This work describes the preparation, characterization and certification of a set of uranium isotope reference materials ranging from 0.5 to 20.0 % of 235 U in mass. The most important concepts of metrology in chemical measurements were applied so that the certified quantities in these materials could be directly traceable to the International System of Units (SI). As a consequence of this approach, these materials can be used in the instruments calibration, estimation of measurement uncertainty, method validation, assessment of performance of analysts, quality control routines and interlaboratory comparison programmes. The most advanced methods and techniques in mass spectrometry, that is, gas source mass spectrometry (GSMS), thermal ionisation mass spectrometry (TIMS) and inductively coupled plasma mass spectrometry (ICPMS) were investigated to identify which are the dominant components in the uncertainty and to quantify its contribution to the final value of the measurement uncertainty of the isotopic ratio. The results obtained were then compared to verify which are the methods and techniques associated to the lowest measurement uncertainty values. The isotope amount ratio n(235 U)/n(238 U) was certified in the materials produced to expanded uncertainties ranging from 0.02 to 0.10 % and the ratios n(234 U)/n(238 U) and n(236 U)/n(238 U), to uncertainties ranging from 0.03 to 2.20 %. These values fully comply to the requirements of the isotopic characterization of nuclear fuel as well as the analysis of environmental samples for nuclear safeguards. n(236 U)/n(238 U 5.3 Certificação dos materiais de referência isotópicos 5.3.1 Valores certificados de razão isotópica 5.3.2 Valores certificados de fração molar, fração mássica e massa molar 5.4 Acondicionamento dos materiais de referência isotópicos 5.5 Estabilidade dos materiais de referência isotópicos 5.6 Homogeneidade dos materiais de referência isotópicos 5.7 Certificados dos materiais de referência isotópicos de urânio 6 CONCLUSÕES APÊNDICES Certificados dos materiais de referência isotópicos Procedimento de análise razão isotópica pela técnica de GSMS Procedimento de análise razão isotópica pela técnica de TIMS por meio do método tradicional Procedimento de analise razão isotópica pela técnica de TIMS por meio do método da evaporacao total da amostra
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