1.1 本试验方法描述了使用热电离质谱仪（TIMS）通过改进的总蒸发（MTE）方法测定硝酸盐溶液中铀材料的同位素量比。 1.2 在确定 235 用户/ 238 MTE的U主要同位素量比类似于（“经典”）总蒸发（TE）方法，如 第1672页 然而，在MTE方法中，蒸发过程定期中断，以允许测量和随后对峰尾的背景进行校正，执行二次电子倍增器（SEM）检测器相对于法拉第杯的内部校准，峰对中，以及离子源重新聚焦。 在测量过程中定期进行这些校准和校正，提高精度，并显著降低小同位素量比的不确定性 234 用户/ 238 U和 236 用户/ 238 与TE方法相比。 1.3 原则上，MTE方法可以产生主要的同位素量比，而不需要质量分馏校正。然而，根据测量条件，在样品炮塔之间观察到微小的变化。因此，建议根据经认证的参考材料的测量值进行小的校正，以提高一致性。 质量分馏修正系数的不确定性通常包括单位。 1.4 单位- 以国际单位制表示的值应视为标准值。SI单位后括号中给出的值仅供参考，不视为标准值。 1.5 本标准并不旨在解决与其使用相关的所有安全问题（如有）。本标准的使用者有责任在使用前建立适当的安全、健康和环境实践，并确定监管限制的适用性。 1. 6. 本国际标准是根据世界贸易组织技术性贸易壁垒（TBT）委员会发布的《国际标准、指南和建议制定原则决定》中确立的国际公认标准化原则制定的。 =====意义和用途====== 5.1 铀材料被用作某些类型核反应堆的燃料。为了适合用作核燃料，起始材料应符合某些规范，如规范中所述 C753型 , C776年 , C787型 , C833 , C967年 , 996年 和 2008年 ，或买方规定。铀材料的同位素量比可以按照该测试方法通过质谱法测量，以确保它们符合规范。 5.2 MTE方法可用于各种样品尺寸，即使在含铀量低至20µg的样品中也是如此。如果铀样品为六氟化铀，则必须将其转化为硝酸铀溶液，以便通过MTE法进行测量。MTE加载溶液的浓度必须在1mg/g至6mg/g的范围内 µg至6µg铀。 强烈建议每根细丝的最小装载量为3µg铀。这是需要有足够和稳定的离子信号，特别是对于两种次要同位素( 234 U和 236 U） 因此能够使用 234 测量期间的U离子束信号。 5.3 迄今为止，MTE方法的仪器功能仅在TRITON上实现™ TIMS仪器。 5. 因此，本测试方法中的所有测量参数建议均针对TRITON仪器进行了规定。其他TIMS仪器（例如，IsotopX和Nu仪器）的制造商已表示计划对其仪器进行必要的修改，以使用MTE方法。 5.4 这里描述的MTE方法也可以扩展到铀以外的元素的测量。注意，已经对钚和钙实施了MTE方法。

1.1 This test method describes the determination of the isotope amount ratios of uranium material as nitrate solutions by the modified total evaporation (MTE) method using a thermal ionization mass spectrometer (TIMS) instrument. 1.2 The analytical performance in the determination of the 235 U/ 238 U major isotope amount ratio by MTE is similar to the (“classical”) total evaporation (TE) method as described in C1672 . However, in the MTE method, the evaporation process is interrupted on a regular basis to allow measurements and subsequent corrections for background from peak tailing, perform internal calibration of a secondary electron multiplier (SEM) detector versus the Faraday cups, peak centering, and ion source refocusing. Performing these calibrations and corrections on a regular basis during the measurement, improves precision, and significantly reduces uncertainties for the minor isotope amount ratios 234 U/ 238 U and 236 U/ 238 U as compared to the TE method. 1.3 In principle, the MTE method may yield major isotope amount ratios without the need for mass fractionation correction. However, depending on the measurement conditions, small variations are observed between sample turrets. Therefore, a small correction based on measurements of a certified reference material is recommended to improve consistency. The uncertainty around the mass fractionation correction factor usually includes unity. 1.4 Units— The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided for information only and are not considered standard. 1.5 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. 1.6 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. ====== Significance And Use ====== 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 isotope amount ratios of uranium material can be measured by mass spectrometry following this test method to ensure that they meet the specification. 5.2 The MTE method can be used for a wide range of sample sizes even in samples containing as low as 20 µg of uranium. If the uranium sample is in the form of uranium hexafluoride, it has to be converted into a uranium nitrate solution for measurement by the MTE method. The concentration of the loading solution for MTE has to be in the range of 1 mg/g to 6 mg/g to allow a sample loading of 2 µg to 6 µg of uranium. A minimum loading of 3 µg uranium per filament is strongly recommended. This is needed to have a sufficient and stable ion signal especially for the two minor isotopes ( 234 U and 236 U) thus enabling the internal calibration of SEM versus the Faraday cups using the 234 U ion beam signal during the measurement. 5.3 Until now, the instrument capabilities for the MTE method have only been implemented on the TRITON™ TIMS instrument. 5 Therefore, all recommendations for measurement parameters in this test method are specified for the TRITON instrument. The manufacturers of other TIMS instruments (for example, IsotopX and Nu Instruments) have indicated plans to implement the modifications needed in their instruments to use the MTE method. 5.4 The MTE method described here can also be extended to measurement of elements other than uranium. Note that the MTE method has already been implemented for plutonium and calcium.