Latent Print Development - National Criminal Justice Reference ...

C H A P T E R 7 Latent Print Development
compromised by surface type or impracticality, such as
dark or patterned carpets (Sears et al., 2005, p 741).
The major concern with the catalytic tests for blood is that
they can produce false-positive results in the presence of
chemical oxidants and catalysts; salts of heavy metals such
as copper, nickel, and iron; and plant peroxidases such as
those found in horseradish, citrus fruits, and numerous root
vegetables (Lee and Pagliaro, 2000, p 1334). A two-stage
test can obviate this. The reduced colorless dye is applied
initially and if no color change is observed, then the hydrogen
peroxide is added. A color change at this point is more
likely to indicate the presence of blood.
It is generally accepted that a negative result with a catalytic
test proves the absence of blood; however, strong
reducing agents, such as ascorbic acid, may inhibit such
tests (Eckert and James, 1989, p 121).
7.12.4 Tests for Protein and Its
Breakdown Products
There are two types of techniques for proteins—those
that stain proteinaceous material and those that react
with amines. Blood contains more protein than any other
material, so these techniques are inherently more sensitive
than those for heme, although they are not at all specific
for blood.
The most effective protein dyes for the enhancement of
fingerprints in blood are a group known as acid dyes. They
are often characterized by the presence of one or more
- + sulphonate (-SO ) groups, usually the sodium (Na ) salt.
3
These groups function in two ways: first, they provide for
solubility in water or alcohol, the favored major solvents
from which to apply these dyes; and, second, they assist
the reaction by virtue of their negative charge (anionic). If
acidic conditions are used (acetic acid being the favored option),
the blood protein molecules acquire a positive charge
(cationic) and this attracts the acid dye anions. Also, hydrogen
bonding and other physical forces, such as van der
Waals, may play a part in the affinity of acid dyes to protein
molecules (Christie et al., 2000, pp 19–20). The presence of
a short-chain alcohol in the dyeing solution helps to prevent
the blood from diffusing during the dyeing stage (Sears and
Prizeman, 2000, p 470). Ethanol is preferred because this
offers lower toxicity and flammability than methanol. The
use of water as the major solvent gives the solution a flash
point of around 30 °C, enabling this formulation (containing
water, ethanol, and acetic acid) to be used at crime scenes
with a few simple precautions (Kent, 2004).
7–40
If acid dye formulations are applied directly to fingerprints
in blood without a fixing stage, the blood will solubilize
and ridges will diffuse or be completely washed away. A
number of different fixing agents have been used, but the
most effective are 5-sulphosalicylic acid and methanol.
Which one is used depends on the major solvent used in
the dyeing process: if water is the main solvent, then a solution
of 5-sulphosalicylic acid is most effective, whereas if
the main dyeing solvent is methanol, then methanol is the
best fixing agent (Sears and Prizeman, 2000, p 470). These
fixing agents act in different ways; the 5-sulphosalicylic
acid precipitates basic proteins, and methanol dehydrates
the blood.
The use of solutions based on methanol has waned for
a number of reasons, including its toxicity, flammability,
and tendency to cause damage to surfaces (e.g., paints,
varnishes, and some plastics), which has a negative effect
on fingerprint development. This fixing stage gives the
protein dyes another advantage over the presumptive tests
for blood: as well as being a more sensitive test, it often
produces more sharply defined fingerprint ridges and the
detail is clearer.
A washing stage is required post-dyeing. On nonporous
surfaces, this just removes excess dye; however, on
porous surfaces, this also acts as a destainer, removing
dye that has been absorbed by the background surface.
The wash solution has to be carefully constructed so that
it solubilizes the dye, does not diffuse or wash away the
dyed fingerprint, and retains the intensity of color of the
dye in the fingerprint. For this reason, the same solvent
mix as that used for the dyeing process, or some small
variation of it, is generally most effective (Sears and Prizeman,
2000, p 470).
Ninhydrin and DFO react with amines and are the two
most widely used techniques to develop latent fingerprints
on porous surfaces (Figure 7–22). They are also very effective
for the enhancement of blood (Sears et al., 2005,
p 741). They both react with amino acids similarly to form
products that contain two deoxygenated molecules of
the starting product, bridged by a nitrogen atom that is
donated from the amine (McCaldin, 1960, p 39; Wilkinson,
2000a, p 87).
Although the reaction mechanisms and products have
similarities, the method of their visualization is entirely different.
Ninhydrin, under the right conditions, produces an
intensely colored product (Ruhemann’s purple), and DFO