Oxide ion formation of long-lived radionuclides in double-focusing sector field inductively coupled plasma mass spectrometry and their analytical applications

The oxide ion formation of the long-lived radionuclides Ra-226, Th-230, Np-237, U-239, Pu-239, and Am-241 was investigated in a mixed aqueous solution by double-focusing sector held inductively coupled plasma mass spectrometry (DF-ICP-MS) with a shielded torch under hot plasma conditions. The measurements of the relative oxide ion intensities by DF-ICP-MS were performed using different nebulizers [Meinhard nebulizer, microconcentric MicroMist nebulizer and a direct injection high-efficiency nebulizer (DIHEN)] for the solution introduction of aqueous standard solutions into the ICP. The highest oxide ion formation rate was observed for thorium in DF-ICP-MS under hot plasma conditions with a shielded torch (ThO+/Th+ = 0.13, 0.26, and 0.41 for Meinhard nebulizer. MicroMist nebulizer and DIHEN, respectively). The application of the shielded torch in DF-ICP-MS yielded an increase in oxide ion formation for the Meinhard nebulizer by a factor of 2.7-7, for MicroMist nebulizer 1.6-13 and for DIHEN 1.7-2.8, compared to the original configuration without plasma shielding. The direct injection of sample solution by DIHEN-DF-ICP-MS results in a relatively high oxide ion formation rate (e.g. PuO+/Pu+ of about 0.16 and 0.35 without and with a shielded torch, respectively). For the relative oxide ion intensities using the Meinhard and the microconcentric MicroMist nebulizers in DF-ICP-MS with shielded torch a decrease in the following sequence is observed: ThO+/Th+ > UO+/U+ > NpO+/Np+ > PuO+/Pu+ > AmO+/Am+ >> RaO+/Ra+. The observed correlation of measured relative oxide ion intensities (ThO+/Th+, PuO+/Pu+, and UO+/U+) and bond energies of these oxides allows for the estimation of unknown bond energies for AmO and NpO (730 and 670 kJ/mol, respectively). High oxide ion intensity of long-lived radionuclides in DF-ICP-MS could be used in some cases for analytical purposes, e.g. the oxide ions are applied as analyte ions for interference-free isotope analysis and thus for checking the analytical results using the atomic ions of the analyte. (Int J Mass Spectrom 202 (2000) 69-79) (C) 2000 Elsevier Science B.V.