What makes fingerprints?
Fingerprints are sometimes visible, such
as when someone gets blood, ink, or grease
on their fingers and then touches a surface.
These types of visible fingerprints are known
as patent fingerprints.
If the fingerprints make a 3-D impression
in a soft material, such as clay or a freshly
painted surface, they are known as plastic
fingerprints. A great way to make a plastic
fingerprint is to push your finger into a glob
of Silly Putty.
Latent fingerprints are those that are not
readily visible to the naked eye. The word
latent means hidden.
These are fingerprints
left behind every time
you touch an object. To
understand why you
leave behind fingerprints,
let’s examine a fingertip
closely. Under magnification, you will see a row of
tiny holes along each friction ridge. Each of these
holes is a pore connected
to a sweat gland.
The most common type of sweat gland,
found in virtually all skin, with the high-
est density on the hands and feet, is the
eccrine gland. You have several million of
these glands, explaining why your hands
and feet often feel sweaty. Several hundred
compounds have been isolated from human
sweat. While the primary component of sweat
is water, many other substances—such as
amino acids, salts, and lipids—are present.
Lipids are molecules that contain hydrocarbons and make up the building blocks
of the structure and function of living cells.
Examples of lipids include fats, oils, waxes,
certain vitamins, hormones, and most of the
non-protein molecules in cell membranes.
Sweat is not the only source of lipids found
in fingerprints, however. Sebaceous glands
on your face and scalp secrete an oily substance known as sebum, which keeps your
skin waterproof. It acts as a barrier that keeps
too much water from getting into your body
and prevents you from losing too much water
through your skin. Sebum also protects skin
from bacterial and fungal infections. When
you touch your face or hair, you are transferring some of this sebum to your fingers.
This combination of sweat and sebum
makes up the oily residue that coats our fingers. When we touch a surface, this oily substance is left behind, along with an impression
of our fingerprints. This fingerprint residue
is primarily nonpolar in nature, as it contains
compounds that do not mix with water. Nonpolar molecules have their charges evenly
distributed, whereas polar substances, such
as water, have distinct regions of positive and
negative charges within each molecule.
Many factors can contribute to the quality
of a fingerprint left behind. If your skin is dry,
there will be fewer oils on the skin, and the
fingerprint left behind will be less distinct.
The type of surface on which the fingerprint is
deposited plays a big role in whether a
fingerprint can be recovered. A porous surface, such as paper or wood, will tend to
absorb the fingerprint, making it last longer
but also making it harder to visualize. A
nonporous surface, such as glass, metal, or
plastic, will leave a more distinct fingerprint.
But because a fingerprint is only deposited on
a surface, it can be wiped off easily.
Revealing latent fingerprints
Unlike crime shows on television, where
perfect fingerprints are uncovered with relative ease, the actual process of revealing latent
fingerprints is a tedious and time-consuming
Lisa Miller (middle), a scientist working at
Brookhaven National Laboratory, Upton, N. Y.,
shows two students why adults’ fingerprints can
last longer than children’s fingerprints.
6 ChemMatters | OC TOBER/NOVEMBER 2016 www.acs.org/chemmatters
Figure 1. Schematic representation of dispersion-force attractions between nonpolar molecules in fingerprints (right) and fingerprint powder (left) at three
times, a few nanoseconds apart (a–c). In nonpolar molecules, slightly negative and slightly positive charged areas of the molecules (indicated by the “+” and
“–” signs) form and disappear continuously anywhere in the molecules due to slight fluctuations in the positions of the electron clouds within the molecules.
At any instant, a few molecules may not have charged areas, because such charged areas are not permanent.
hole along a
ridges is a