Accurate determination of ²³⁴U/²³⁸U and ²³⁰Th/²³²Th for U-Th disequilibria studies by MC-ICP-MS with simple bracketing

Accurate determination methods for ²³⁴U/²³⁸U and ²³⁰Th/²³²Th using multiple collector (MC) ICP-MS with a single secondary electron multiplier through a retarding potential quadrupole filter (RPQ-SEM) have been developed. The RPQ-SEM yield and the instrumental mass discrimination were simultaneously corrected by the simple bracketing method which utilizes accurately determined standard solutions measured by TIMS. For the bracketing and as an evaluation of the precision of the method, four standard solutions, NBL-145 U and U-m for U, and Th-I and Th-II for Th, were prepared, and NBL-145 U and Th-I were used as bracketing standards. The U and Th amounts of 0.5-21 ng and 0.6-21 ng used in U-m and Th-II analyses gave intermediate precisions (2RSD) of 0.85-0.06 and 0.88-0.19%, respectively, and vary from TIMS measurements by <0.35 and <1.1%, respectively, which agrees with error values in TIMS analyses. To achieve similar intermediate precisions of 0.2 and 0.6% (2RSD) in the U and Th isotope analyses to that of TIMS, 3 and 2 ng of U and Th are sufficient. When only the isotope analysis is performed, the required amounts of U and Th can be reduced by half. The precision of the isotope analyses, as well as the concentration determinations, were further confirmed by analyses of U and Th separated from the GSJ standard basalt, JB-2.