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Fingerprints

 

Fingerprints

Introduction

This paper is designed to give a general overview of the use of fingermarks in the investigation of criminal activity.  It does not deal with the matter in depth and further study is required before a level of expertise can be achieved.

History

Fingerprint identification is at least 4,000 years old; King Hammurabi used fingerprint impressions in seals on contracts, and the 5th Century law book of Yung-Hwui in China required a husband in a divorce decree to seal the document with a fingerprint.  In 1823 the student Johannes Purkinje classified fingerprints into nine major groups in his doctoral thesis whilst 65 years later Sir Francis Galton and Sir William Herschal devised the Galton Details which used various identifiable points within a fingerprint.  Galton published Fingerprints in 1892.  The Henry System arose when Sir Edward Richard Henry devised a further classification system (Smyth et al 2008, 9; Becker 2009, 120-121).

Formation of fingerprints

Friction ridges arise on the hands and feet and run in patterned ridges of different shapes and characteristics.  As well as differing between individuals, these patterns differ from digit to digit.  Close inspection reveals that the ridges are broken at irregular intervals by sweat pores associated with eccrine glands.  Sebum, an oily substance, is secreted from the human sebaceous glands which are found on the forehead, chest, back, and abdomen; finger contact with these areas results in the deposit of fingerprints on touched surfaces (Becker 2009, 121).

 

Fingerprint individuality

It is often said that no two individuals have the same fingerprints; however, there has not been any real attempt at empirical testing of that hypothesis, nor has there been any extensive research conducted on population frequencies (National Research Council 2009, 139).  More properly it could be said that ‘no two fingers have yet been found that have identical characteristics’ (Becker 2009, 121).

Individuality is not dependant on factors such as age, size, gender, or race; it is the minutiae (or ridge characteristics) that individualise fingerprints by their shape, location, and number (Becker 2009, 121).

From birth to death a person’s fingerprints will remain unchanged except for size (Becker 2009, 122).

Injuries to the hands that cause changes to fingerprints act as a further point of comparison provided the injury occurs before the fingerprint is laid down.

There are several characteristics that go to make up fingerprints –

  • ridge ending
  • bifurcation
  • dot
  • island (short ridge)
  • lake (enclosure)
  • hook (spur)
  • bridge
  • double bifurcation
  • trifurcation
  • opposed bifurcation
  • ridge crossing
  • opposed bifurcation/ ridge ending (Lennard and Patterson 2008).

In addition to the above, fingerprints are also composed of loop and whorl patterns.  Loop patterns are of two types ‘radial’ and ‘ulna’ drawn from the names of the forearm bones.  A radial loop points towards the radial bone (the same side as the thumb) and an ulna pattern away from the thumb, together they account for 60 to 70 percent of the patterns encountered, and whorls about 25 to 35 percent; there are several types of whorl patterns (Lennard and Patterson 2007).

 

Developing fingerprints

It is noted that the application of fingerprint developers can adversely affect the gathering of blood-related evidence.

Adhesive tape

Use of an adhesive side powder mixed with Photo-Flo (a film developer manufactured by Kodak) can reveal fingerprints on adhesive tape that has been balled or folded (Becker 2009, 132).

Chemical developers

Ninhydrin (or triketohydrindene hydrate) is commonly sprayed on porous surfaces (such as paper and cloth) to develop (render visible) prints; it reacts with the amino acids in the sweat and produces a purple-blue print with a development time of one to 24 hours, shorter if heating is applied at 100o C (Becker 2009, 126).

Diaza-fluoren one (DFO) produces a similar result to ninhydrin; it is more reliable and is also useful on porous surfaces but requires ultraviolet light to fluoresce (Becker 2009, 126-127).

Fluorescence and alternative light sources

Various applications can be used to cause fingerprints to fluoresce, for example the blue-green light from an argon-ion laser causes sweat to fluoresce.  The treatment of fingerprints with ninhydrin and then zinc chloride or dye rhodamine 6G after superglue fuming causes florescence and sensitivity to laser light (Becker 2009, 132).

Fuming

Cyanoacrylate ester (superglue) is using in fuming cabinets to develop fingerprints on non-porous surfaces.  The glue is heated for at least five hours after which a hard whitish deposit is formed on the fingerprint which is then dusted (Becker 2009, 127-128).

Prints on paper can be developed with iodine fumes or crystals but must be photographed or fixed because the image quickly fades.  Iodine should be used before ninhydrin and silver nitrate (Smyth et al 2008, 10; Becker 2009, 130).

Silver nitrate adheres to chlorides in skin secretions to form silver chloride which turns grey when exposed to light; however, the developed colour turns black very quickly.  It is useful on paper, cardboard, plastics, and unvarnished light coloured wood.  It should be used after ninhydrin and iodine (Becker 2009, 130).

