Chemical Detection Techniques

Silver nitrate is probably the oldest known chemical technique for fingerprint detection on porous surfaces such as paper. Silver nitrate reacts with the chloride component of the latent fingerprint deposit (eccrine secretion) to form light sensitive silver chloride. On exposure to light, silver chloride, which is white in colour, is decomposed to silver metal, producing a black image of the fingerprint. The technique is effective for detecting fresh fingermarks on most paper surfaces and untreated wood. However, loss of fingerprint detail tends to be observed with relatively old prints (> one week) developed with silver nitrate due to the diffusion of chlorides through the substrate. Another disadvantage with the reagent is its strong background reaction which results in a darking of the substrate with time.

A typical working solution of the reagent is two per cent (w/v) silver nitrate in methanol. The solution may be applied by immersion or with a spray. After treatment, contrast between the developed fingerprints and the substrate is observed. Due to the disadvantages discussed above, and the more sensitive procedures now available, eg, ninhydrin and DFO, silver nitrate is seldom employed in practice. However, special circumstances, such as fresh fingerprints on untreated wood, may sometimes justify consideration of the technique.

The reaction of amines with ninhydrin to form the coloured reaction product known as Ruhemann’s purple was discovered by Siegfried Ruhemann in 1910. Since that time, the reaction of amines, amino acids, peptides and related compounds with ninhydrin has found extensive use in the qualitative and quantitative analysis of such compounds in chemistry and biochemistry. However, the value of ninhydrin for the development of latent fingerprints was not realised until 1954 when Odén and von Hofsten suggested its use in criminal investigations. Ninhydrin is now the most widely used method for developing latent fingermarks on paper surfaces.

Ninhydrin, as well as its analogues, reacts with the amino acid compound of the latent fingerprint deposit (eccrine secretion) to give the dark purple product known as Ruhemann’s purple (RP). The chemical reactions involved are complex and, as a result, the development conditions need to be controlled if optimum results are to be obtained. The method is very effective for the development of fingerprints on porus surface such as paper. However, some paper surfaces (certain bank notes, for example) react strongly with the reagent and its use is limited in such cases.

Amino acids are stable compounds that, due to an affinity for cellulose, do not tend to migrate through a dry paper substrate with time. The amino acid content of the eccrine secretion also appears to remain relatively constant. As a result, very old latent prints can be developed with ninhydrin on documents stored under favourable conditions. The development of 30-year -old prints has been recorded.

A ninhydrin solution of approximately 0.5% weight per volume in concentration is required to develop fingerprints on paper. Many different formulations have been published in the literature, with the more popular formulations based on the use of the solvent 1,1,2-trichlorotrifluoroethane (sold under various names: “Arklone”, “Fluorisol”, “Freon 113”, etc). This solvent, a chlorofluorocarbon (CFC), is ideal for fingerprint work as it is non-toxic, non-flammable, and does not cause ink-running on documents. Unfortunately, production of this solvent has been phased out due to its harmful effects on the Earth’s ozone layer. Several alternatives have been suggested, including CFC substitutes and petroleum fractions such as ether and heptane.
The ninhydrin treatment is the same regardless of the formulation employed: the item is briefly immersed in the solution, removed and air dried. The development should be performed at room temperature, preferably in the dark, with a relative humidity of 50 to 80 per cent, over 24 to 48 hours. Heating the document to accelerate development is not generally recommended, as this favours reaction with the background, which is particularly damaging on some paper surfaces if the secondary metal salt treatment is to be employed. However, for reasons such as operational efficiency, ninhydrin development cupboards may be employed where the temperature and humidity are precisely controlled (80°C with 65 per cent relative humidity; development time approximately five minutes). The re-treatment of the document with the ninhydrin solution, the use of the enzyme trypsin to enhance fingerprint development, are strongly discouraged as these processes tend to increase background coloration and therefore reduce fingerprint contrast.

To obtain the best contrast, ninhydrin developed prints should be photographed under white light with a green-yellow filter (560-580nm) fitted to the camera.

