Carbon-13 solid-state NMR is used to determine, via the temperature dependence of paramagnetic shifts, the spin ladder of magnetic levels and the related spin exchange parameters in [4Fe−4S]2+ clusters. This study has been carried out between 180 and 330 K in three model compounds of the active sites of 4Fe−4S proteins:  [(C2D5)4N]2[Fe4S4(S13CD2C6D5)4], [(C2H5)4N]2[Fe4S4(SC(CH3)3)4], and [(C2H5)4N]2[Fe4Se4(SC(CH3)3)4]. The temperature dependencies of both the isotropic and anisotropic parts of the 13C paramagnetic shift tensors have been measured and analyzed. Antiferromagnetic exchange coupling constants among iron atoms of J ≈ 430, 380, and 370 cm-1 have been deduced from fits of their temperature dependencies. Additional contributions of the double-exchange term B favoring electronic delocalization and of a ΔJ term associated with the compression of the cubane geometry are also discussed. We also show that connections between these measurements and those of magnetic susceptibility are especially close in the case of the [4Fe−4S]2+ redox state.

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The Joint Research Centre, in cooperation with the Commissariat à l’Energie Atomique et aux Energies Alternatives, produced a novel 243Am spike reference material for mass spectrometry. Americium solution with an isotopic composition of 88% 243Am and 12% 241Am was used as the source for the preparation of the spike material. The certified value of 5.696 (11) nmol g−1 for the amount content of 243Am and 0.136138 (54) for the n(241Am)/n(243Am) amount ratio were assigned. The assigned values from mass spectrometry were confirmed by alpha-particle spectrometry, alpha-particle counting at a defined solid angle, and high-resolution gamma-ray spectrometry. Furthermore, an external validation of the certified values was obtained from the results of an interlaboratory comparison exercise, using this americium reference solution as the test sample.