Direct quantification of femtogram per liter (fg L−1) level 90Sr in rainwater using thermal ionization mass spectrometry

Abstract

Thermal ionization mass spectrometry (TIMS) is a specialized technique that precisely measures isotopic ratios. This paper proposes the direct quantification of ⁹⁰Sr at femtogram per litter (fg L⁻¹) levels using isotope dilution (ID)-TIMS of preconcentrated samples. Although preconcentration is ineffective when measuring isotope ratios in classical TIMS, the magnification afforded by preconcentration improves the limit of detection (LOD) of ⁹⁰Sr in ID-TIMS. After fixing the isotope ratio between stable Sr isotopes and ⁹⁰Sr, the fg L⁻¹-level ⁹⁰Sr concentration was successfully obtained by the ID method (recovery R% = 100 ± 11.1%). The obtained LODs are improved by 10⁵ times from those of conventional radiometric analysis such as oxalic acid precipitation/low background gas flow counters, which depend on the amount of coexisting natural Sr. Under the optimized conditions, the LOD was 9.03 attograms per liter (ag L⁻¹), equivalent to a radioactivity concentration of 46.1 micro becquerels per liter (μBq L⁻¹). The sample volume (10 mL containing 10 μg L⁻¹ of natural Sr) roughly corresponds to a rainwater-level sample with 10⁴-fold preconcentration. Spike and recovery tests were conducted on rain water and related environmental waters (stagnant stock water collected from a swimming pool immediately after the Fukushima Daiichi Nuclear Power Plant accident). The ⁹⁰Sr levels were successfully quantified by the proposed method.