Silicon isotope ratio measurements by inductively coupled plasma tandem mass spectrometry for alteration studies of nuclear waste glasses

• Published in: Analytica chimica acta 2017-02, Vol.954, p.68-76
• Main Authors: Gourgiotis, Alkiviadis, Ducasse, Thomas, Barker, Evelyne, Jollivet, Patrick, Gin, Stéphane, Bassot, Sylvain, Cazala, Charlotte
• Format: Article
• Language: English
• Subjects: Abundance; Aqueous solutions; Dilution; Dissolution; Fractionation; Fuel reprocessing; Geologic formations; Glass; ICP-MS/MS; ICP-QQQ; Inductively coupled plasma mass spectrometry; Integration; Isotopes; Isotopic enrichment; Mass spectrometry; Mass spectroscopy; Nuclear fuel reprocessing; Nuclear fuels; Nuclear glasses; Optimization; Oxygen isotopes; Physics; Quadrupoles; Radioactive wastes; Reproducibility; Sensitivity; Silicon; Silicon dioxide; Silicon isotope ratios; Silicon isotopes; Species; Tandem mass spectrometry; Triple quadrupole; Waste disposal
• ISSN: 0003-2670; 1873-4324
• DOI: 10.1016/j.aca.2016.11.063

High-level, long-lived nuclear waste arising from spent fuel reprocessing is vitrified in silicate glasses for final disposal in deep geologic formations. In order to better understand the mechanisms driving glass dissolution, glass alteration studies, based on silicon isotope ratio monitoring of 29Si-doped aqueous solutions, were carried out in laboratories. This work explores the capabilities of the new type of quadrupole-based ICP-MS, the Agilent 8800 tandem quadrupole ICP-MS/MS, for accurate silicon isotope ratio determination for alteration studies of nuclear waste glasses. In order to avoid silicon polyatomic interferences, a new analytical method was developed using O2 as the reaction gas in the Octopole Reaction System (ORS), and silicon isotopes were measured in mass-shift mode. A careful analysis of the potential polyatomic interferences on SiO+ and SiO2+ ion species was performed, and we found that SiO+ ion species suffer from important polyatomic interferences coming from the matrix of sample and standard solutions (0.5M HNO3). For SiO2+, no interferences were detected, and thus, these ion species were chosen for silicon isotope ratio determination. A number of key settings for accurate isotope ratio analysis like, detector dead time, integration time, number of sweeps, wait time offset, memory blank and instrumental mass fractionation, were considered and optimized. Particular attention was paid to the optimization of abundance sensitivity of the quadrupole mass filter before the ORS. We showed that poor abundance sensitivity leads to a significant shift of the data away from the Exponential Mass Fractionation Law (EMFL) due to the spectral overlaps of silicon isotopes combined with different oxygen isotopes (i.e. 28Si16O18O+, 30Si16O16O+). The developed method was validated by measuring a series of reference solutions with different 29Si enrichment. Isotope ratio trueness, uncertainty and repeatability were found to be