Sensitive redox speciation of neptunium by CE–ICP–MS


Capillary electrophoresis (CE) was used to separate the neptunium oxidation states Np(IV) and Np(V), which are the only oxidation states of Np that are stable under environmental conditions. The CE setup was coupled to an inductively coupled plasma mass spectrometer (Agilent 7500ce) using a Mira Mist CE nebulizer and a Scott-type spray chamber. The combination of the separation capacity of CE with the detection sensitivity of inductively coupled plasma mass spectrometry (ICP-MS) allows identification and quantification of Np(IV) and Np(V) at the trace levels expected in the far field of a nuclear waste repository. Limits of detection of 1 × 10-9 and 5 × 10-10 mol L-1 for Np(IV) and Np(V), respectively, were achieved, with a linear range from 10-9 to 10-6 mol L-1. The method was applied to study the redox speciation of the Np remaining in solution after interaction of 5 × 10-7 mol L-1 Np(V) with Opalinus Clay. Under mildly oxidizing conditions, a Np sorption of 31% was found, with all the Np remaining in solution being Np(V). A second sorption experiment performed in the presence of Fe2+ led to complete sorption of the Np onto the clay. After desorption with HClO4, a mixture of Np(IV) and Np(V) was found in solution by CE–ICP–MS, indicating that some of the sorbed Np had been reduced to Np(IV) by Fe2+.

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The authors are grateful to the Deutsche Forschungsgemeinschaft (DFG graduate school 826) and to BMWi (contracts 02E10166 and 02E10981) for funding. Support from the German Academic Exchange Service (DAAD-RISE program) for K.M. is also gratefully acknowledged.

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Correspondence to Nils Stöbener.

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Published in the topical collection Modern Aspects on Elemental Speciation with guest editors Thorsten Hoffmann and Klaus G. Heumann.

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Stöbener, N., Amayri, S., Gehl, A. et al. Sensitive redox speciation of neptunium by CE–ICP–MS. Anal Bioanal Chem 404, 2143–2150 (2012).

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  • Capillary electrophoresis
  • Inductively coupled plasma mass spectrometry
  • Neptunium
  • Actinides
  • Speciation
  • “Hyphenated” techniques
  • Mira Mist CE nebulizer
  • Opalinus Clay
  • Sorption