²³⁸U and ²³²Th are ubiquitous in the earth’s crust and interest in their concentration in various fields including environmental monitoring, human exposure, disposal of radioactive waste, etc. The analysis of ²³⁸U and ²³²Th can be performed by various methods such as gamma spectrometry, alpha spectrometry, liquid scintillation counting(LSC) and inductively coupled plasma-mass spectrometry(ICP-MS). ICP-MS is universal and sensitive analytical method for rapid, accurate, and precise determination of ²³⁸U and ²³²Th. In ICP-MS analysis, internal standard is used to correct signal fluctuation by matrix effect or instrumental drift. In order to utilize the internal standard method, the same amount of internal standard is added to the sample and standard solution for calibration curve. The internal standard have similar mass with analyte and must absent in the sample.

In this study, an experiment was conducted to select a suitable internal standard for determination of ²³⁸U and ²³²Th in environmental certified reference material(CRM). ¹⁰³Rh, ¹¹⁵In, ¹⁸⁷Re, ²⁰⁵Tl, ²⁰⁹Bi were selected as internal standard. Several types of CRM were measured by ICP-MS to identify the signal intensities of five internal standards originally contained in CRM and the variation of the recovery percent of the nuclide after adding internal standard was predicted. In order to determine ²³⁸U and ²³²Th in CRM, a quantitative analysis was conducted using five internal standards, and the recovery percent measured by ICP-MS was compared with the predicted value. Finally, ²³⁸U and ²³²Th concentrations were compared with the reference value.