Fingerprints developed with ninhydrin may be further treated with a metal salt solution which produces a colour change. Zinc salts produce an orange colour while cadmium salts produce a red colour. The colour change, which may help improve contrast on some surfaces, is due to the formation of a coordination complex between the Ruhemann’s purple (the product from the ninhydrin reaction) and the metal salt. These complexes, particularly those formed with zinc(II) and cadmium(II), show good luminescence properties. A considerable enhancement of ninhydrin developed fingerprints can be obtained by exploiting this luminescence.

The document, after ninhydrin treatment, is immersed in a solution of zinc or cadmium metal salt. A colour change occurs during the evaporation of the solvent, indicating that the reaction is complete. The sample is then inspected under a laser or suitably filtered light source. To favour the luminescence, the sample is cooled to liquid nitrogen temperature (-196°C); this being achieved by placing the sample in an insulated container such as a polystyrene foam tray and covering it with a thin layer of liquid nitrogen. The complex formed with zinc(II) has a maximum absorption in the blue region while the complex formed with cadmium(II) absorbs in the blue-green. The emission of light (luminescence) occurs in the green-yellow for the first complex and in the yellow-orange for the second. Filters which transmit these wavelengths are placed in front of the camera, or the eye, to record the resulting luminescence. The filters used are costly and specially made for this work (interference filters with precisely defined characteristics are typically employed). The treatment with a cadmium salt is preferred since the complex formed is more stable than the corresponding zinc complex, and the final result is less dependent on the conditions used for the ninhydrin development. The resulting fingerprint luminescence is often weak and therefore long exposure times are required for the photographic recording of prints, this may be from a few seconds to several minutes.

Other amino acid specific reagents are known, and some offer advantages over ninhydrin for the detection of fingerprints on paper. Compounds of this type include ninhydrin analogues such as benzoninhydrin and 5-methoxyninhydrin. These reagents develop latent fingerprints on paper in a manner similar to that of ninhydrin but, after metal salt treatment, offer a considerably stronger luminescence, even at ambient temperature. This is an advantage, particularly on surfaces that are luminescent at liquid nitrogen temperature such as manilla paper and yellow envelopes.

DFO is an amino acid sensitive reagent that gives a pale pinkish-purple reaction product (lighter in colour than that obtained with ninhydrin). The advantage of the reagent is that, without any secondary treatment, developed prints show a strong room temperature luminescence. In addition, results are obtained within a very short period of time (less than 30 minutes). The chemical reaction involved has not yet been clearly defined, but is probably similar to the reaction between ninhydrin and the amino acids present in fingerprint deposit.
The development process is simple and rapid: the document is dipped in a solution of DFO, dried, then heated in a laboratory oven at 100°C for 20 minutes. Alternatively, development may be achieved in a clothes press at 180°C for 10 seconds. Treated prints may be visible under white light as pale pinkish-purple ridges; however, the detection is much more sensitive in the luminescence mode with an excitation at 530nm (or, in certain cases, at 470nm or 550nm) and observations at 570-600 nm, depending on the surface. The quantum yield is excellent and the detection extremely sensitive at room temperature (cooling in liquid nitrogen does not improve the results). It must be noted that the luminescence is at its maximum immediately after the heat treatment and then decreases slightly with time due to the absorption of ambient humidity. The luminescence may be restored to its original intensity by reheating the prints. Operational trials on casework material have shown that DFO reveals approximately two to three times more latent fingerprints than ninhydrin. However, a high-powered light source with appropriate filtration must be available for the visualisation of developed prints. If this is not the case, the advantages of using DFO over ninhydrin may be lost as weak fingerprints will not be detected.

Fingerprints developed with DFO can be subsequently treated with ninhydrin, or one of its analogues. DFO is generally inefficient if used after ninhydrin treatment. Although DFO is generally more sensitive than ninhydrin, ninhydrin may sometimes give better results, particularly on luminescence surfaces where DFO loses its advantage. The application of DFO is also problematic on heat sensitive articles, eg, thermal paper, window envelopes, labels on polyethylene objects etc. If lower temperatures are used for the development process, significantly longer development times are required.

