Advances in Fingerprint Technology.pdf

Figure 5.9 Formation of Ruhemann’s purple-metal complex. (From Lennard,
C.J., Margot, P.A., Sterns, A., and Warrener, R.N., J. Forensic Sci., 32, 597, 1987.
With permission.)
coincidence, and that it was not the outcome of any theoretical design. They
proposed a modification of the ninhydrin molecule in an attempt to enhance
the color and fluorescence of the corresponding Ruhemann’s purple complexes.
It was assumed that compounds analogous to ninhydrin, containing
the same active moiety — the cyclic vicinal triketone but with different
groupings on the aromatic ring — might also react with amino acids to give
colored products, and some of them could become new fingerprint reagents
with improved properties. Expansion of the conjugated system and introduction
of electron-donating or electron-withdrawing substituents can, in
principal, modify the color by increasing the molar absorption coefficient
(epsilon), or by shifting the absorption maximum towards longer or shorter
wavelengths (red or blue shift). They named this group of compounds “ninhydrin
analogues.” Their initial experiments included the preparation of
three ninhydrin analogues, all of which gave colored products with amino
acids. One analogue in particular, benzo[f]ninhydrin (Figure 5.10), showed
great promise. It developed latent fingerprints as dark green impressions,
with a sensitivity similar to that of ninhydrin. 99 * The ability to develop a
print of a specific color and then treat the developed print with zinc chloride
to afford a complex that exhibits fluorescence or another color would aid in
print visualization on a variety of backgrounds. In 1985, Menzel and Almog
reported that the zinc complex of benzo[f]ninhydrin-developed prints
showed luminescence properties that were superior to those obtained from
the zinc complex of prints developed with ninhydrin. 92 This result stimulated
interest in the preparation of ninhydrin analogues, which would serve as
more sensitive fingerprint reagents. In 1986, Lennard et al. synthesized and
* Benzo[f]ninhydrin was prepared for the first time in 1957 by Meier and Lotter, who also
reported the formation of a dark green color upon reaction with amino acids. They did not
characterize the color spectroscopically and this reaction had no practical use. 100