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C H A P T E R 7 Latent Print Development High-intensity light sources capable of emitting wavelengths between 510 and 570 nm must be used to excite fluorescence from blood reacted with DFO. The fluorescence emitted is between 550 and 650 nm. Benefit may also be gained by using shorter wavelengths, between 350 and 450 nm, to excite background fluorescence after ninhydrin treatment. The three recommended acid dyes, acid black 1 (CI 20470), acid violet 17 (CI 42650), and acid yellow 7 (CI 56205), should all be applied to blood fixed for at least 5 minutes with a solution of 5-sulphosalicylic acid. Dyeing of fixed blood is most effective if the area of interest is immersed in the dyeing solution for at least 3 minutes for acid black 1 and acid violet 17 and for at least 5 minutes in the case of acid yellow 7. Areas heavily contaminated with blood require longer dyeing times. If it is not possible to immerse the bloodied fingerprints, then the dyeing solution should be applied above the area of interest and allowed to flow down over it, keeping the area damp for the specified time. A well may be constructed around the area of interest on horizontal surfaces, which may be flooded and drained as appropriate. Areas of interest will then need to be washed or destained to remove excess dye. The most effective solution for doing this is the same solvent composition as the dye solution, washing as required to remove excess dye or destain the background. High-intensity light sources capable of emitting wavelengths between 420 and 485 nm must be used to excite fluorescence from blood dyed with acid yellow 7. The fluorescence emitted is between 480 and 550 nm. The use of shorter wavelengths between 350 and 450 nm, to excite background fluorescence after acid black 1 or acid violet 17 treatment, may be beneficial. Work carried out by the U.K. Home Office has demonstrated that positive DNA identification may be made after fluorescence examination and any single chemical treatment, provided that simple guidelines are followed. If more than one fingerprint development technique is used in sequence, then the chances of successfully carrying out DNA identification are much reduced (Kent, 2004). The U.K. work has shown that the most effective formulation for the acid dyes is as follows (Sears et al., 2005, p 741): 7–42 Fixing Solution—46 g 5-sulphosalicylic acid dehydrate dissolved in 1 L water. Staining Solution—1 g acid dye dissolved in 700 mL distilled water, 250 mL ethanol, and 50 mL acetic acid. Washing Solution—700 mL water, 250 mL ethanol, and 50 mL acetic acid. The staining and washing solutions are flammable. Safety precautions must be taken if these solutions are used outside a fume cupboard with ambient temperatures above 28 °C (Kent, 2004). 7.13 Aqueous Techniques This section covers four commonly used aqueous metal deposition methods: those involving silver nitrate reagents, silver physical developers, multimetal deposition processes, and gun blueing reagents. Each of these methods involves reagents with metal salts dissolved in an aqueous carrier (or an alcohol, as in the case of some silver nitrate reagents). These reagents reveal water-resistant latent prints such as sebaceous prints (except for the silver nitrate reagents used on porous surfaces that target salt). Here, the metal ions are reduced to metal particles on the latent print residue (except for the case of latent prints on metal, where the print residue resists the deposition). 7.13.1 Silver Nitrate Reagents 7.13.1.1 History and Background. One of the first reagents used for developing latent prints on porous surfaces was a 1–3% aqueous solution of silver nitrate, AgNO . 3 It was used as early as 1891 for this purpose (Forgeot, 1891; Rhodes, 1940, p 10). Most formulations now include an alcohol to hasten drying and to increase the wetness (reduce the surface tension) (Lee and Gaensslen, 2001, pp 105–175). The silver ions in silver nitrate react with the chloride ions in salt (sodium chloride, NaCl) contained in the latent print residue to form silver chloride (AgCl), a highly insoluble salt (Ksp = 1.8x10-10 ) (Dean, 1985). Ag + + Cl – AgCl → K formation = 1/K dissociation = 1/K sp = 5.6 x 10 7 There are at least two reasons the silver nitrate treatment works well on porous surfaces. One is that the precipitation process is much faster than the dissolution process; that is, the reaction to form the insoluble AgCl is quicker
