ASTM C1871-2018 通过使用热电离质谱计的双同位素稀释剂法测定铀同位素组成的方法 Standard Test Method for Determination of Uranium Isotopic Composition by the Double Spike Method Using a Thermal Ionization Mass Spectrometer

1.1 This test method describes the determination of the isotope amount ratios of uranium material as nitrate solutions by the double spike (DS) method using a thermal ionization mass spectrometer (TIMS) instrument. 1.2 The analytical performance in the determination of the 235U/238U major isotope amount ratio by the DS method is five to ten times better in terms of the internal and external reproducibility compared to the (“classical”) total evaporation (TE) method as described in Test Method C1672 and the “modified total evaporation” (MTE) as described in Test Method C1832. This is due to the use of an internal rather than external mass fractionation correction by using a double spike material with a known or certified 233U/236U isotope ratio, which is mixed with the sample prior to the measurement, either during the sample preparation or directly on the TIMS filament. 1.3 The DS method cannot be applied for the determination of the 236U/238U minor isotope amount ratio, and is also not recommended for the determination of the 234U/238U minor isotope amount ratio. 1.4 In case the uranium amount concentration of the double spike is known or certified, the uranium amount concentration of the sample can be determined using the isotope dilution mass spectrometry (IDMS) method as described in Test Method C1672, by blending the sample gravimetrically with the double spike and performing a DS measurement. 1.5 An external mass fractionation correction by measurements of a certified reference material loaded on different filaments and measured in the same measurement sequence, as recommended for TE and required for MTE measurements, is not necessary for the DS method. However, for quality control (QC) purposes it is recommended to perform DS measurements of low enriched or natural uranium isotopic reference materials on a regular basis. 1.6 The DS method can only be applied to uranium samples with rel