A new series of uranium isotope reference materials for investigating the linearity of secondary electron multipliers in isotope mass spectrometry

摘要

A new series of gravimetrically prepared uranium isotope reference materials, the so-called IRMM-074 series, with the n(~(235)U)(~(238)U)isotope ratio held constant at unity and the n(233U)(238U) isotope ratios varying from 1.0 to 10~(-6) has been prepared and certified. This series is suited for calibration of sec_ondary electron multipliers used widely in isotope mass spectrometry, in particular for techniques such as thermal ionization mass spectrometry (TIMS), inductively coupled plasma mass spectrometry (ICPMS), accelerator mass spectrometry (AMS) and resonance ionization mass spectrometry (RIMS). The new IRMM-074 was prepared as a replacement for the already exhausted IRMM-072 predecessor series. Uranium materials with high isotopic enrichments of ~(233)U, ~(235)U and ~(238)U were purified using identical methods involving separation on anion and cation column followed by a precipitation as peroxide. The oxides were calcined to convert them to U_30_8 simultaneously, in an oven installed in a glove-box that provided a controlled low-humidity environment. The oxides of ~(235)U and ~(238)U were weighed and mixed with a mole ratio n(~(235)U)(~(238)U) = 1.0 and then dissolved. The 233U oxide was dissolved to form a separate solution with the same concentration and from this primary solution three dilutions were made by weighing. A weighed amount of the n(~(235)U)(~(238)U) solution and weighed amounts of the ~(233)U solutions were mixed in various proportions in order to achieve n(~(233)U)(~(238)U) isotope ratios varying from 1.0 to 10~(-6). The methods for the preparation, the mixing and the mixing calculations are described. The expanded uncertainties (coverage factor k = 2) of the certified isotope ratios for the IRMM-074 series are 0.015% for the n(~(235)U)(~(238)U) ) ratio and 0.025% for the n(~(233)U)(~(238)U) ratios, which constitutes an improvement compared to those of the predecessor IRMM-072 series. In addition, recent observations regarding the linearity response of secondary electron multipliers (SEMs) and suitable reference materials for investigating detector linearity are reviewed. Two measure_ment procedures for applying the IRMM-072 and IRMM-073 (diluted from the remaining fraction of IRMM-072) series as well as the new IRMM-074 series for assessing SEM linearity are suggested. The procedures are tailor-made for the specific instrumental characteristics of thermal ionization mass spectrometers (TIMS) and multiple-collector inductively coupled plasma mass spectrometers (MC-ICPMS) but can be adapted also for further types of isotope ratio mass spectrometers.