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Physical Chemistry

Studying Chemical Warfare Agents Directly

A surface science instrument adapted for highly toxic compounds permits study of environmental and decontamination effects

by Jyllian Kemsley
May 5, 2014 | A version of this story appeared in Volume 92, Issue 18

Photo of scientist Erin Davis works at the hood next to the surface-science instrument.
Credit: Tom Faulkner/U.S. Army
Davis prepares to transfer a sample through a port in the side of the hood into the reaction chamber.

A new instrument at the Army’s Edgewood Chemical Biological Center is now allowing direct studies of how chemical warfare agents such as sarin interact with surfaces of environmental materials or coatings designed for protection or decontamination. Previously, scientists could work only with less toxic analogs of the agents.

The ability to explore how these chemical warfare agents decompose and are neutralized on surfaces “offers exciting opportunities to develop new purification and decontamination technologies,” says John T. Yates Jr., a leading surface science researcher and chemistry professor at the University of Virginia. Yates was involved in neither the instrument development nor experiments conducted so far.

The apparatus combines infrared spectroscopy to track bond making and breaking on surfaces, mass spectrometry to detect gas-phase products, and X-ray photoelectron spectroscopy to study the elemental composition of surfaces. Using it, researchers can monitor reactions in real time with IR spectroscopy and MS and take before-and-after images of surfaces using photoelectron spectroscopy.

The combination of techniques is not unusual for surface science studies, says Virginia Tech chemistry professor John R. Morris. Morris led the instrument development and experiments with former students Amanda Wilmsmeyer, now a chemistry professor at Illinois’s Augustana College; Wesley O. Gordon, now a chemist at the Edgewood center; and Erin Durke Davis, now a chemist employed by defense contractor Excet.

What is unusual is how the instrument, which operates at ultrahigh vacuum, was engineered for safe study of chemical warfare agents (Rev. Sci. Instrum. 2014, DOI: 10.1063/1.4846656). Samples are loaded in and out from within a hood equipped with exhaust filters designed to handle the most toxic of substances. Pumps and backup pumps, along with an overpressurization burst disk, also exhaust into the hood. Aluminum connector seals reduce leaking orders of magnitude below standard polymer seals, and the system undergoes a quarterly leak-check with helium, Gordon says.

Warfare agents sarin and soman and a commonly studied, less toxic analog hydrogen-bond to surfaces such as silica.
Structures of sarin, soman, and dimethyl chlorophosphate can hydrogen-bond to surfaces such as silica.
Warfare agents sarin and soman and a commonly studied, less toxic analog hydrogen-bond to surfaces such as silica.

After getting everything set up, the biggest challenge with the instrument is ensuring that maintenance is done safely, Davis says. “It’s not trivial to keep ultrahigh vacuum systems running,” she says. But given the nature of the experiments, “you can’t just unbolt a flange to fix something,” she adds. A custom-designed thermal jacket enables thermal cycles and hot-gas purges that eliminate residual contamination. The apparatus incorporates equipment to enable a wet-swab test on the walls of the experimental chamber to confirm decontamination. And then, when any flange is finally opened for maintenance, a blower sucks laboratory air through the instrument into the hood at 1,500 cu ft per minute, ensuring constant negative pressure.

Samples can be loaded onto a surface in two different ways. First, a surface sample on a small plate is transferred into the reaction chamber. For an agent with a low vapor pressure, such as VX, a separate cartridge loaded with an inert, solid sorbent is charged with the compound. The cartridge is then sent into the reaction chamber, where it is cooled below the compound’s sublimation temperature. The cartridge cap is removed, the cartridge is positioned within about 3 mm of the surface sample, and the sorbent is heated to allow the agent to desorb from the carrier and interact with the surface.

For agents with higher vapor pressure, such as sarin and soman, the sample is loaded into a cartridge as a liquid. It is subsequently heated to produce a vapor, which is directed toward the surface sample by a tube. In both sample-dosing mechanisms, a mere 10–40 µL of sample is enough for an experiment.

In initial sarin and soman experiments, Morris and colleagues examined how the agents interacted with amorphous silica surfaces. “We chose silica because of its abundance on Earth—it’s the first- or second-most abundant material in nature,” Morris says. Silica is usually covered by –OH groups that may serve as hydrogen-bonding sites for warfare agents. No one had directly studied the gas-surface hydrogen-bonding energy of such agents previously, Morris explains. Such studies are important because hydrogen-bonding energy determines how long the agents stick to a surface, he says.

By exposing the silica to sarin and soman, then heating the surface to get the agents to desorb, the team found that the desorption energy is 50 kJ/mol for sarin and 52 kJ/mol for soman, in line with those previously determined for analogs such as dimethyl chlorophosphate (J. Phys. Chem. Lett. 2014, DOI: 10.1021/jz500375h). The hydrogen-bonding energy depends on the electron-withdrawing ability of functional groups surrounding the central phosphoryl group.

The results will help benchmark computational studies and point to the best analogs to use in other experiments. They also provide a basis for future experiments with sarin, soman, and other warfare agents. The researchers would like to explore the effects of water or other atmospheric contaminants on the hydrogen-bonding interactions. They also hope to study how these agents interact with surfaces other than silica.

“One of the biggest advantages of this setup is that we can investigate actual reaction mechanics and the kinetics that are taking place,” Davis says. What the researchers learn from the fundamental science will help guide improvements to technology for protecting soldiers and others from chemical weapons.  



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