The Boston marathon bombers Tamerlan and Dzhokhar Tsarnaev reportedly purchased several pounds of black powder explosive before the bombing. Used in fireworks and bullets, the explosive substance is both deadly and widely available. It's also very hard to detect. Now, researchers have modified one bomb-sniffing device to accurately spot very small amounts of black powder, an advance that could make us safer from future attacks.
Invented in China as early as the 7th century, black powder is a mixture of charcoal, sulfur, and potassium nitrate. It's also used as a fertilizer and food additive. To detect explosives, drugs, and other compounds, security agents often use ion mobility spectrometers (IMSs), the machines that analyze cloth swatches that Transportation Security Administration agents wipe across luggage and clothing. Inside an IMS, a sample's volatile atoms and molecules receive an electrical charge, ionizing them in the process. An electric field carries these ions across a chamber several inches long against a headwind of purified air. Each ion's characteristic size and charge dictates how quickly it reaches a detector at the chamber's end, allowing experts to distinguish the components of a sample.
Conventional IMS technology is popular because it can analyze a sample in thousandths of a second, and thousands are deployed in airports worldwide. And though bomb-sniffing dogs can easily find black powder, these machines can work without breaks, naps, or treats. What has prevented detection of black powder by IMS in the past, however, is that sulfur and oxygen—which composes 20% of air—hit the detector at almost the same time. A strong oxygen signal can thus mask a small amount of sulfur, like what a bombmaker's dirty fingers might leave on a luggage strap.
A group led by chemist Haiyang Li at the Dalian Institute of Chemical Physics in China modified an IMS to eliminate the oxygen signal. Researchers had previously tried doing so by injecting the sample into a carrier gas of dichloromethane that captures oxygen ions before the sample reaches the device's analysis chamber. But Li showed that this inhibits the initial formation of sulfur ions, sabotaging the device's purpose. So Li's team used an approach called titration region IMS (TR-IMS) that first ionizes the sample and then passes it through dichloromethane. In tests reported online last month in Analytical Chemistry, the TR-IMS knocked down the oxygen signal and easily spotted a strong sulfur signal when it analyzed black powder and fireworks.
The scientists also showed that reducing the oxygen background improved the device's sensitivity, enabling it to detect very small amounts of black powder. "We have tested the sensitivity of TR-IMS, and its limit of detection of black powder can reach as low as 0.05 nanograms," Li says. One nanogram is the average weight of a human cell. "Experimental results taken in our lab showed that it was sensitive enough to detect trace amounts on luggage and clothing," he continues. The inherent problems of conventional IMS mean that it could miss 100 times that much sulfur or more. Li estimates that only about $10 in parts would be needed to retrofit a conventional IMS, which he says cost tens of thousands of dollars. Li reports that the TR-IMS would work as quickly and efficiently as current devices.
"The titration method they propose is novel and looks promising," says Herbert Hill, a chemist at Washington State University, Pullman. "But [it] would need lots of testing before it could be employed as a routine black powder detector." One concern he raises is that sulfur is a fairly common ingredient in everything from rubber to pesticides, increasing the chances of a false positive reading. Still, he notes that a signal from the machine could alert security agents to search a bag more carefully.