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C H A P T E R 7 Latent Print Development 7.7 5-Methylthioninhydrin (5-MTN) 5-Methylthioninhydrin (5-MTN) was first prepared and applied as a fingerprint reagent in 1990 as part of a U.S. Secret Service project (Cantu et al., 1993, pp 44–46). This analogue reacts with amino acids in a manner identical to ninhydrin because the reactive, chromogenic core of the molecule is not changed by the addition of the sulfur group (Figure 7–9) (Elber et al., 2000, pp 757–760). As a result, 5-MTN-developed fingerprints are a shade of purple similar to ninhydrin-developed fingerprints. Development of 5-MTN-treated fingerprints requires heat and humidity, much the same as ninhydrin development. This can be delivered in the same manner described previously for ninhydrin or by microwaving the treated exhibit for 2–3 minutes alongside a container of water (Almog et al., 1992, pp 688–694). Care must be taken to avoid overheating the sample because significant background development may occur. The resulting fingerprint should appear deep purple in color, similar to a ninhydrin-developed fingerprint. On treatment with a zinc salt, the 5-MTN-developed fingerprint changes color from purple to pink (Almog et al., 1992, pp 688–694). Accompanying this change is a strong fluorescence at room temperature when excited by light at 520 nm and viewed through a 590 nm filter (Cantu et al., 1993, pp 44–66; Almog et al., 1992, pp 688–694), with an intensity that is comparable to that of DFO. This is an obvious advantage over the continued use of ninhydrin (Cantu et al., 1993, pp 44–66). A recent study confirmed that 7–22 FIGURE 7–9 Structure of 5-methylthioninhydrin and its reaction product with amino acids. CH 3 5-MTN could outperform ninhydrin but produced poorer fluorescent results than DFO or 1,2-indanedione (Wallace- Kunkel et al., 2006, pp 4–13). The fluorescence becomes even more intense if the exhibit is cooled to liquid nitrogen temperatures, but this step is not necessary (Almog et al., 1992, pp 688–694). 5-MTN can be synthesized in small-scale operations in the forensic laboratory following methods reported in the literature (Heffner and Joullié, 1991, pp 2231–2256; Della et al., 1999, pp 2119–2123). Alternatively, it can be sourced from commercial forensic suppliers. However, some suppliers provide the ethanolic hemiketal of 5-MTN, which dissolves more readily but may require some alteration of the given formulation (Section 7.14) to ensure the appropriate concentration of 5-MTN in the solution (BVDA, 2010). 7.8 Modifications for Use on Chemically Treated Papers 7.8.1 Chemically Treated Papers Chemically treated paper is a class that encompasses thermal paper and carbonless specialty papers (Stimac, 2003a, pp 185–197). These papers cannot be treated with the conventional amino acid reagent formulations described previously because the polar solvents react unfavorably with the chemical treatments applied to the paper during manufacture. This undesired interaction frequently causes the surface of the paper to blacken, obliterating the documentary evidence the paper contained (Stimac, S CH 3 S --- - - O O - - O 5-MTN N H + - - - - - - - O O O O + amino acid, 5-MTN S H 3 C
2003a, pp 185–197). To address this limitation, several solvent-free or low-polarity formulations have been devised for the treatment of these difficult substrates. 7.8.2 Application of DFO to Chemically Treated Paper DFO may be applied to chemically treated paper by a process known as “DFO-Dry” (Bratton and Juhala, 1995, pp 169–172). This technique does not require the application of a solvent to the exhibit under examination. Instead, filter paper is impregnated with a solution of 1 g DFO in 200 mL methanol, 200 mL ethyl acetate, and 40 mL acetic acid. The dried filter paper is applied to the exhibit, a towel is placed on top, and a steam iron filled with 5% acetic acid solution is applied for one minute. This transfers DFO onto the exhibit and provides the heat for development. This technique results in a less prominent color change but equal fluorescence to solvent-based methods (Bratton and Juhala, 1995, pp 169–172). 7.8.3 Ninhydrin Techniques 7.8.3.1 “Nin-Dry”. This method was described in 1996 (McMahon, 1996, pp 4–5) and is similar to the previously described “DFO-Dry” process. Blotter or filter paper is soaked in a solution of 30–50 g ninhydrin dissolved in 1.5 L acetone and allowed to dry. An exhibit is placed between two sheets of the impregnated paper and then sealed into a plastic bag for 3 days to 1 week. This technique develops high-contrast fingerprints while preserving the integrity and appearance of the document and is applicable to any fragile paper types, including chemically treated papers. 7.8.3.2 Ninhydrin Fuming. The method proposed by Schwarz and Frerichs (2002, pp 1274–1277), and described above, can be applied to chemically treated papers with no loss of document detail. 7.8.3.3 Nonpolar Solution. A ninhydrin solution can be prepared in a mixture of the nonpolar solvents HFE 71IPA and HFE 7100. The exhibit is immersed in the working solution and allowed to develop in dark, humid conditions for 2–3 days, avoiding high temperatures (Stimac, 2003a, pp 185–197). 7.8.4 Indanedione Formulation Indanedione is sufficiently soluble in nonpolar solvents that it can be effectively applied to thermal paper without causing any blackening (Stimac, 2003b, pp 265–271). The exhibit is immersed in the prepared solution and allowed to develop for at least 1 day in dark, cool conditions. Fluorescence is induced as described previously. 7.8.5 2-Isononylninhydrin (INON) 2-Isononylninhydrin, also known as INON, or commercially as ThermaNin, is a derivative of ninhydrin with greatly increased solubility in nonpolar solvents (Takatsu et al., 1991; Joullié, 2000). This compound, which is a product of the reaction between 3,5,5-trimethyl-1-hexanol and ninhydrin (Almog, 2001, pp 177–209; Hansen and Joullié, 2005, pp 408–417; Takatsu et al., 1992), has the chemical structure shown in Figure 7–10. Solutions of this reagent do not have a long shelf life, so working solutions should be prepared as needed (BVDA, 2010). The 2-isononylninhydrin solution is applied to the chemically treated paper by immersing the exhibit in the solution in an aluminum or plastic tray. The exhibit is allowed to dry and develop in dark, humid conditions for 24–48 hours. Under these conditions, the ninhydrin hemiketal reacts with water absorbed by the paper to form ninhydrin and 3,5,5-trimethyl-1-hexanol. The freed ninhydrin reacts slowly with the residues in the fingerprint to develop a fingerprint that is somewhat less intensely colored than a traditionally ninhydrin-developed print. This may be due to the relatively lower concentration of ninhydrin present after the hydrolysis reaction occurs (Al Mandhri and Khanmy-Vital, 2005). 7.9 Cyanoacrylate Fuming 7.9.1 Background Latent Print Development C H A P T E R 7 The liquid commercial adhesive, super glue, was inadvertently developed in the 1950s by researchers who were trying to develop an acrylic polymer for the aircraft industry. Besides its use as a glue, CA adhesive also found use as a field dressing in Vietnam in the 1960s, although it never received FDA approval for this use. In the late 1970s, researchers in Japan and the United Kingdom almost simultaneously discovered the latent fingerprint development capabilities of the fumes of the liquid adhesive. Shortly thereafter, latent print examiners from the U.S. Army Criminal Investigation Laboratory in Japan and the Bureau of Alcohol, Tobacco, and Firearms introduced this technique to North America. Once CA fuming proved practical, with the introduction of methods to make the technique faster 7–23
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