Physico-Chemical Detection Techniques

Physical Developer (PD) is a fingerprint development reagent that is effective on most paper surfaces and on untreated wood. The reagent is an aqueous solution of silver ions, a ferrous/ferric redox (reduction/oxidation) system, a buffer and a detergent. The detergent prevents the premature deposition of silver ions. When the paper is immersed in this solution, silver metal is deposited on its surface. The silver metal is deposited preferentially on any fingerprints present and such prints become visible as dark grey ridges against a light grey background.

Contrary to ninhydrin, the reagent is sensitive to components in the latent print which are not soluble in water, i.e., sebaceous secretions. On wet paper or paper which has been wet, PD is one of the only techniques which can permit the satisfactory development of latent prints. The reagent may also be used as a treatment after ninhydrin if the latter has failed to reveal useful prints on a dry paper surface. PD can improve fingerprints partially developed with ninhydrin or develop prints not observed after ninhydrin treatment.

The PD technique presents some major inconveniences. It is destructive, components in the paper may react strongly with the reagent, and no other fingerprint development technique is effective after PD treatment, except for the radioactive enhancement technique discussed below. The PD reagent is delicate to prepare, it is a solution which is unstable since its efficiency is dependent on its very instability. A reasonable amount of experience is required for its successful preparation and utilisation. Despite these reservations, PD should be systematically applied at the end of any detection sequence on paper.

When prints developed by the physical developer process are weak or show insufficient contrast due to background patterns or colourations, radioactivity may be introduced by transforming the metallic silver (deposited by the physical developer) into radioactive silver sulfide.
The article, after treatment by PD, is immersed in a solution of bromine and potassium bromide which transforms the metallic silver into silver bromide. After being rinsed several times in water, the article is then treated with a solution of thiourea or sodium sulfide which converts the silver bromide into radioactive silver sulfide. It is then possible to develop an image of the fingerprint by autoradiography, treated fingerprint is placed in contact with a film sensitive to radioactive emissions. The exposure time depends on the degree of radioactivity of the treated print and varies, in general, from a few hours to several weeks.

Radioactive methods of fingerprint detection present several disadvantages which are inherent to the use of radioactive materials, the reagents are expensive and specific safety measures must be taken. The use of such techniques is therefore restricted to specialised laboratories. However, the remarkable results that can be obtained with these methods justify their application under certain circumstances.

This technique combines the principles of a small particle reagent and of a physical developer and was developed under the name of “Multimetal Deposition” (MMD). The development is achieved in two steps, the first being immersion of the object to be treated in a solution of tetrachloroauric acid (gold chloride) where the active constituent is colloidal gold. The use of colloidal gold is an established technique in biochemistry for the detection of proteins, peptides and, by extension, amino acids. It is now known that colloidal gold, at a certain pH (2.5 and 3.9), binds to the amino acids, peptides, and proteins in the latent fingerprint deposit to give a weakly gold metallic outline of the ridges. In some cases, identifiable prints are obtained after this first step.

The second step involves the treatment of prints developed by the “colloidal gold” process with a modified Physical Developer solution. The bound colloidal gold provides a nucleation site around which silver precipitates from the PD solution. This step greatly amplifies the visibility of the print which, after this second treatment, can vary in colour from light grey to almost black. One advantage of the Multimetal Deposition technique is that it can be incorporated into fingerprint reagent sequences, as will be discussed later.

Many types of surfaces, porous or non-porous, wet or dry, can be treated by this technique. Examples are, metal objects, wet and dry paper, plastic, glass, styrofoam, both sides of adhesive tape, and floppy disk surfaces. Fingerprints in blood may also be enhanced by the treatment.

Tests have shown that the method is particularly effective on plastic, glass, white paper, adhesive tape and expanded polystyrene. Prints up to three years of age have been detected on plastic and white paper. On certain paper surfaces, MMD gave better results than the conventional physical developer. On dry surfaces, however, MMD was generally not as sensitive as conventional techniques such as cyanoacrylate (non-porous surfaces) or DFO (porous surfaces).

