Detecting Gunshot Residue In The Field

A detective walks up to a crime scene and finds a victim shot to death and three possible shooters. Now researchers have developed a portable electrochemical sensor that allows the detective to quickly detect gunshot residue on a suspect’s hands to separate the guilty from the innocent (Anal. Chem., DOI: 10.1021/ac302361z).

Currently, investigators swab suspects’ hands to collect samples and then bring those samples to a laboratory to detect gunshot residue. “They use big, bulky equipment,” such as mass spectrometers, says Joseph Wang of the University of California, San Diego. Police would prefer a device they could use at the crime scene, he says.

To build such a device, Wang and his colleagues adapted field-friendly electrochemical sensors, which identify chemicals based on how easily the compounds can be oxidized or reduced. Their portable gunshot residue detector consists of two parts: a removable electrode and a sensor. The electrode is a small strip of plastic coated with wires made of carbon nanotubes. A detective would rub the strip directly on a suspect’s hands to collect a sample. Then the investigator would insert the strip into the sensor, which measures changes in current between the strip and an electrode within the sensor as the voltage between electrodes changes. The resulting plot of current versus voltage, called a voltammogram, acts as an electrochemical signature that may match that of the mix of metals and organic chemicals that make up gunshot residue.

To test the device, the researchers headed to a shooting range. Six volunteers each went through a six-step procedure: They arrived at the laboratory, stood in the lobby outside the shooting range, stood in the room while others fired guns, loaded a gun, shot the gun, and then washed their hands after firing the gun. After each step, the researchers swabbed the participants’ hands and recorded a voltammogram, which took just two minutes, Wang says.

To the naked eye, the voltammograms for the 36 samples looked pretty similar, Wang says. But he and his team then ran a fraction of the 36 data sets through a statistical analysis program that teased out subtle variations. Through this analysis, the researchers determined which features in a voltammogram correspond to each of the six steps. They then tested its prediction ability on the rest of the voltammograms. The program could accurately assign each trace to the correct step.

Ronen Polsky of Sandia National Laboratories expresses amazement that the technique can distinguish between a shooter and bystander, a distinction mass spectrometry can’t make. “Plus, it’s inexpensive,” he says.

Wang plans to continue to validate the approach with samples from people, such as smokers, who may have other chemicals on their hands that could complicate gunpowder detection.