Arsenic alarm
Safe sounds
Among the problems the old Soviet Union leii trie new" ivussia is more man ouuu tons of the unstable chemical wanare agent Lewisite (fra«s-dichloro[2-chloroviny1] arsine)) Joseph Aldstadt and hii solleagues at Argonne National Laboratory, working with Global FIA (Gig Haaborr WA), hope to introduce early next year a commercial lieldportable instrument to detect Lewisite in ambient air. The commercial instrument will weigh around 35 lb, be the size of a small suitcase, and have no installation requirements, according to Aldstadt. Its design uses flow injection to integrate gas permeation membrane sampling with electrochemical detection. Air samples flow across a thin-walled (-0.10 mm) silicone rubber tube. Lewisite diffuses across the tubing wall and into a carrier stream of 10 mM NaOH with a KN03 electrolyte where it hydrolyzes instantly in a two-step reaction to arsenite. The arsenite ion is plated out onto a gold-film electrode and is detected by potentiometric stripping analysis. The electrode potential is set to only measure As (III) and thereby differentiate from other arsenic compounds in the environment which typically As(V) Other possible interferences were also
One way to disperse a toxic warfare agent is to place it inside an artillery shell. But how does an inspector safely monitor what could be inside a live shell? Dipen Sinha and his colleagues at Los Alamos National Laboratory think they have found a way using sound waves. Swept-frequency acoustic interferometry (SFAI) is a noninvasive method that can determine the properties of fluids (as well as particulates and solids) inside containers like artillery shells. According to Sinha, with data on sound attenuation and speed plus the calculated fluid density from the data, it is possible to uniquely identify a chemical warfare agent. The prototype SFAI instrument weighs six pounds and runs on a rechargeable battery. Measurements take only 10-20 s. Although the instrument generates sound waves in the KHz-MHz range, the chemical weapon measurements are based on sound waves of 1-15 MHz. Depending on the container geometry, certain frequencies will set up standing waves in the fluid, creating a resonance phenomenon that is recorded by a piezoelectric crystal transducer. "The spacing between peaks [in the resulting spectrum] gives the sound speed, and the width of the peaks are related to sound attenuation," says Sinha. Measurements of the sound attenuation as a function of frequency, in turn, yield the fluid density. The composition of the metal container is also a factor but Sinha says that can also be determined and corrected by the instrument. The data on sound attenuation, sound speed, and liquid density are plotted together to uniquely identify the chemical agents. Sinha's group has already generated these types of three-dimensional plots for all the common chemical warfare agents and 43 of their precursors. They also measure shell temperature with the probe transducer, and this procedure allows correction of the liquid property measurements in the three-dimensional plots. Sinha points out that chemical warfare agents are just one of many potential applications for SFAI techniques. SFAI can work with emulsions, suspensions, gases, and even solids. He cites potential uses for SFAI in determining spoiled milk in containers, arthritis in bones, water quality, parts-per-million changes in concentration, Salmonella in nggs, aad intercranial pressures. Several companies are now negotiating for licenses to commercialize the technique. "It is too many applications for just one company to handle" he says
SERS detector Organophosphorus compounds constitute one important group of chemical warfare agents, such as Savin and VX. "Their Raman spectra are dominated by a peak for the phosphorous-carbon bond, which is fortuitous for us because it is characteristic for these compounds, allowing them to be differentiated from many common phosphorous-containing species," says John Haas of EIC Laboratories (Norwood, MA). With that observation, Haas and his colleagues have turned to surfaceenhanced Raman spectroscopy (SERS) to detect these agents and their degradation products. "SERS can be used to analyze extracts from [surface] 'swipes' and solid samples such as soil," he relates. Because these agents are typically acidic, the extract samples are applied to a basic substrate, which binds the analytes and also serves to enhance the Raman signal. Their best results are with silver foil that has an electrochemically generated oxide coating on the surface. As little as 0.5 uL of solution is required for a spectrum. "We can use any type of extract aqueous or organic"
Lewisite is detected with the aid of a membrane sampling device.
considered. We looked at over a dozen metals. Antimony was the only [one] that was kind of a curve ball." To avoid problems with Sb, the arsenic compound is plated onto the electrode, and the solution medium is changed to one with a high CI" concentration, which shifts the potentials. However, Sb compounds also endanger worker health, and it may be worthwhile not to discriminate against this element, argues Aldstadt. In actual experiments with Lewisite at an Army-certified laboratory, the prototype instrument succeeded in measuring the analyte in 10 min with an estimated detection limit well below the Army's eight-hour time weighted exposure limit of 3 ug/m3 in ambient air. The same instrument could also be used to screen slurried soil samples for Lewisite or other environmental contaminants. "We are looking to extend the system to other volatile organometals," says Aldstadt.
Haas says. The sample absorption is irreversible, and the silver oxide substrates are designed as throwaways. A near-IR diode laser, connected to a fiber-optic cable, illuminates a 100-um spot on the coated silver substrate. Spectra are collected by a small spectrograph with a multichannel charge-coupled device detector, which records the entire spectrum simultaneously. According to Haas the key phosphorous-carbon band is found in the region of 500 to 750 cm-1; another strong band belonging to the carbon-hydrogen stretch is seen around 2900 cm"1. Integration times of only a minute are needed to qualitatively or semi-quantitatively show whether ganophosphorus compound is present at concentrations as low as 1 ppm Haas says that they are still developing the instrument, which they envision will be used for rapid screening of samples. He expects that the instrument will ultimately weigh around 30 lb. Haas says that they are also looking at other applications for the system such as screening samples for unexploded ordinances and environmental testing.
Analytical Chemistry News & Features, November 1, 1997 6 5 7 A
