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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 below 10^–5 and ²³⁶U/U below 5 × 10^–4, 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.

5.1 Uranium material is used as a fuel in certain types of nuclear reactors. To be suitable for use as nuclear fuel, the starting material shall meet certain specifications such as those described in Specifications C753, C776, C787, C833, C967, C996, and C1008, or as specified by the purchaser. The ²³⁵U/²³⁸U isotope amount ratios and the amount content of uranium material can be measured by mass spectrometry following this test method to ensure that they meet the specification.

5.2 The double spike method has been used for studies of uranium fractionation effects in isotope geochemistry and cosmochemistry, for uranium source attribution in nuclear forensics and for investigation of conversion or sampling processes in nuclear industry and nuclear safeguards (7-11). Most recently, the double spike method has been used for the validation of the Cristallini sampling method of UF₆ (12 and 13). The double spike method can be used for a wide range of sample sizes even in samples containing as low as 50 μg of uranium. The concentration of the loading solution for the DS method has to be in the range of 1 to 6 mg/g to allow a sample loading of 4 to 6 μg of uranium. A minimum loading of 4 μg uranium per filament is recommended.

5.3 The measurement of ²³⁶U/²³⁸U ratios using this method is not possible due to the large isobaric interference from the ²³⁶U ion beam of the double spike onto the ²³⁶U ion beam from the sample (>50.000 times for close to natural material, for example, like IRMM-184).

5.4 The application of the double spike method for measurements of ²³⁵U/²³⁸U ratio is limited by the isobaric interference between the ²³⁶U from the double spike material and the ²³⁶U contained in the sample. As a consequence, the method is not suitable for samples which contain significant amounts of ²³⁶U due to prior neutron capture from ²³⁵U in the predecessor materials. For samples with ²³⁶U/²³⁸U ratios higher than about 10^–6, the double spike method should be applied with care for the isobaric correction. For an appropriate isobaric correction, the ²³⁶U/²³⁸U ratios of the samples should be determined separately using a suitable measurement method, for example, the modified total evaporation MTE method (Test Method C1832, Ref (5) and (6)).

5.5 The measurement of ²³⁴U/²³⁸U ratios using this method is very limited in the analytical performance due to the isobaric interference of the ²³⁴U from the double spike with the ²³⁴U from the sample (range from 5 to 15 %). The correction algorithms are presented in 14.3, but statements for precision and bias are not given. Other methods like MTE (Test Method C1832, Ref (5) and (6)) are better suited and more reliable for measurements of ²³⁴U/²³⁸U ratios.

5.6 The DS method described here can also be extended to measurement of elements other than uranium, if a suitable double spike material is available.