In a paper being presented today at the 227th national meeting of the American Chemical Society in Anaheim, Calif., scientist David Wayne of the Laboratory's Nuclear Materials Technology Division describes his analyses of desert varnish samples from the southwestern United States.
Using a technique called Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS), Wayne and his colleagues vaporized the top layer of desert varnish from samples taken from a site near Fallon, Nev., from another location just outside of Las Vegas, Nev., and from the San Juan River in southeastern Utah. The technique is so sensitive that only a 2 to 4 millimeter-square area of varnish is removed during the analysis. The vaporized rock varnish aerosol was transported in a stream of pure argon gas into a mass spectrometer -- a very sensitive instrument that identifies and quantifies the chemical elements present in the sample. The focus of the investigation was on the trace elements--elements that are present at levels less than 1000 parts per million--in the varnish.
Desert (or rock) varnish forms in arid and semi-arid regions as a composite of alternating layers of materials, each with different compositions. Some of these layers are rich in windblown clays and dust-sized particles, while others are rich in iron and manganese oxides and hydroxides. The components of desert varnish are very adept at capturing heavy metals, like lead, arsenic, zinc, cobalt, uranium and tungsten, that have been dispersed into Earth's atmosphere through natural phenomena, and through industrial activities like mining, smelting, oil refining, chemical processing and nuclear plant operations.
Wayne and his university colleagues discovered that desert varnish collected near Fallon contained 10 to 100 times more tungsten than the varnish from other localities, and that the top-facing surface of entirely varnish-coated pebbles from the San Juan River contained five to 10 times more lead, arsenic and cadmium than the bottom-facing surfaces.
Wayne believes that if desert varnish is as good at capturing and preserving airborne heavy metals and other elements as it appears, scientists could analyze it to infer what sorts of activities have been going on nearby. For example, since the minerals in desert varnish are also known to adsorb and hold radionuclides such as uranium and plutonium, the relatively quick and simple LA-ICPMS technique could also be used to determine if nuclear materials had ever been released (either intentionally or unintentionally) into the environment. This makes the technique not only a good tool for earth science research, but also a potential environmental and nuclear nonproliferation analysis tool for understanding the recent past.
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