As military and security experts devise strategies to reduce risks of chemical attacks on civilians and military personnel, scientists are developing materials and procedures for detecting and quantifying hazardous substances that might be used in those attacks. A number of scientists active in that area of research gathered at a Pittcon symposium earlier this month to discuss the latest findings.

The symposium on detection methods for chemical warfare agents was one segment of a three-part session focusing on detection of biological and chemical weapons and explosive materials. The symposium was coorganized by two chemistry professors: David R. Walt of Tufts University and Michael J. Sailor of the University of California, San Diego.

"We'd like to take a large piece of laboratory instrumentation, make it smaller, and move it out of the lab so it can be mounted on a fence post and ultimately in the palm of your hand," Sailor remarked in his introductory comments. He acknowledged that miniaturizing sophisticated instruments for use in detecting chemical weapons in the field "poses significant challenges not found on a lab bench." Indeed, at the present time, a number of instruments offer high sensitivity but not all of them are field-tested and readily available in compact size.

At Colorado State University, chemistry professor Steven H. Strauss measures parts-per-billion quantities of hazardous materials in water with Fourier transform infrared (FTIR) spectroscopy methods. Specifically, Strauss uses a technique known as attenuated total-reflectance (ATR) in which an IR beam is reflected from the surfaces of an ATR crystal as the beam propagates along the length of the crystal. The IR spectrum of a thin sample--for example, a solid film or a thin layer of liquid--in contact with the crystal can be probed with this method because as the light bounces off the crystal surfaces, it penetrates the film slightly.

To maximize the method's detection sensitivity, Strauss, graduate student Gretchen N. Hebert, and their coworkers coat ATR crystals with metal-complex extractant films that serve as analyte concentrators. For example, the group uses films made from salts of a ferrocenium complex to detect perchlorate ions, nerve agents, and other weakly hydrated anion species. Strauss noted that the coating procedure is simple and that the films are easily recycled via redox chemistry.

Presenting results of detection limit studies, Strauss reported that perchlorate ion (used in rocket propellants) can be detected in concentrations as low as 4 ppb. Data were measured in roughly 30 seconds following a 10-minute ion-exchange period during which the coated crystal was exposed to perchlorate solution. "If we wait longer, we can detect even lower concentrations," he asserted. But in keeping with analytical chemistry practices common to military applications, Strauss reported the results as a 10-minute limit of detection. He added that compared with uncoated ATR crystals, the organometallic compound, which was synthesized by the Colorado State group, improves the method's sensitivity by a factor of 6,000.

APPLYING THE ATR-FTIR spectroscopy method to nerve agents, Strauss's group determined that the 10-minute limit of detection for PMPA, a long-lived hydrolysis product of GD (also known as soman), is 125 ppb. At present, however, the calibration curve is linear only in the range of 1.8 to 180 ppm, he said. The group has developed a similar method for detecting cyanide ions. By exchanging Cl^– for CN^– in a film of a diphosphine-chelated NiCl₂ compound, the researchers can detect as little as 2.3 ppb of CN^– in 10 minutes. Strauss reported that the ATR-FTIR method has also been used to measure parts-per-billion levels of perfluorinated sulfonate ions and other analytes.

Vibrational spectroscopy is also used by military scientists. Steven D. Christesen, a research chemist at Edgewood Chemical Biological Center, Aberdeen Proving Ground, in Maryland, discussed the use of Raman spectroscopy to detect nerve agents, mustard gas (a blister agent), and cyanide (a blood agent) in water samples. Christesen noted that the U.S. military is aiming to develop detection methods for these types of substances based on portable instruments that can be used to complete analyses in 10 minutes or less.

To maximize detection sensitivity, Christesen deposits the analytes on roughened surfaces of gold and silver and probes them with surface-enhanced Raman spectroscopy (SERS), a highly sensitive variation of the Raman technique. He noted that the time required for analysis--including dipping a metal probe tip into an analyte solution, allowing it to dry, and recording spectra--is just a few minutes.

