Determination of europium isotope ratios in natural waters by MC-ICP-MS

Abstract

An analytical protocol for the accurate and precise determination of the europium isotope ratio in low level natural waters is presented for the first time by using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Parameters affecting the separation of Eu from the matrix by employing a single-step column separation with di(2-ethylhexyl) orthophosphoric acid (HDEHP) resin were investigated in detail. Matrix elements (e.g., alkali, alkali-earth and transition metals) were efficiently removed and Eu was quantitatively separated from other rare earth elements (REEs) by sequential elution using gradient HCl concentrations (from 0.25 to 0.75 mol L⁻¹). Fractionation of Eu isotopes in the column was not observed, since quantitative europium recovery (101.2 ± 0.5% mean and 1 SD, n = 3) was obtained (data based on 150 mL SLRS-6 river water CRM test portions spiked with 1.0 ng mL⁻¹ Eu standard solution). A combined standard-sample bracketing with internal normalization method was employed for the mass bias correction, wherein Gd was added to both the sample and standard as an internal standard. The developed method was validated by processing 0.02 ng mL⁻¹ Eu spiked river water CRM SLRS-6 (wherein Eu was removed first by using the above-mentioned column separation method) using the proposed method, and satisfying results (δ^151/153Eu value of 0.01 ± 0.02‰; 1 SD) relative to Eu standard solution were obtained. No significant differences were found in the δ^151/153Eu values in river water SLRS-6, drinking water AQUA-1 and collected groundwater samples from Brazil, which ranged from −0.07 ± 0.02 to 0.12 ± 0.24 (mean ± 1 SD, n = 5).