Physical Detection Techniques
One of the oldest, most common and most readily available methods for the development of latent fingerprints is that of dusting a surface with a fine powder of contrasting colour. The use of powders depends on the object upon which the search is being conducted. A wide range of powders is commercially available and their formulation differs with each manufacture.
Fingerprint powder is applied at the crime scene on smooth, non-absorbent surfaces and, in general, only to objects that cannot be transported back to a laboratory. The powder adheres to the humid, sticky, or greasy substances in the latent fingerprint deposit. The application of powder is relatively simple and inexpensive and little experience is necessary to obtain satisfactory results. However, compared to techniques such as cyanoacrylate fuming, powdering is an insensitive detection method and only relatively fresh fingerprints, up to several months of age, are normally developed. Difficulties also arise when the nature of the surface and the composition of the fingerprint vary. Among the multitude of powders and brushes available, the choice is often made according to experience or personal preference.
The Ideal powder is one of contrasting colour, good adherence properties and sensitivity, possibly incorporating a luminescent material. Magnetic powders, generally made by mixing iron grit with either aluminium or copper flake powder, are applied using a magnetic wand. Their use avoids the brushing, and hence destruction, of fragile prints, however the technique is difficult to apply on vertical surfaces.
Fingerprints developed by powdering can be collected from a surface using a suitable transfer medium. Elastic or rubber fingerprint lifters or transparent cellophane tape are normally used for this purpose. The use of fingerprint lifters is of particular advantage on curved surfaces such as doorknobs on which developed prints are difficult to photograph. These lifters are commercially available items made of a thin, rubbery material coated with an adhesive. The adhesive is protected by a transparent celluloid material removed prior to use and replaced onto the lifter after use. The colour of the lifter may be either black, white or transparent for use with different fingerprint powders.
|Brush it with aluminum powder||The marks became visible.||Put a black rubber lifter on it.|
|The marks are on the lifter.||Put a plastic sheet over it.||Ready to search!|
Fingerprint lifting tape is the most common method of collecting latent fingerprint evidence after powdering. After the surface is dusted with fingerprint powder, the adhesive tape is placed over the print and smoothed down with the finger. Particles of fingerprint powder adhere to the sticky surface of the tape and thereby transfer a mirror image of the fingerprint pattern. The tape is finally placed onto a card of suitable colour, contrasting with the powder used.
The lifting method is simple and easy to master, and requires no knowledge of photography and no photographic equipment. Its use, however, requires greater accuracy in specifying the exact position where the prints were located.
SMALL PARTICLE REAGENT
Latent fingerprints may be made visible by immersion in a aqueous suspension of an insoluble powder followed by rinsing with water. Small particle Reagent (SPR) may be considered as a wet powdering method. The reagent is sensitive to the sebaceous (non-water soluble) compounds of the latent fingerprint and may be used on a wide range of non-absorbent surfaces. SPR is effective on surfaces which are wet – a condition which excludes the use of conventional powders or reagents sensitive to the eccrine (water soluble) components of the latent print. Treatment with the suspension is by immersion or vapourisation (using a garden spray, for example) – the sample is then washed with water in order to remove any excess reagent.
Conventional SPR is a suspension of molybdenum disulfide particles, the fine crystalline structure of which is critical for fingerprint development. The results obtained using molybdenum disulfide from different sources are strongly influenced by variations in this structure. Good results have been obtained using “ROCOLÂ® AS powder” – molybdenum disulphide obtained from other sources should be tested and compared with a SPR of known quality.
There is now available a new SPR formulation based on white zinc carbonate powder. This formulation is designed for use on dark surfaces. As with molybdenum disulfide, it was found that the dimensions of the zinc carbonate particles had a significant influence on the quality of the fingerprint development. Aerosol sprays containing this white SPR formulation have been developed in Israel and have given excellent results in the field. More recently, a luminescent SPR has been developed.
VACUUM METAL DEPOSITION
Fingerprint contamination on a surface can hinder the deposition of metallic films following metal evaporation under vacuum. This phenomenon has been known for a long time but it is only recently that it has been applied to the detection of latent fingerprints. It is now accepted that Vacuum Metal Deposition (VMD) is an extremely sensitive and useful technique for fingerprint detection on a variety of surfaces and it may be employed in conjunction with other development techniques, such as cyanoacrylate. Unfortunately, a large VMD units are prohibitively expensive for most laboratories, and significant experience is required in order to obtain the best results from this technique.
Gold is evaporated under vacuum to form a very thin layer of metal on the surface under examination (this layer is invisible to the naked eye). A second layer of zinc or cadmium (the latter is rarely used because of its toxicity) is deposited in the same manner. The gold film is uniformly deposited across the surface of the sample and penetrates the fingerprint deposit. The zinc is deposited preferentially on the exposed gold but does not penetrate the fingerprint deposit – the ridges are therefore left transparent while the background becomes plated with a layer of zinc. Excellent fingerprint detail can be obtained in this way with the best results on surfaces such as plastic and glass. Fresh fingerprints, less than 48 hours old, have also been developed on cloth and banknotes using this technique. See below.
Vacuum Metal Deposition can sometimes reveal fingerprint detail when all other techniques have failed. Excellent results have been obtained using metal deposition after cyanoacrylate development followed by luminescent staining.