Cd isotopic composition measured by plasma source, magnet sector mass spectrometry in natural and anthropogenic materials

Cadmium is a trace metal that is extensively used as a geochemical tracer of natural processes, like biological productivity and paleo-productivity, and also of anthropogenic pollution, as Cd is known to be a toxic heavy metal that has become a major environmental and health concern. For these purposes, an outstanding issue is to determine whether Cd, like a number of metallic elements (e.g. Fe, Cu, Zn, Mo, Tl), may display variable isotopic compositions in natural and/or industrial compounds. It is known that Cd may display variable isotopic composition. Indeed, isotopic fractionation processes have been documented in some meteorites and in lunar soils. Consequently, due to its relatively low boiling point (767 deg C) and also to the large mass range covered by its isotopes (10 mass units, from masses 106 to 116), Cd might fractionate isotopically, for example during the outpouring of acidic volcanic magmas and/or the emplacement of granitoids, since the temperature of granitic magmas is close to the boiling point of Cd. On another hand, isotopic fractionation could also occur during human activities that may involve a comparable range of temperatures: refuse incineration, industrial manufacturing, for instance. Finally, biologically-induced isotopic fractionation should not be ruled out, as it is more and more clearly evidenced for other metals, like Fe. A high precision method has been developed for determining the isotopic composition of Cd by plasma source mass spectrometry (Neptune). This method holds on the standard-bracketing technique, owing to the availability of different Cd solutions of known isotopic composition, synthesized at the University of Munster. This allows to correct precisely for the mass fractionation processes that occur in the plasma source mass spectrometer. It is also critical for the analysis to be possible to work with Cd solutions of very high purity. For solid materials, chemical isolation of Cd involves 3 successive steps. Usually, gram quantities of sample have to be processed for solid samples, since Cd concentrations are typically in the range or below 100 ppb. For waters, Cd contents lie in the range or below 10 ppt (for both seawater and continental waters), except in the case of polluted waters. Thus water amounts of at least one liter have to be processed. This however does not require to carry out the complex and time-consuming standard pre-concentration technique featuring a chelating resin, as Cd hydroxide may be co-precipitated with Fe at pH > 7. This co-precipitation step allows in addition to skip over the first chemical separation step required for solids at least if the water loads are not too high. It should finally be kept in mind that these two chemical separation procedures may be also suitable for Cd isotopic measurements using solid source mass spectrometry. Preliminary results have been obtained on different solid materials, and suggest that Cd is likely to fractionate during smelting activities, as indicated by isotopic measurements on mining waste. The calibration of a 110Cd-116Cd double spike is underway for absolute, true isotopic ratios to be derived. This double spike technique allows the isotopic analysis of Cd using a plasma source, but also a solid source mass spectrometer, as Cd ionizes efficiently in such mass spectrometers owing to the standard silica gel technique. (author)