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 spectrometr y (DF-ICP-MS) with a shielded torch under hot plasma conditions. The measur ements of the relative oxide ion intensities by DF-ICP-MS were performed us ing different nebulizers [Meinhard nebulizer, microconcentric MicroMist neb ulizer and a direct injection high-efficiency nebulizer (DIHEN)] for the so lution introduction of aqueous standard solutions into the ICP. The highest oxide ion formation rate was observed for thorium in DF-ICP-MS under hot p lasma conditions with a shielded torch (ThO+/Th+ = 0.13, 0.26, and 0.41 for Meinhard nebulizer. MicroMist nebulizer and DIHEN, respectively). The appl ication 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 n ebulizer 1.6-13 and for DIHEN 1.7-2.8, compared to the original configurati on without plasma shielding. The direct injection of sample solution by DIH EN-DF-ICP-MS results in a relatively high oxide ion formation rate (e.g. Pu O+/Pu+ of about 0.16 and 0.35 without and with a shielded torch, respective ly). For the relative oxide ion intensities using the Meinhard and the micr oconcentric MicroMist nebulizers in DF-ICP-MS with shielded torch a decreas e 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 o f these oxides allows for the estimation of unknown bond energies for AmO a nd 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 analytica l purposes, e.g. the oxide ions are applied as analyte ions for interferenc e-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.