Determination of 129I in soil samples by DRC-ICP-MS

Abstract

The use of ICP-MS for determination of ¹²⁹I in environmental samples is complicated by interference from ¹²⁹Xe⁺ originating from Xe present as an impurity in Ar plasma gas and by ¹²⁷IH₂⁺ derived from ¹²⁷I in samples. Collision cell and reaction cell techniques have been used to reduce such interference. Here, we used ICP-MS with the dynamic reaction cell (DRC) and axial field technology (AFT) to improve ¹²⁹I determination. The response characteristics of DRC-ICP-MS to test solutions with various ¹²⁹I and ¹²⁷I concentrations were investigated after instrument optimization. We eliminated ¹²⁹Xe⁺ interference by using O₂ as the reaction gas, and we effectively reduced ¹²⁷IH₂⁺ interference by applying a negative AFT voltage and removing traces of H₂ and H₂O from the reaction gas. Consequently, the background count ratio (mass 129/mass 127) decreased to the 10⁻⁸ level. However, the relationship between the signal at mass 129 and ¹²⁹I concentration was nonlinear, and we attributed the nonlinearity to sensitivity degradation due to abundant ¹²⁷I in the matrix. Using a Rh internal standard, we corrected the sensitivity degradation and obtained a linear relationship between mass 129/mass 103 and ¹²⁹I concentration. The detection limit for ¹²⁹I in a 1 mg mL^{−1 127}I matrix was 15.2 pg mL⁻¹, which is equivalent to an ¹²⁹I/¹²⁷I ratio of 1.5 × 10⁻⁸. The developed method was used to determine ¹²⁹I in soil samples obtained from the land surrounding the nuclear fuel reprocessing plant in Tokai, Japan. The measured ¹²⁹I/¹²⁷I ratios (2.6 × 10⁻⁸ to 4.7 × 10⁻⁷) agreed with the ratios determined by tandetron accelerator mass spectrometry.