Latent Print Development - National Criminal Justice Reference ...

O
O
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of drastically increasing the number of identifiable latent
fingerprints developed by the ninhydrin process. With the
current ubiquity of forensic light sources, both absorption
bands of Ruhemann’s purple can be exploited to produce
high-contrast fingerprints (Champod et al., 2004, p 117).
N
OH 2
7.4.2.5 Post-Treatment with Metal Salts. The reaction
between Ruhemann’s purple and metal salts such as zinc,
cadmium, cobalt, and copper was used in a biochemical
context to preserve ninhydrin spots on chromatograms
(Kawerau and Wieland, 1951, pp 77–78). Formation of a
metal-salt complex alters the color of Ruhemann’s purple
from deep violet to red or orange, depending upon the salt
used (Stoilovic et al., 1986, pp 432–445). The lighter hue
may provide a greater contrast against a dark-colored background,
especially when observed at 490–510 nm, where
the metal–Ruhemann’s purple complex has an absorption
maximum (Stoilovic et al., 1986, pp 432–445).
M
It has been reported that viewing zinc-complexed ninhydrintreated
fingerprints under an argon ion laser could induce
fluorescence of even weakly developed prints (Herod and
Menzel, 1982b, pp 513–518). This discovery had a profound
impact on fingerprint development because fluorescent
reagents are more sensitive than chromogenic ones and
can be viewed more clearly against colored backgrounds
(Champod et al., 2004, p 120). Subsequent studies revealed
that intense laser light was not necessary if the zinc-treated
samples were cooled to the temperature of liquid nitrogen
(-196 °C or 77 K); the fluorescence could be observed under
a xenon arc lamp. This technique required submersion of
the document in liquid nitrogen, a glass plate being placed
between the sample and the light source and camera, and a
heat source to prevent condensation on the glass (Kobus et
al., 1983, pp 161–170). Later research showed that cadmium
complexes provided an improved luminescence under these
conditions (Stoilovic et al., 1986, pp 432–445).
X
O
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O
H 2 O
M: Cadmium or zinc
X: Anion from the metal salt
FIGURE 7–4
Structure of Ruhemann’s
purple–metal salt complex.
Structural studies of the Ruhemann’s purple–metal salt
complexes have identified the structure in Figure 7–4
(Lennard et al., 1987, pp 597–605; Davies et al., 1995a, pp
565–569; 1995b, pp 1802–1805).
7.4.3 Application
Latent Print Development C H A P T E R 7
7.4.3.1 Ninhydrin Formulations. Several ninhydrin formulations
have been reported in the literature (Crown, 1969,
pp 258–264; Odén and van Hofsten, 1954, pp 449–450;
Speaks, 1964, pp 11–13, 23; Champod et al., 2004, pp
117–120; Almog, 2001, pp 177–209; Everse and Menzel,
1986, pp 446–454; Clay, 1981, pp 12–13). Ninhydrin solutions
are typically prepared in two steps: first, a stock solution
is prepared that has a high proportion of polar solvent
to facilitate the stability of the mixture; second, a portion
of the stock solution is diluted with a nonpolar carrier
solvent to produce a reagent suitable for application to
evidential items.
Application of ninhydrin working solutions can be performed
by dipping, spraying, or brushing (Odén and van
Hofsten, 1954, pp 449–450; Speaks, 1964, pp 11–13, 23),
with the dipping method preferred in most instances. The
item to be examined is briefly submerged in the working
solution and allowed to air-dry to evaporate the solvent
(Champod et al., 2004, pp 116–117).
Following treatment with ninhydrin solution, development
should ideally proceed at room temperature, in a dark and
humid environment (50–80% humidity), for a period of 1–2
days (Champod et al., 2004, pp 116–117). If ambient humidity
is low, development in a specialized, humidity-controlled
fingerprint development chamber may be necessary (Almog,
2001, pp 177–209). The development may be accelerated
by the application of steam or heat, but this may result in a
greater degree of background development, reducing the
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