Iodine Fuming
The development of latent fingerprints with iodine fumes involves a simple method of application, is inexpensive, fast and reversible, and does not generally preclude the subsequent application of other techniques. Iodine treatment may possibly be the oldest procedure for the development of latent prints. It is a physical process that relies principally on the absorption of iodine vapour by the lipids present in the fingerprint deposit to give a brown coloration. Prints developed with iodine are difficult to record because of the usually poor and transient contrast unless they are chemically fixed, and the effectiveness of the method decreases rapidly with time. The technique may be applied to a wide range of surfaces such as paper, wood, plastic and glass, but, due to its limited sensitivity, prints older than three to five days are unlikely to be detected by exposure to iodine. In addition, iodine vapour is toxic and corrosive, therefore prolonged exposure to iodine fumes must therefore be avoided.

The application of iodine followed by transfer to a silver plate has had limited success for the detection of fresh fingerprints on skin and leather (< 12 hours old for the latter) but the results are difficult to reproduce and depend on factors difficult to control. To apply the technique, prints are fumed with iodine then immediately transferred to a highly polished silver plate by firm contact over approximately 5 seconds. The transferred prints are then visualised by exposure of the silver plate to a strong light source. This results in the formation of a dark image of the latent print against the shiny surface of the silver plate.

Benzoflavone Post-Treatment
Fingerprints developed with iodine fade quickly and usually show poor contrast, it is therefore advantageous to fix the prints chemically by the application of a reagent that will increase both the contrast and the stability. A variety of reagents has been proposed (eg, starch, tetrabase), but the best results have been achieved with the organic fixative 7,8-benzoflavone (also known as naphthoflavone). Prints fumed with iodine are immediately treated with a solution of 7,8-benzoflavone, a dark blue-purple coloration generally results.

Iodine- Benzoflavone Spray
The use of a mixed solution of iodine and 7,8-benzoflavone has been suggested for the development of latent fingerprints at the scene of crime on surfaces such as wallpaper, emulsion painted walls and aged gloss painted surfaces. The solution may be applied with a spray, a paint brush, or a paint roller, and is currently used with good success by the Serious Crimes Unit (SCU) of Scotland Yard. A few minutes after application of the reagent, treated prints become visible as dark blue ridges. As the mixed reagent is not stable, it is prepared at the crime scene by mixing a solution of iodine with a solution of 7.8-benzoflavone just before use. The technique is particularly effective for revealing fresh marks less than a week old. Prints more than a week old are less efficiently developed. A further study has shown that the spray technique is more effective for revealing fingerprints at the crime scene on the surfaces mentioned above, then either ninhydrin or iodine/Benzoflavone solution applied with a brush.

Cyanoacrylate Fuming
Cyanoacrylate esters (generally the methyl or ethyl ester) are colourless, monomeric liquids sold commercially rapid, high-strength glues eg, “Superglue”. Cyanoacrylate liquid forms a vapour which reacts with moisture and certain eccrine and sebaceous components in a latent fingerprint. The vapour selectively polymerises on the fingerprint ridges to form a hard, white polymer known as polcyanoacrylate. Greasy fingerprints i.e., prints with a high sebaceous component, appear to be particularly sensitive to cyanoacrylate vapour although the glue probably also reacts with the moisture and some water-soluble (eccrine) components in the print. The technique is effective on most non-porous surfaces including metal, glass and plastic. Originally developed in Japan in the late 1970’s, the cyanoacrylate fuming process is now the most widely used fingerprint detection technique for non-porous objects treated in the laboratory.

Cyanoacrylate Development
Numerous methods for cyanoacrylate treatment have been published, and several commercial fuming chambers are available on the market. A cost-effective fuming chamber involves the use of a large glass container, such as a fish aquarium modified to allow the introduction of an appropriate heat source (eg, soldering irons). The samples to be treated are suspended in the tank, a few drops of liquid cyanoacrylate vaporised by heating to approximately 100° C, and a cover placed over the chamber to contain the fumes. Solid cyanoacrylate polymer, i.e., polycyanoacrylate, may also be used to develop fingerprints, but higher temperatures (160-180° C) are required for its vapourisation. The objects are removed when sufficient fingerprint detail is observed.
Development must be tightly controlled as fingerprints over-exposed to cyanoacrylate vapour suffer from a loss of detail that is particularly damaging after treatment with a luminescent stain.

A vacuum cyanoacrylate fuming technique has been developed by the National Research Council of Canada. Development is achieved by placing the evidential objects in a large metal chamber, together with about 0.5 ml of liquid cyanoacrylate glue, then the pressure reduced to approximately 200 mtorr (0.2 torr; standard atmospheric pressure equals 760 torr). At this pressure, the evaporation of the cyanoacrylate is accelerated and the development time subsequently reduced. No heat or humidity is employed. The contents of the chamber are kept under reduced pressure, in the presence of cyanoacrylate, for about 20 minutes, then air is admitted and the objects checked for fingerprint development.

