Electron energy-loss safe-dose limits for manganese valence measurements in environmentally relevant manganese oxides.

ABSTRACT

Manganese (Mn) oxides are among the strongest mineral oxidants in the environment and impose significant influence on mobility and bioavailability of redox-active substances, such as arsenic, chromium, and pharmaceutical products, through oxidation processes. Oxidizing potentials of Mn oxides are determined by Mn valence states (2+, 3+, 4+). In this study, the effects of beam damage during electron energy-loss spectroscopy (EELS) in the transmission electron microscope have been investigated to determine the "safe dose" of electrons. Time series analyses determined the safe dose fluence (electrons/nm(2)) for todorokite (10(6) e/nm(2)), acid birnessite (10(5)), triclinic birnessite (10(4)), randomly stacked birnessite (10(3)), and δ-MnO(2) (<10(3)) at 200 kV. The results show that meaningful estimates of the mean Mn valence can be acquired by EELS if proper care is taken.