New instrument effective in detecting chemical weapons
Chemistry—Military commanders and national leaders need to know if chemical warfare agents are present before sending troops into hostile territory.
Garold Gresham leads an INEEL research team in the development of a new ion-trap mass spectrometry instrument that could benefit national security and peacekeeping efforts.
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Researchers at DOE’s Idaho National Engineering and Environmental Laboratory can now detect part-per-million levels of chemical warfare agents chemical warfare agents such as the blister agent HD or the nerve agent VX on soil or plant surfaces within 5 to 10 minutes using a new ion-trap secondary ion mass spectrometer (IT-SIMS). Samples as small as 3 to 4 milligrams—about 40 grains of salt—are identified by the ions that the instrument sputters off the sample surfaces.
Using IT-SIMS, researchers can isolate trace amounts of the chemical warfare agents within the instrument and then break them apart to obtain unequivocal chemical identification. Each chemical present is identified by a detectable spectrum as unique as a fingerprint.
INEEL researchers are developing surface analysis instrumentation specifically for environmental samples such as soil or plant surfaces. Better analytical techniques for these kinds of materials support both environmental restoration and national security Department of Energy missions. Chemical warfare agent detection is just one possible application of IT-SIMS. Results are published in the International Journal of Mass Spectrometry, volume 208.
Using IT-SIMS, researchers bombard the surface of a sample with a polyatomic projectile to lift or "sputter" off molecules adhering to the sample surface. The sputtered molecules, called secondary ions, retain the chemical characteristics of the chemical warfare agent stuck to the surface of the soil. The secondary ions are filtered by mass and then counted. That data is displayed as a spectra (a bar graph that plots the number of ions versus their mass) that researchers then use to identify the chemicals. Each chemical has its own unique, signature spectra—similar to that of a fingerprint.
The current standard for verifying the presence of chemical warfare agents is gas or liquid chromatography in conjunction with mass spectrometry. Samples must be extracted from soils, plants or concrete prior to analysis—a time-consuming, labor intensive process that can destroy or bias the original sample. Ion Trap SIMS offers distinct advantages to this conventional process.
One advantage of IT-SIMS is that once a surface contaminant is sputtered into the gas phase, researchers capture and retain the ions specific to HD and VX agents, for example, and screen out everything else. Then, researchers subject the target ions to precisely chosen electromagnetic frequencies-called tickle voltages-to fragment the ions into still smaller pieces. "With IT-SIMS, we can essentially do mass analysis to the nth degree. We just keep pounding the ions apart and analyzing the product ions," explains retired INEEL scientist David Dahl. "This gives us the ability to get incredible specificity in our chemical analysis."
"Using ion trap SIMS helps us get around the need for time consuming sample preparations," explains INEEL scientist Gary Gresham. "This is particularly critical when investigating suspected Chemical Weapons Convention issues of non-compliance. Investigators may have a mere 72 hours to sample for chemical agents and determine if violations have occurred. Clearly, being able to process more samples in that period of time would be beneficial to the investigators."
Secondly, researchers have developed a bombarding projectile, called the primary ion, which lends an additional edge to the INEEL IT-SIMS design. The team uses a rhenium/oxygen molecule (called a perrhenate ion) instead of an atomic particle such as cesium or gallium. The substantial size of the perrhenate ion makes the sampling process more efficient. "Think of this as the difference between shooting a bullet or throwing a large rock at the surface. The bullet penetrates deeply into the target, and doesn't disturb the surface very much. The rock, however, will impact a much broader surface area and more efficiently sputter adsorbed chemical agents into the gas phase," said Gresham. The perrhenate ion can sputter off intact molecules from a sample surface, giving researchers more accurate molecular information.
A third advantage of the INEEL IT-SIMS is a design feature that allows researchers to overcome the problem of sample charging. Researchers need to collect both positively and negatively charged secondary ions in order to make an accurate chemical identification of the many chemical warfare agents and their degradation products. However, environmental surfaces such as soils and plants are naturally insulating. Over time, bombarding the sample surface with the negatively charged perrhenate projectile causes the sample to build up a charge--hampering the collection of both positively and negatively charged secondary ions. INEEL researchers overcame this problem by taking advantage of a phenomenon known as self charge stabilization optics, which keeps the ion trap either more negatively or positively charged than the sample itself.
IT-SIMS is particularly suited to applications such as chemical weapon agent detection because such chemicals are designed to be both adsorptive and persistent—to stick to any and all surfaces and stay there. Using IT-SIMS, researchers can collect large numbers of intact ions from the sample surface and accurately identify the chemical substances. Researchers can analyze samples as small as 3 to 4 mg with minimal sample preparation. Analysis times are fast—on the order 5 to 10 minutes—and researchers can analyze a sample surface without the need for sample extraction.
The purpose of the current two-year study is to test the feasibility of using IT-SIMS to detect chemical warfare agents, and then develop portable instrumentation. "We have demonstrated that this analytical approach gives complementary results to the standard mass spectrometry techniques. And it brings additional benefits of increased specificity in analysis and speed," said Gresham. "The barriers to making such a system field-portable will be substantial, however."
The bulky and heavy electronics, and a vacuum system that safeguards sample integrity are a major barrier to an easily field-portable system. The next step, already in progress, is a completely rewritten computer code to relocate nearly all of the computational processing to a computer workstation instead of a processor within the ion trap itself. "Each new capability that we add means new computerized controls," says INEEL’s John Olson. The research team wants to automate as many functions as possible in order to give technicians even greater control over operating parameters. Computerizing and automating functions reduces the possibility for human error during sample analysis.
The design and fabrication work, chemical agent degradation product and precursor testing is being carried out at INEEL facilities. Testing of live chemical warfare agents is being conducted under controlled conditions at the U.S. Army West Desert Test Center chemistry laboratory, Dugway Proving Ground, Dugway, UT.
This research is supported by the U.S. Army Chemical Materiel Destruction Activity, Non-Stockpile Program, Defense Threat Reduction Agency and by the DOE Office of Research & Development (DOE-NN).
The complete reference for a recent paper covering this research is "Static secondary ionization mass spectrometry and mass spectrometry/mass spectrometry (MS2) characterization of the chemical warfare agent HD on soil particle surfaces," G. L. Gresham, G. S. Groenewold, A. D. Appelhans, J. E. Olson, M. T. Benson, M. T. Jeffery, B. Rowland, and M.A. Weibel, International Journal of Mass Spectrometry 208 (2001), 135-145.