The vacuum cyanoacrylate method is claimed to give more uniformed fingerprint development and better contrast than the traditional cyanoacrylate procedure. Fingerprints developed by the vacuum process tend to show better ridge and pore detail. There is less risk of fingerprint over-development and the operator is not exposed to cyanoacrylate vapour. In addition, fingerprints may be developed on the inside of sealed plastic bags and therefore it is unnecessary to open and suspend such items of evidence.

The vacuum cyanoacrylate chambers manufactured by the National Research Council of Canada cost around $US10,000 each. Other commercial units are also available, however a low-cost system can be readily constructed using a bench-top vacuum dessicator.

Enhancement of Cyanoacrylate Developed Prints
The contrast produced by fingerprints developed with cyanoacrylate, particularly on white-coloured surfaces, may be enhanced by the application of a coloured or luminescent stain. To obtain the best results with these stains, the cyanoacrylate development must be precisely controlled and over-exposure rigorously avoided. In addition, to limit the risk of washing cyanoacrylate developed prints off the surface with the staining solution, the prints should be left overnight, after the cyanoacrylate treatment, to allow the white polymer to harden on the fingerprint ridges before application of the stain. The choice of a particular staining technique will depend on the colour and luminescence properties of the surface to be treated. If in doubt, tests should be conducted on a similar surface before treatment of the evidential article. The following stains are particularly recommended.

Gentian Violet.
Fingerprints developed with cyanoacrylate and treated with a solution of gentian violet are stained dark purple in colour. The prints need to be thoroughly rinsed with water after application of the stain to avoid any background coloration. Application of gentian violet does not require a specialised light source, since no luminescent is produced, however, its use should be restricted to lightly coloured or highly luminescence surfaces. For most cases, the luminescent stains described below should be used in preference due to the higher sensitivity that is generally achieved.

Ardrox 970-P10.
Ardrox 970-10 is a fluorescent penetrant sold by Ardrox Limited, Canada. The product is a highly luminescent oily liquid that can be diluted to give an efficient staining solution for cyanoacrylate prints. Prints treated with the stain are left to dry, then rinsed with water to remove any background luminescence. The luminescent prints that result can be visualised using an excitation in the 250 (UV) to 500 nm range with observation in the 450 to 650 nm region. An advantage with Ardrox is that a simple UV lamp (short or long wavelength) can be used for the detection of treated prints. The UV excited luminescence is stronger than that observed with rhodamine 6G or Basic Yellow 40.

Rhodamine 6G.
Rhodamine 6G is a multipurpose luminescent compound that has a particularly high quantum yield (luminescence efficiency). Cyanoacrylate developed prints are strongly luminescent after treatment with a solution of this reagent. In most cases, the article need not be rinsed with water after application of the stain. The luminescent prints that result can be visualised using an excitation in the 450 to 550 nm range with observation in the 550 to 600 nm region. Prints treated with rhodamine 6G are only weakly luminescent under UV light. Initial reports suggesting that rhodamine 6G is carcinogenic have proved to be largely unfounded according to recent studies.

Basic Yellow 40.
Basic Yellow 40 (BY40; Panacry Brilliant Flavine 10 GGF; Maxilon Flavine 10 GFF) is a non-toxic dye currently being used as a cyanoacrylate stain in a number of countries including the United Kingdom. It produces about half the emission intensity of rhodamine 6G. The stain has an excitation maximum at 440 nm with a luminescence emission peak at 490 nm.

Basic Red 28.
Basic Red 28 (BR28; Bayer AG, Leverkusen, Germany) produces a strong red luminescence emission centred at 585 nm with an excitation maximum at 495 nm. The stain may be mixed with Basic Yellow 40 to obtain a significant increase in the Stokes shift. A mixture of BY40 and BR28 gives a combined signal representing a Stokes shift (difference between excitation and emission maxima) of about 150 nm.

Thenoyl Europium Chelate.
Canadian workers have proposed a cyanoacrylate staining procedure based on the europium complex “Thenoyl Europium Chelate” or TEC. This complex absorbs in the long-wave UV region (350 nm) with a strong narrow emission in the red at 614 nm. Good results have been reported for the enhancement of cyanoacrylate developed prints even on highly luminescent surfaces and human skin.