Television shows and movies often feature people wiping their fingerprints off surfaces
like tabletops, steering wheels, doorknobs and guns—basically anything that they may
have touched, to avoid being identified by the investigators.
However, is wiping a surface enough to ensure that fingerprints can’t be pulled?
Humans develop fingerprints long before we are born and they stay with us throughout
our lifetime.
Those grooves and ridges on our fingertips not only improve our touch perception and
ability to grip things, but also provide us with a completely distinctive identification
mark.
Every single person has a unique set of fingerprints.
Not even identical twins with the same DNA code share the same fingerprints!
This peculiar property of friction ridge impression, commonly known as fingerprints, has been a
part of crime-solving for over 100 years.
So, how do we end up leaving fingerprints on things we touch?
Even if your fingertips are not soiled by blood, dirt, or Cheeto dust, they will still
leave behind chemical imprints on things you touch because the sebaceous glands under our
skin secrete a cocktail of sweat, oils, and amino acids.
This phrase is known as the “Exchange Principle” and forms one of the basic principles of forensic
science.
It was popularized by Edmund Locard, one of the pioneers in the field of forensics, who
showed that every time two objects came in contact with each other, there was an exchange
of material between them.
At this point, you might be wondering how fingerprints are lifted from a crime scene…
Well, Crime Scene Investigators (CSIs) look for three different types of fingerprints:
Patent, Plastic and Latent.
Patent Fingerprints are visible to the naked eye, for instance, if a finger was soaked
in blood.
Plastic Fingerprints are those left on soft surfaces like soft wax or wet paint.
And Latent Fingerprints are the invisible impressions created by skin secretions on
various surfaces.
Investigators usually begin with patent fingerprints because they are usually visible under normal
light and UV blacklight.
Then they look for plastic fingerprints left on soft surfaces; these can be lifted off
by pouring a liquid into them, such as plaster, molten alloys, or silicon rubber, which can
later harden and create an impression of the print.
Finally, there comes the most challenging part… the hunt for lifting latent fingerprints.
Since these are invisible or barely visible, CSI’s must use chemistry to enhance such
prints, if any exist.
The most common technique and the one many of us have likely seen on TV shows is the
use of fingerprinting powder.
CSIs use their trusty brush and a palette of fingerprint powders to visualize prints
from both smooth and textured surfaces.
The fingerprint dusting powders stick to oils and moisture left by the ridges on our fingertips
and create a visible print that can then be lifted off using transparent tape.
The powders they use vary from surface to surface.
For example, aluminum powders are preferred for glass surfaces and brass, whereas granulated
jet black powder is used on silvery surfaces to create better contrast.
Fluorescent powders are used to enhance prints on multicolored surfaces.
For porous surfaces like paper or walls, fingerprints are detected by spraying a solution called
ninhydrin on the surface where a print may be found.
This ninhydrin solution reacts with amino acids present in sweat and turns purple due
to the formation of Ruhemman’s purple.
In some cases, CSIs will also use cyanoacrylate or super glue fumes to enhance a latent fingerprint
from a rough surface.
The cyanoacrylate molecules react with skin residue and polymerize to create a 3D matrix
of the fingerprint.
If the CSIs still have no luck, they can try the most recent and advanced method—vacuum
metal deposition, or VMD.
This method deposits a thin layer of gold and zinc on the site in question.
Part of the gold layer diffuses with the skin oils, while the rest remains free.
When the zinc is coated, it attaches itself to the undiffused free gold, thus creating
a negative contrast of the fingerprint.
Though quite expensive, this is one of the fastest methods for developing a fingerprint
on extremely tricky surfaces like wood or fabric.
Now, let’s circle back to the common movie trope of an actor wiping their fingerprints
off a gun.
It seems like a simple wipe with a cloth eliminates any trace of a criminal’s guilt, but that
isn’t actually the case!
Scientists have developed a method that can visualize prints from even the faintest fingerprint
deposits left after the metal of a gun is wiped off.
This technique uses a colored-electro active film that uses fingerprint residues as a stencil
for blocking the electric current.
Once a voltage is passed through, the oily regions deflect the colored substances surrounding
bare regions, which gives rise to a negative ridge pattern, similar to the VMD method.
There’s also the bullet casing to consider.
Scientists can now identify who handled the bullet before it fired by visualizing corrosion
on the brass casing.
The oil or sweat marks left by finger ridges can cause the brass casing to corrode, which
is further accelerated by the heat produced while firing the bullet.
Thus, scientists coat the casing with a conducting powder and pass an electric current through
it.
The powder moves and sticks to the corroded spots… and voila!
We have a fingerprint!
There have been many phenomenal advances in forensic technology, so unfortunately, the
days of wiping down weapons to remove fingerprints will soon be a myth from a bygone era.
Eventually, movie makers will also have to do away with criminals wiping their fingerprints
off surfaces to escape detection!