Isotope labeling eyed for tracing smoke particles
Ashley: studies are at lab stage silk are used in spinning webs. An chor silk attaches corners of the web to bushes, trees, and other objects. And dragline silk, as the name implies, supports the spider. It is the dragline silk that is of most interest to the PA Technology team because of its physical proper ties. Citing data from the literature, Ashley points out that the tensile strength of dragline silk for three species of spiders ranges from 1420 million to 1550 million newtons per square meter (pascals). That's gen erally similar to the corresponding values of Kevlar 49, a Du Pont aramid fiber (2000 million Ν per sq m); of a high-tensile nylon (1600 million Ν per sq m); and of carbon fiber (1750 million Ν per sq m), he explains. The same three dragline silks have an elongation-at-break rating ranging from 16 to 30%. For Kevlar 49, it is 4%; for high-tensile nylon, 16%; and for carbon fiber, 1%. The dragline silk protein mole cule consists of crystalline regions of repeating hexapeptide subunits interspersed by amorphous regions. The crystalline regions give the strands their strength; the amor phous regions provide elasticity. Ashley plans to study how the prop erty of the silk might be tailored by altering the ratio of the crystallineto-amorphous regions along the fi ber's chain. Dermot O'Sullivan
In an effort to track emissions from coal-fired power plants to their sources, researchers at the National Bureau of Standards and the Uni versity of Maryland are developing a technique that shows promise as a simple and definitive way of trac ing smoke particles. The researchers say the method, which uses stable isotopes of rareearth elements to tag emission par ticles (fly ash), may be the first one suitable for tracing fine particles over long distances. It would com plement techniques based on tracer gases, which detect only gaseous components of stack emissions, not particles. The NBS/Maryland work is sponsored by Electric Power Re search Institute and the Maryland Department of Natural Resources. Several rare-earth elements, such as neodymium and samarium, are well suited for the project because they have as many as seven stable isotopes whose relative abundances are constant in nature. Moreover, the rare-earth "background" in the environment is very low. Thus, re searchers can artificially enrich the natural abundance of any of these isotopes (such as neodymium-148) to create an altered isotopic ratio that can later be measured with high precision using thermal ionization mass spectrometry. Since there are no industrial sources of isotopically enriched rare-earth elements, de tecting an enrichment would defin itively identify the tagged source, say the developers of the technique, W. Robert Kelly, a research chemist at NBS, and John M. Ondov, an associate professor of chemistry at the University of Maryland. At the plant site, the rare-earth tracer may be injected into the stack emission stream by one of two meth ods. In one, a solution of the rareearth element is atomized into the stream, causing the solvent to evap orate and the rare-earth particles to bind to the fly ash. This method is feasible, Ondov tells C&EN, but the amount of tracer that can be re leased using it may be too limited. The second method, now being fieldtested, involves injection of a vola tile organic chelate of the rare earth. The chelate is thermally degraded,
leading to a rare-earth oxide, which then would be expected to adsorb to the fly ash. The researchers have performed model calculations that suggest that only about 140 mg per hour of the enriched isotopic tracer need be injected into a 425-MW power plant to permit detection hundreds of ki lometers away. According to Kelly, the tracer would cost about $1.00 per milligram. "We anticipate that tagging will be especially effective in the submicrometer particle range," says Kelly, a mass spectrometrist who thought up the idea. "This is im portant because these tiny particles are the ones that travel the farthest." Ondov, whose specialty is atmo spheric aerosols, says it should be possible to track particles 1000 km or more from their source. The two researchers envision the simultaneous tagging of several emission sources in a region. If, for example, a rare-earth element has five usable isotopes, each one could be used to tag a different emission source, they say. That would enable scientists to assess the emission con tribution from each source at a sin gle deposition site. Thus, analysis of, say, leaf samples from a wooded area near several sources would reveal where the fly ash came from. Because the analysis is quantita tive, the results would allow scien tists to gauge a source's environ mental impact, according to NBS. So far, work on the project has been limited to in-stack tests at a single coal-fired plant site to deter mine the feasibility of the two tag ging methods. The researchers hope to actually begin tracking emissions from more than one power plant by next summer, Ondov says. The NBS/Maryland scientists also hope to combine their study results with those of researchers examin ing gaseous emissions from coalfired plants to produce a more com plete picture of total emissions. In addition, data derived from this study likely will be used to improve computer models that predict dis tribution of acidic deposits from in dustrial plants, NBS notes. Ron Dagani July 25, 1988 C&EN
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