Powders

Powders differ in colour, consistency, density, and polarity for use in different situations and are applied using brushes, magnetic wands, or blowing across visible or latent prints where the powder adheres to the bodily fluids responsible for the laying down of the print.  When fluorescent powders are used they are rendered visible using ultraviolet light and an orange filter (Becker 2009, 125-126).

Thermal paper

Visualising fingerprints on thermal paper has long been problematic due to solvent turning the whole of the paper black; however, a new process developed in 2014 by the University of Leicester has addressed that issue.  The process uses a specially tailored UV light source, heat and commercially available equipment; it is known as the Hot Print System (University of Leicester 2014).

Tyres

Because of the environment in which tyres are usually used (wet, abrasive, and hot and cold conditions) the visualisation of latent fingerprints can be difficult, that is, if they persist at all.  Success in the endeavour has been found using black magnetic powder, although visualisation might be difficult because of the often black surface of the tyre.  Further, the tyre can be ‘superglued’ and an orange fuming agent employed.  Some success with yellow fluorescent powder has also been reported.

Wet surfaces

Should a fingerprint be encountered on a wet nonporous surface, it is possible to use Small Particle Reagent (SPR) as a type of liquid fingerprint lifting powder.  The SPR is sprayed across the fingerprint after which the print is lifted in the usual manner.  What happens is that the SPR adheres to the fingerprint. (Becker 2009, 131-132; Forensic Magazine 2014).  A word of caution, the active ingredient of SPR is molybdenum disulfide which is a hazardous substance; caution should be exercised in its use (North Carolina State Crime Laboratory 2013).

Lifting fingerprints

The traditional method of lifting fingerprints has, to a degree, succumbed to the age of digital photography.  In cases where fingerprints are sufficiently visible they can be photographed for comparison purposes, in other cases the methods of using powders, chemical developers, fuming, fluorescence and alternative light sources is resorted to before photography.

Once a print has been developed it can be lifted be means of one of the many specialised adhesive lifters that are commercially available, generally these are transparent tapes that are placed adhesive side down over the developed print and which, when removed, bring the print with the tape.  The tape is then placed on a coloured cardboard background to render it more easily visible (Becker 2009, 133-134).

Fingerprint matching

An individual fingerprint may have as many as 100 points that can be matched (Lennard and Patterson 2008) which leads to the question of how many matching points are required before identification can be made.  Following a three year study, the International Association for Identification determined that ‘no valid basis exists for requiring a predetermined minimum number of friction ridge characters which must be present in two impressions in order to establish a positive identification’ (Saferstein 2007, 435).

In Western Australia there is not a set number of comparison points (there is in some places) and the declaration of a match or otherwise is left to the skill and experience of the fingerprint examiner.  Even when a match is made, before such is declared, a confirmatory opinion is sought from another fingerprint examiner.  The value of the match can be measured by the –

  1. number of comparable ridge characteristics
  2. knowledge of the examiner
  3. experience of the examiner
  4. ability of the examiner to explain how the comparisons were performed
  5. quality of the evidence of another person whose view is in conflict with the finding of the examiner (Becker 2009, 122).

To that list ought to be added the number of non-comparable ridge and other characteristics, for example if there is an apparent injury to the finger of an identified person that is not present on the lifted visible or latent fingerprint then such needs to be explored before a match is declared.  Also relevant is whether or not there is more than one matching fingerprint found at the offence scene that also matches the accused person, and whether any fingerprints at the offence scene do not match the person.

Accreditation

Accredited fingerprint examiners in Australia undergo an assessment by the National Institute of Forensic Science’s Australian Field Forensic Science Accreditation Board (AFFSAB) before they can call themselves ‘experts’ and individual laboratories are accredited, examined, and audited by the National Association of Testing Authorities (NATA), a nationally recognised accreditation body.

Automated searching

In Australia, the National Automated Fingerprint Identification System (NAFIS) is housed in Canberra and operated by CrimTrac; it is a computer based system into which all jurisdictions contribute ‘tenprints’.  NAFIS can be searched by all contributing jurisdictions for fingerprint matches and agreements with international law enforcement organisations allow the release of information in certain cases.  By 2008 NAFIS housed 4.4 million tenprints (CrimTrac 2008) by comparison in the same year the Federal Bureau of Investigation held 200 million fingerprints (Smyth et al 2008, 10) in that year the comparative populations were USA 304.5 million: Australia 21.3 million, meaning that in the USA every 1.5 citizens were recorded but only 4.84 in Australia if all of the prints held were unique.

In 2011 NAFIS holdings had grown to 5.6 million tenprints representing 3.3 million people (CrimTrac 2011).

Lip prints

Lip prints can be of use and were successful in matching the victim of a hit and run traffic crash with a vehicle when they were left on the vehicle’s bumper during impact (Smyth et al 2008, 10).  However, the author has not been able to discover any published research on the reliability of using this method of comparison.

 

© Hadyn R Green 2014

 

 

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