Year
2019
Abstract
Certified reference material (CRM) standards characterized for the isotope ratios are used to calibrate the mass spectrometer instruments, to establish traceability of the isotopic measurements to the S. I. units and to support safeguards including material control and accountability 1. Compared to the mass spectrometer instruments used for certification of the uranium (U) and plutonium (Pu) CRMs, modern multi-collector instruments demonstrate large improvement in measurement capabilities. Analytical instrumentation and measurement techniques have improved so much so that the uncertainties in the CRM standards used for instrument calibration and for estimation of the small correction factors (like mass fractionation), dominate the overall uncertainty realized in an analytical laboratory. As the precision of modern mass spectrometric data for isotope ratios is an order of magnitude better than the certified uncertainty of the Pu and U isotopic standards, it is important to know at what levels the systematic bias between standards start to correct data away from the true values. This question becomes especially important when systematic biases in the range of 0.02% to 0.08%, smaller than the certified uncertainties of 0.1% for most CRM standards, are observed in multiple U and Pu CRMs from New Brunswick Laboratory (NBL)2. The results obtained on the sample is expected to be independent of the standard used for calibration of the instrument. Bias of one CRM in relation to other CRMs implies different correction factors and therefore different results depending on the standard used. Within the uncertainties realized by modern mass spectrometers and analytical methodologies, systematic biases between U and Pu CRMs have been identified. The magnitude of these biases and the implications of using them for instrument calibration are presented. References: 1. Srinivasan B. et al. (2009) J. Radioanal. Nucl. Chem., 282(3), 963; 2. Mathew K. J. et al. (2018) J. Radioanal. Nucl. Chem., 318(1), 395.