than the ability of the aqueous carrier to dissolve away the soluble NaCl salt. The second reason is that the insoluble AgCl gets trapped within the structure or “micro-roughness” (Kerr et al., 1981, pp 209–214) of the porous surface; that is, the fresh latent print residue is in an aqueous or semiaqueous form that soaks into the porous surface, carrying its constituents with it. An ethanol-based 3% (w/v) silver nitrate reagent (90% ethanol and 10% water) develops prints on water-repelling surfaces such as waxed paper, cardboard with a wax finish, and Styrofoam (Trozzi et al., 2000). Here, the ethanol is used to reduce the dissolution of the NaCl in the fingerprint residue, to better wet the surface (because these surfaces are usually water-repellent), and to give faster evaporation. As expected, because of the low porosity of such surfaces, developed prints on these surfaces are more fragile than those on porous surfaces like paper and wood. Under ordinary room light, the silver chloride gradually converts by photo-reduction to elemental silver; however, this is hastened with UV radiation. The most efficient development occurs with short-wavelength UV radiation (254 nm); however, the safer, long-wavelength UV radiation (366 nm) also develops prints, but less efficiently (Goode and Morris, 1983). AgCl + hν Ag + ½Cl 2 The elemental silver formed is colored dark brown to black (not a silver color). The reason for this is that the silver deposits as an aggregate of tiny (colloidal-size) silver particles, which makes for a highly porous surface that traps much of the light that strikes it. The formation of dark, light-trapping silver happens because the silver ions are reduced very quickly. 7.13.1.2 Application (Porous and Water-Repelling Surfaces). The silver nitrate reagent is usually applied to specimens by dipping them in the solution or by spraying the solution on the specimens. The FBI (Trozzi et al., 2000, pp 38–39) recommends the 3% AgNO water-based formula- 3 tion for porous surfaces and the 3% AgNO ethanol-based 3 formulation for water-repellent surfaces. Champod et al. (2004, pp 153–154) recommend the 2% AgNO methanol-based reagent for porous surfaces. After 3 drying, the specimens are exposed to a high-intensity light source, UV light, or sunlight to develop the prints. As soon as the prints develop, they are photographed and the specimens are stored in the dark. Over time, the background Latent Print Development C H A P T E R 7 darkens because of the gradual reduction of any residual silver nitrate in the specimens (this reduction is accelerated if exposed to light). Rinsing the specimens after development and then drying them in the dark does little to slow down the background development. Goode and Morris (1983) reported in 1983 that immersing specimens in disodium ethylenediaminetetracetic acid (Na EDTA) complexes excess silver ions, which are then 2 easily rinsed away with water. Their modified silver nitrate (MSN) procedure uses a 1% aqueous silver nitrate solution that also contains 5% Na EDTA and 3% K CO . The MSN 2 2 3 procedure involves (1) treating the specimens with this modified reagent for just enough time to wet the surface, (2) transferring them to a 1% (w/v) Na EDTA solution and 2 leaving them in for 1 minute, (3) removing and washing thoroughly with distilled water, and finally (4) placing this in a 5% thiourea solution containing 1% KOH for about 30 seconds to 2 minutes. The first step creates the silver chloride from the chloride ions in the latent print, and the last step converts this to black silver sulfide. Later in 1998, Price and Stow (1998, pp 107–110) recommended dipping the specimens in a “stopping solution” consisting of an aqueous solution of 40% methanol, 20% acetic acid, and 2% glycerol to suppress the further development of the background. 7.13.1.3 Enhancement. According to Lennard and Margot (1988, pp 197–210), weakly developed prints could be enhanced by treating the specimens with a diluted silver physical developer solution. The dilution factor is 1:10. Goode and Morris (1983) discuss a radioactive enhancing method that converts a silver print to a radioactive, β-emitting silver sulfide print, which is then imaged using radiographic film (this image-recording process is sometimes called autoradiography or β-radiography). If the original silver nitrate treatment did not significantly stain the background with silver, then this method will bring out only the developed prints with little or no interfering background. The process, described by Goode and Morris (1983) and reviewed by Cantu (2001, pp 29–64), involves converting the silver in the silver image to silver bromide (AgBr), using brominating (bleaching) methods, and then treating this with either sodium sulfide or thiourea (where the sulfur is radioactive 35 35 S) to convert AgBr to Ag S. The 2 process is called radioactive toning. If the MSN procedure is used, which yields a silver sulfide print, then radioactive 35 thiourea is used to form Ag S. 2 7–43
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