process. The most common method of revealing latent fingerprints—especially when they
are left on a nonporous surface—involves
dusting with fingerprint powder. Because like
dissolves like, nonpolar fingerprint powder
readily binds to a nonpolar latent fingerprint.
Nonpolar substances bind this way through
dispersion-force attractions, which are a type of
intermolecular force that occurs when areas of
opposite charges form between two molecules.
These charged areas do not last long, and new
ones form in other parts of these molecules. It
is these relatively weak and short-lived forces
that enable the nonpolar fingerprint powder to
adhere to the oily fingerprint (Fig. 1 on p. 6).
The most common type of fingerprint
powder is composed of finely divided carbon
particles. Fingerprint powders come in black,
white, and silver colors to provide maximum
contrast between the dusted fingerprint and
the surface. But if you dust a black surface
for fingerprints, it would not make sense to
use a black powder. Instead, a white or silver
powder would be preferred.
Also, some fingerprint powders are
tagged with a fluorescent dye, causing the revealed fingerprint to glow
brightly when illuminated by a black
light, which emits ultraviolet light.
These fluorescent dyes are highly sensitive to ultraviolet light and may be
used if fingerprints are faint and hard
to read or if a dusted print is difficult
Once the fingerprint is revealed by
dusting, it needs to be preserved. First,
a photograph of the revealed fingerprint
is taken. Then, a wide square of adhe-
sive tape is applied directly over the
dusted fingerprint. The piece of tape
containing the powdery fingerprint is
placed on a card that will provide maximum
contrast. If using a black powder, the piece
of tape would be placed on a white card. This
card is then carefully labeled and catalogued
Finding a match
Once fingerprints have been revealed, the
next step is to match them with those of a
known offender. Because many perpetrators
are repeat offenders, there is a good chance
that fingerprints revealed from a crime scene
are already on file from a previous arrest. The
U.S. Federal Bureau of Investigation houses
the largest computerized inventory of fingerprints in the world, known as the Integrated
Automated Fingerprint Identification System
(IAFIS), which contains the fingerprints of
more than 70 million people. These people
include not only convicted criminals and terrorists but also members of the military and
those employed by the federal government.
Unlike TV shows, in which the com-
puter returns an exact match, when a
suspect’s fingerprints are run through
IAFIS, the computer will return several
fingerprints that are the closest match,
and then a trained fingerprint analyst will
make the final determination.
This determination is made by
first classifying the fingerprint
into one of three basic classifica-
tions—loop, whorl, or arch. Next,
the analyst will look for the various
factors that make each fingerprint
unique. Matching fingerprints is as much of
an art as it is a science, and it takes many
hours of practice before one becomes pro-
Recent analysis of fingerprints’ oily residues
has revealed a method that can determine the
age of fingerprints, providing a vital piece of
information for forensic investigators who
are attempting to determine the time when a
crime was committed. Palmitic acid, which
is found in a fingerprint’s oily residue, will
migrate out of the fingerprint at a constant
rate, allowing for a determination of the fingerprint’s age.
A test was also developed to detect traces
of cocaine in fingerprints and cocaine metabolites—products resulting from the breakdown
of cocaine in the body—revealing not only
that cocaine is present in fingerprints but also
whether it was metabolized by the body.
The science of fingerprinting has
made incredible strides in the past 100
years. Whereas fingerprinting began as
an afterthought, fingerprints are now
the first piece of evidence investigators
look for when examining a crime scene.
Millions of fingerprints are run through
IAFIS each year, and numerous cases
are solved using fingerprint evidence.
Fingerprints are also being used for a
lot more than just catching criminals. If
you own a smartphone, there is a good
chance you have already unlocked it
using your fingerprint. There’s no telling
what your fingerprints may be used for
in the future!
Noble, D. The Disappearing Fingerprints.
ChemMatters, Feb 1997, pp 9–12.
Chemist’s Technique Can Identify a Fingerprint as
Male or Female. Phys.org, Science X Network,
Nov 26, 2015: http://phys.org/news/2015-
[accessed July 2016].
Yamashita, B.; French, M. et al. Latent Print
Development. U.S. Department of Justice, Office
of Justice Programs, National Criminal Justice
Reference Service; Chap 7, pp 7-1–7-67:
[accessed July 2016].
Brian Rohrig is a science writer who lives in
Columbus, Ohio. His most recent ChemMatters
article, “A Close-Up Look at the Quality of Indoor
Air,” appeared in the April/May 2016 issue. S H U T T
Fingerprints and Gender
The gender of the person who left a finger- print can be determined by analyzing the ratio of certain amino acids in a fingerprint’s oily residue. Amino acids are excreted in
sweat, which shows up in fingerprints. Women
have about twice as many amino acids in
their sweat as men. Also, due to hormonal
differences, women and men have different
ratios of certain amino acids, which can be
analyzed to determine whether a fingerprint
came from a woman or a man.
men. Also, due to hormonal