Development of Latent Fingerprints
Latent fingerprints can be made by sweat and oil on skin surface
·
During the course of crime the culprit’s hands
get exposed to oil, dirt and grease from touching the tools skin or hair also it’s a common
tendency of a human
being to get nervous while committing a wrong deed
which leads to excessive perspiration.
·
The contents of the
perspiration (Water, Salts and Amino Acids) along with the external
contamination happen to leave a thin film of ridge patterns upon any surface
the
perpetuator’s hands come in contact
with.
·
These patterns are invisible to unaided eyes and if visible may be unrecognizable and need development.
Development of Latent Fingerprints
There are various Physical
and Chemical methods
employed to develop
latent fingerprints.
·
Physical Methods
rely on the ability of reagents used to adhere
to the materials of prints
without fusing into them.
·
Chemical Methods
rely on the ability of reagents to react with the contents
of the fingerprints (Amino Acids, Salt, External Contaminants and Water)
Physical Methods
1. Powders
·
This method of developing latent fingerprints is based on the simple physical phenomenon of adhering. The powder substances when applied to the
latent fingerprints in a proper
and systematic manner ,adhere to the secretions left on the object
bearing latent fingerprints.
·
One of the oldest and most common
methods of latent
print detection (1891).
·
They have affinity
for moisture and oily components in a print causes adhesion
·
The most commonly
used powders are black and grey powders.
·
Black powders are used for developing prints
on white or light colored
surfaces and grey powders are used on dark colored
surfaces
·
A magnetic-sensitive powder
can be spread over a surface with a magnet
in the form of a Magna
Brush
·
A Magna Brush does not have any bristles to come in contact with the surface,
so there is less chance that
the print will be destroyed or damaged
·
The magnetic-sensitive powder comes in black and
gray and is especially useful on such items as finished
leather and rough
plastics, where the minute texture of the surface tends to hold particles of
ordinary powder.
·
Can be used on both porous and non-porous surfaces
3.
Fluroscent Powder
·
When the latent
impressions are to be developed on a multicolored surface,
the use of powder of one
color does not prove useful because of lack of contrast of the developed print
against multicolored background. Such type of impressions are developed with
fluorescent powders which can be applied like other powders.
· The prints
developed with fluorescent powders glow under
ultraviolet light and they
can be photographed on an ordinary film.
· These powders
are available in a variety
of colors like green, red and yellow.
4.
Fuming Methods
·
Iodine is a solid crystal
that, when heated,
is transformed into a vapor
without passing through a liquid phase; such a transformation is called
sublimation
·
Most often, the suspect material is placed in an
enclosed cabinet along with iodine crystals as the crystals are heated, the resultant vapors fill the chamber
and combine with constituents
of the latent print to make it visible.
· The vapors
are absorbed by the
sweat deposition of the
latent imprint thereby revealing the ridge pattern.
·
The mechanism
of the reaction involved physical absorption of iodine on the fatty acid content. The iodine-fatty acid interaction imparts a yellowish
brown color to the
ridge pattern.
· Unfortunately, iodine
prints are not permanent and begin
to fade once the fuming
process is stopped. Therefore, the examiner must photograph the prints
immediately on development in order to retain a permanent record
Various methodology is adapted for iodine fuming
like :
v
Fuming Gun Method
Ø
Made of glass/plastic; contains iodine crystals
and dehydrating agent (e.g., calcium
chloride).
Ø Air is blown into the mouthpiece to push vapors over latent prints.
• Gun is held ~2 cm from
surface.
Ø
Developed prints are photographed immediately.
v
Fuming Cabinet Method
Ø
Iodine crystals are placed in a chin dish (porcelain dish) and heated
(~50°C).
Ø
The object is suspended inside
a sealed cabinet.
Ø Vapors develop
the print; when contrast is sufficient, the object is removed and photographed.
v
Powder Dusting Method
Ø
Iodine is powdered
and spread with a camel hair brush.
Ø
Excess is blown
off.
Ø
Simple and low-cost, but hazardous due to direct
exposure to iodine
fumes.
Fingerprints
developed by iodine are not permanent in nature. These tend to fade out on
standing. In presence of air the fading of prints is accelerated.For this reason the iodine developed prints have to be
photographed immediately
Iodine is known to react with starch giving
a stable, deep blue complex.
Thus, posttreatment of iodine-developed fingerprints with
starch solution gives long-lasting blue impressions.
Chemical Methods
NINHYDRIN
·
The development of latent prints
with ninhydrin depends
on its chemical reaction
to form a purple-blue color with amino acids present in trace amounts
in perspiration.
·
Ninhydrin (triketohydrindene hydrate)
is commonly sprayed
onto the porous surface
from an aerosol can
·
A solution is prepared by mixing the ninhydrin powder
with a suitable solvent, such as
acetone or ethyl alcohol; a 0.6 percent solution appears to be effective for
most applications.
·
Generally, prints begin to appear
within an hour or
two after ninhydrin application; however, weaker prints may be visualized after 24
to 48 hours.
·
The development can be hastened
if the treated specimen
is heated in an oven or on a hot
plate at a temperature of 80–100°C.
The ninhydrin method has developed latent prints
on papers as old as 15 years