Osmium tetroxide is a volatile oxidant that reacts with the double bonds present in the unsaturated organic components of the fingerprint deposit (sebaceous gland secretion) to give a blank product. The treatment is by simple exposition of the object to the vapour given off by the crystals of the reagent in an enclosed glass container. Development times may be from one to 12 hours and dark grey-black fingerprint images are formed. Good results have been achieved on both porous and non-porous surfaces, but the technique is particularly useful on problem surfaces such as banknote paper which normally reacts with ninhydrin.

Unfortunately, osmium tetroxide is extremely toxic and must be used with great care and only in specialised laboratories. Exposure to the vapour may be fatal if the compound is inhaled, swallowed or absorbed through the skin. In addition, the reagent is prohibitively expensive.
Ruthenium tetroxide has also been proposed as a sensitive latent print fuming technique. The reaction with fatty substances in the fingerprint deposit is the same as for osmium tetroxide. However, unlike osmium tetroxide, ruthenium tetroxide does not become gaseous at or near room temperature. Previous fuming methods called for a potentially hazardous application of heat in order to volatilise the crystals. Ruthenium tetroxide decomposes explosively at 108°C. More recently, Japanese workers have proposed a safe procedure for the generation of ruthenium tetroxide vapour. Equal volumes of 0.1% ruthenium (III) chloride hydrate solution and 11.3 per cent ceric ammonium nitrate solution are mixed together at room temperature in a closed container, ruthenium tetroxide fumes are generated chemically by the oxidation of ruthenium chloride. Any latent prints that come into contact with these fumes are developed as dark grey images after about 10 to 20 minutes, depending on the surface. Longer development times may be required for surfaces such as wood or aluminium. The developed prints have a similar aspect to those obtained by osmium tetroxide treatment. RTX works well when sebaceous material is present in the latent fingerprint, but is generally ineffective on eccrine secretions.

Small objects may be enclosed in a glass or plastic container and treated with the RTX fumes generated by the mixing of the two solutions. Relatively large surfaces, such as doors, may be fumed using several millilitres of each solution added to a plastic wash bottle. By gently squeezing the wash bottle, the ruthenium tetroxide fumes that are produced may be directed from the nozzle towards the surface under investigation. Using either of these methods, prints can be developed on both porous and non-porous surfaces. Good results with the RTX technique have been reported on a variety of surfaces including paper, plastic and human skin.

As urea is a major component of eccrine sweat, dimethylaminocinnamaldehyde (DMAC) has been considered as a fingerprint development technique on porous surfaces such as paper. Documents may be treated with a solution of DMAC; the urea in the fingerprint deposit reacts rapidly with the DMAC to produce a dark red, unstable product that must be photographed immediately. In some past research it has been described that the scope and limitations of the regent, tests concluded that its main advantage was for the quick development of relatively fresh fingermarks (up to 72 hours old). Urea migrates rapidly through paper and blurred images are generally the rule for fingerprints older than a few days developed by this method.

More recently, it was reported that DMAC used as a fuming agent provides good ridge detail visualisation on a wide selection of substrates, and can be included in routine sequential examination procedures. Items are treated by passing them slowly through the fumes produced by heating DMAC to 175°C. Samples are then left for at least 24 hours at room temperature before visualising developed prints in the luminescence mode with excitation in the range 490 – 530 nm and observation with a filter transmitting above 550 nm. Fingermarks up to three months old have been developed on paper. In addition, DFO or ninhydrin will function after DMAC treatment. The technique is less effective on non-porous surfaces such as plastic, and it interferes with cyanoacrylate development. DMAC fuming shows particular promise as a reagent for visualising fingermarks on thermal paper, a substrate that has hitherto been problematic. The chemistry of the DMAC vapour reaction is not clearly understood, but it appears that, in this case, the reaction does not rely on the urea component of the latent fingerprint deposit.