Fingerprints revealed by metal dust
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A bloody palm print. A single smudge left on the television knob. Fingermarks left on the neck of a lamp.

Each of these led to the successful conviction of a cold-case homicide.

These type of prints left on evidence, called latent prints, are often key in investigations and can be recovered from objects like windows, bodies, bags, and letters. Unlike fingerprints taken when a criminal is brought into a police station, called tenprints, they aren’t left under ideal conditions. For this reason, they can prove difficult for forensic chemists, particularly as each surface at a crime scene is different in the way it holds prints.

A newly discovered method may hold the key to unlocking latent prints on porous surfaces like paper. To do so, the crime lab uses a combination of gold nanoparticles and silver chemicals to darken the paper underneath the print, rather than the print itself, to reveal the fingermarks.

Old methods rely on chemicals to darken the print itself, attaching to the oils of the fingers. These can be done either on scene using powders or at the crime lab using more expensive, time-intensive materials.

The new technique reverses the resulting images of these traditional processes by exposing the prints in the negative. “We found a way by which we got the paper to react and it reacts everywhere but in the fingerprint The paper darkens and the fingermarks remain colorless,” says Joseph Almog, forensic chemist at The Hebrew University of Jerusalem and lead author of the paper describing this new technique.

Not only does the process help bypass the difficulties of visualizing prints from small amounts of data, it’s faster than older techniques. Even at this prototype stage, development of a clean print can occur in as little as five minutes.

This is impressive given that current techniques can take hours or longer to develop a print from a crime scene, depending on lab conditions. As Dennis Kern of the Iowa Division of Criminal Investigation Crime Lab explained, “It can take an hour, it can take two days, it depends, part of it’s the humidity, part of it’s the time of year.”

To complete the process, the paper evidence must be dipped in two separate chemicals. First, the evidence is submerged in gold nanoparticles, invisible to the eye and designed to bind to the cellulose fibers of the paper. The nanoparticles, while absorbed into the surface of the paper, leave the oily fingerprints intact. The print essentially acts as a mask, protecting those parts of the paper from being covered in the gold.

Following the gold dip, the paper is submerged in a solution of silver for less than a minute which, when combined with the gold, turns the coated areas black, leaving the uncoated regions of the print intact. Once revealed, the prints are photographed and analyzed on three separate features, determined by the patterns and fine details in the ridges.

The sweat of the human hand, composed of 98 percent water, is what leads to a good fingerprint with clean features. Made of a greasy mix of fats, salts and other chemicals, the composition of a human print can be as different person to person as the patterns of ridges. This composition, as well as the individual patterns and points on the fingers, are what allow criminologists to determine if latent prints match a potential criminal.

Despite what detective television series might suggest, fingerprints can be difficult to obtain precisely because they rely on human sweat or other dirt on the fingertips. If a criminal happens to wash their hands particularly well, the oils of their fingers may not leave a signature that can be lifted. In addition, wet papers can wash away the chemicals needed to create a good print with current techniques. “In many cases … less than fifty percent of the latent fingermarks actually develop in sufficient quality to be identified,” says Almog.

Almog’s technique, because it adheres to the paper instead of the print, could help raise this statistic, particularly for cases where little print residue is left behind. However, while it works for porous surfaces, specifically paper, several other methods must be used for materials like plastic.

Techniques for revealing latent prints on porous and non-porous surfaces are presented each year at the International Association for Identification’s conference, a group begun in 1915 with over 7,000 members worldwide. In 2011, the U.S. Secret Service presented an update on a process for non-porous items like lock-top sandwich bags, sheet protectors, and trash bags, which involves spraying the evidence with a mix of powder and heated glue before gassing the items to reveal latent prints.

A similar method is used by the Boston Police Department and the Iowa Crime Lab, in addition to chemical dyes that fluoresce under special lighting. In Boston, for papers or items like unfinished wood, they vary between four types of fuming using chemicals that cause the prints to appear in color when exposed, depending on their chemical composition. Like Almog’s method, the Boston Crime Lab can also stain the background of an object with a fingerprint left in blood, making the imprint visible in the negative.

Examining this and other difficulties, the Los Alamos National Laboratory in 2011 applied a series of X rays to the fingerprints, drawing on the salts left behind, and collected what was emitted from the sample, effectively creating a picture of the print.

While a side project of the lab, the study contributed a new way to look at prints, leaving the evidence intact—important for other lab tests—and offering a method that could obtain the same results even seven months later (given that the salts remained on the object in question). Unfortunately, many of these new latent printing strategies are still expensive and still being refined for use in labs outside of large research facilities.

Los Alamos National Lab’s x-ray machine is around $200,000 and the negative print method using nanoparticles is infeasible for cash-strapped forensics labs. Explained Almog about his method, “It cannot be applied to every burglary case. It’s too sophisticated and requires manpower and quite expensive chemicals right now.”

Deborah Dobrydney, a senior criminalist with the Boston Police Department, agreed that new techniques are defined by the cost and time. “The less steps and the more readily available the materials are, the better the process is.”

Tags: fingerprint, homicide, nanoparticles