Tracking mercury by its isotopes

Different isotopes of the same element don’t always behave identically in chemical reactions. As a result, naturally occurring samples can have measurably different ratios of stable isotopes. In most observed isotope fractionation, deviations in reactivity vary with the mass difference between isotopes, due either to kinetic effects or to differences in the zero-point vibrational energy of chemical bonds. Last year Bridget Bergquist and Joel Blum of the University of Michigan in Ann Arbor found that photochemical reactions of mercury can result in isotope fractionation that does not fit the mass-dependent pattern: Odd-numbered Hg isotopes behave differently from even-numbered ones. Such mass-independent fractionation, observed in only a few elements so far, may be due to spin–spin interactions between nuclei and the unpaired electrons created in light-initiated reactions. Now, Abir Biswas, working with Blum and other Michigan colleagues, has found that Hg stored in coal deposits shows the effects of both mass-dependent and mass-independent fractionation. Moreover, coal samples from different regions—the US, China, and Russia–Kazakhstan—bear different Hg isotopic signatures. The researchers suggest that those signatures could provide some information about how Hg pollution (produced when the coal is burned) circulates in the environment, a process that is poorly understood. (A. Biswas et al., Environ. Sci. Technol., in press.) — Johanna L. Miller