1.1This 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.

1.2The analytical performance in the determination of the ²³⁵U/²³⁸U major isotope amount ratio by MTE is similar to the (“classical”) total evaporation (TE) method as described in Test Method C1672. However, in the MTE method, the evaporation process is interrupted on a regular basis to allow measurements and subsequent corrections for background form 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 ²³⁴U/²³⁸U and ²³⁶U/²³⁸U as compared to the TE method.

1.3In principle, the MTE method yields major isotope amount ratios without the need for mass fractionation correction. However, depending on the measurement conditions, small variations are seen among 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.

1.4Units—The values stated in SI units are to be regarded as the standard. When non-SI units are provided, they are for information only.

1.5This 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 and health practices and determine the applicability of regulatory limitations prior to use.