ASTM C1871-18a

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 relative isotope abundances ²³³U/U and ²³⁶U/U below 10^-5, the DS method is therefore mainly used for low enriched or close to natural uranium samples.

1.7 Units-The values stated in SI units are to be regarded as the standard. When no SI units are provided, the values are for information only.

1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.9 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.