surfaces such as, rubber, metals and electrical tape. It is also hazardous as the fumes are harmful, thus it is done in a chamber fitted extraction system developed by Foster and Freeman. A hard white polymer is developed due to the vapours when latent prints are present, during the circulation of air and relative 80% humidity to speed the reaction (Jackson et. 2011).
Based on Ramotowski (2013), different fingerprint powders can further enhance patent fingerprints such as, traditional, magnetic, luminescent or nanoparticle powders. Later in the 19th century, powders have been in use as a means of visualising prints from organic and inorganic materials. Despite the decreased attraction between the drying of the fingerprint powder and residue, they are still useful in sticking to moist and latent print residue substances.
Chemical Composition of Latent Fingerprints
Clean and oily prints are two chemical compositions that can be found in fingerprinting. Oily prints are classified as latent due to the deposition of sebum, whereas clean prints are deposited from freshly clean hands. Eccrine glands release eccrine sweat from the palms of hands, while sebum is transmitted from contact of the face, hair or neck. Oily prints have lipids, found on the surface of skin, which originate from sebaceous glands consisting of squalene (10%), sterol esters (2.5%), sterols (1.5%), wax esters (22%), triacyl glycerols (25%), di- and monoacyl glycerols (10%), unesterified fatty acids (25%), and 4% unknown substances (Ramatowski, 2013).
Fingerprinting: Reliability and Validity
Fingerprinting has been a reliable technique used for identification in the field of forensics. Based on the principles of friction ridge analysis, fingerprinting is seen as solid evidence for identification since Galton's time. Based on Gottfried Whilhelm Leibniz's statement 'for in nature there are never two beings which are perfectly alike'' the concept of uniqueness is that it is not possible for two individuals to have identical qualities (Houck, 2007).
Recently, critiques have challenged the reliability and validity of fingerprinting. Their reasons are:
- Uniqueness cannot be proven and is irrelevant if probability cannot be used
- Identification needs probability for a conclusive result
- Relying too much on systems can lead to human errors
- Responsibility of entrusting a fingerprint expert in interpreting a match
- Results are in the form of opinions not statistics
- Knowledge of forensic techniques are still insufficient
- Accuracy in pattern comparisons
- Automated devices cannot replace humans
- Use of correct phraseology in conveying opinions
- Errors in chain of custody in evidence analysis
However, some forensic science practitioners defended the idea of uniqueness based on morphological analysis, and that there is no certainty in chances of probability in forensic science. Despite these arguments, it is of real concern if an error occurred in the methods of forensic science. Nonetheless, this technique has been used for over 100 years and has portrayed benefits in crime scene investigation (Jayaprakash, 2013).
Overall, even though the method of fingerprinting has been challenged in relations to its scientific reliability and validity, with the use of research in recent and more advanced technologies of fingerprinting, improvements can be made to existing techniques to minimise errors.
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