Christesen reported that the lower limit of detection for CN^– is roughly 2 ppb, which is lower than the military's detection goals, but added that the measurements exhibit poor reproducibility. He proposed that the inconsistencies may be due to subtle differences in the surface properties of the metal substrates. Mustard gas can be detected at concentrations as low as 50 ppm, Christesen said, but it hydrolyzes quickly to thioldiglycol, low levels of which are not easily detected by the SERS method.

Although the technique is a sensitive probe for blister and blood agents, "we're having problems with nerve agents," Christesen acknowledged. He presented data from detection limit studies of VX and its hydrolysis products, ethyl methyl phosphonic acid and EA2192, a highly stable and toxic product. He also reported findings from investigations of GD hydrolysis products and other organophosphates. Christesen noted that the SERS method can be used to detect most of the analytes in the low parts-per-million range but added that the goal is to improve sensitivity by three orders of magnitude.

Herbert H. Hill, a professor of chemistry at Washington State University, Pullman, discussed the use of ion mobility spectrometry (IMS) for detecting chemical warfare agents. Hill explained that in his IMS setup, a plume of sample ions is generated in an electrospray source and interacts with a drift gas that strips away solvent molecules. The ions travel under the influence of an electric field to an ion gate that opens briefly, allowing a pulse of ions to separate in a drift tube according to their size and shape. After the separation step, the ions are analyzed in a mass spectrometer--making the technique a "two-dimensional" method, IM-MS.

	THE SENSITIVE KIND University of Pittsburgh chemists (from left) John F. Jackovitz, Walker, Asher, Ward, and Sharma, measure trace levels of analytes with crystalline colloidal array sensors.

THE INSTRUMENTATION in Hill's lab is neither small nor portable, he acknowledged, but the size has been reduced recently in a design that replaces the instrument's quadrupole mass filter with a time-of-flight mass spectrometer.

The 2-D system can be used to analyze gas and liquid samples and positive and negative ions, Hill pointed out. And unlike other analytical equipment, the system is not prone to fouling. For example, with little filtering, river and bog water have been analyzed directly, he noted.

According to Hill, the IM-MS method has been used to analyze aqueous samples of pesticides, nitrates, nerve agents such as VX, and various hydrolysis products. In four-minute detection limit studies, the technique provided parts-per-billion sensitivity for many of the species without special procedures to concentrate the analytes. Hill reported that his research group has also used the technique to study vapor-phase samples of standard simulants for chemical warfare agents.

"The system provides two-dimensional high-resolution identification of numerous compounds with high sensitivity," Hill stressed. He added that although IM-MS is not field-deployable today, progress is being made in that direction.

Advances in instrumentation aren't the only sources of new chemical detection methods. Some researchers are developing new materials that function as sensitive and selective probes for various analytes, including harmful materials. For example, at the University of Pittsburgh, chemistry professor Sanford A. Asher and his research group prepare crystalline colloidal arrays of various particles that can respond to specific chemical species in an easy-to-recognize manner. The ordered materials are a type of photonic crystal.

To make chemical sensors, the Pittsburgh group takes advantage of electrostatically driven self-assembly to order the particles into colloidal arrays and then polymerizes the arrays to form robust hydrogels. By functionalizing the hydrogels with suitable chemical recognition agents, the materials bind particular analytes. Asher explained that the binding events cause the arrays to swell, which, in turn, alters the material's lattice parameters and changes the diffraction conditions. The outcome is a color change that's visible to the eye.

In recent work, Jeremy P. Walker, a graduate student in Asher's group, and coworkers demonstrated that parathion, a highly toxic organophosphate compound, can be detected in low concentrations using hydrogels functionalized with acetylcholinesterase. And postdoctoral associate Anjal C. Sharma and graduate student Michelle M. Ward developed an 8-hydroxyquinoline-based hydrogel sensor capable of detecting nanomolar levels of copper and other metal ions.

Symposium coorganizer Walt commented that chemical weapons detection "requires the most sensitive methods of analysis available today as well as new innovations." Highly sensitive detection methods already exist for some analytes. Yet the all-in-one field-portable detection tool desired by the military has still not been developed. But it may be on its way. With threats of chemical attacks continuing to grow, analytical chemists are working on it.

Chemical & Engineering News
Copyright © 2003 American Chemical Society