Determination of ultra-trace level of ²³²Th in seawater by ICP-SFMS after matrix separation and preconcentration

Highlights: • Analytical procedure for ²³²Th determination at ultra-trace level in seawater samples is completely validated. • Combination of automated pre-treatment seaFAST system and ID ICP-SFMS was used. • Validation is performed according to ISO-17025 and Eurachem guidelines. • The uncertainty budget is used as a tool for evaluation of main uncertainty contributors. • The results obtained in this study agreed with ²³²Th values published elsewhere. - Abstract: This article describes the development and validation of an analytical procedure for the matrix separation, preconcentration and determination of sub-ng kg⁻¹ levels ²³²Th in a small volume (20 mL) of seawater samples. The matrix separation and Th preconcentration was carried out using a commercially available ion-chelation system seaFAST-pico. The acidified to pH 232Th was quantitatively retained on the resin, demonstrating a good affinity (selectivity) for Th in seawater matrix. The element retained on the resin was eluted using only 0.2 mL of 1.8 M HNO₃ enabling to achieve high preconcentration factor. The pretreatment procedure, with two sample loading cycles (each one 10 mL), was accomplished in about 25 min. Determining ²³²Th mass fraction in seawater samples was based on isotope dilution inductively coupled plasma mass spectrometry (ID ICP-MS) method, which was considered as the most accurate calibration strategy for precise quantification of ²³²Th mass fraction in seawater samples. ISO/IEC17025 and Eurachem guidelines were followed to perform the validation of the developed in this study procedure. In the case of seawater samples with natural level of thorium the major contributions to the expanded uncertainty arose from the uncertainty associated with isotopic ratio measurements in the isotopically spiked sample, followed by the correction for procedural blank and the correction for mass discrimination effect. The estimated method detection limit for ²³²Th was 0.005 ng kg⁻¹. The developed method was successfully applied to the determination of ²³²Th mass fraction in seawater reference samples: IAEA-443, SLEW-3, NASS-4, NASS-6, and CASS-5. Although ²³²Th is not certified in any of the seawater reference materials, a good agreement was obtained between the results in this study and data published elsewhere.