Metrological Triangle for Measurements of Isotope Amount Ratios of Silver, Indium, and Antimony Using Multicollector-Inductively Coupled Plasma Mass Spectrometry: The 21st Century Harvard Method

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Abstract
The calibration of measurements of the isotope amount ratio using a log−linear regression method with multicollector-inductively coupled plasma mass spectrometry (MC-ICPMS) is the latest development in isotope amount ratio metrology. This calibration method, however, is often met with criticism. In this work, we evaluate the robustness of this calibration strategy wherein measurements of antimony and indium isotope amount ratios are calibrated against the isotope amount ratio of silver, despite the significant difference in their atomic mass. In addition, a metrological triangle comprising Ag−Sb−In measurement results is constructed from three pairs of interelemental isotope amount ratio calibrations: N(121Sb)/N(123Sb) from the N(107Ag)/N(109Ag) of NIST SRM 978a measurement standard, N(113In)/N(115In) from the N(121Sb)/N(123Sb), and then calibration of N(107Ag)/N(109Ag) from the obtained N(113In)/N(115In) measured ratio values to verify consistency with the known N(107Ag)/N(109Ag). This calibration method revives one of the salient features of the classical “Harvard method”, a network of relationships among the isotopic compositions of various elements. The atomic weights of antimony and indium, reported here for the first time using MC-ICPMS, 121.7590(22)k=2 and 114.818 27(35)k=2, are in good agreement with their current standard atomic weights. In addition, this study provides the first calibrated mass spectrometric isotope amount ratio measurements for indium.

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