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

• Published in: Analytica chimica acta 2018-02, Vol.1000, p.144
• Main Authors: Wysocka, Irena, Vassileva, Emilia
• Format: Article
• Language: English
• Subjects: Acetic acid; Acidification; Ammonium; Ammonium acetate; Calibration; Chelation; Chemical analysis; Dilution; Functional groups; Inductively coupled plasma mass spectrometry; Mass spectrometry; Mass spectroscopy; pH effects; Pretreatment; Reference materials; Seawater; Separation; Thorium; Uncertainty; Water analysis
• ISSN: 0003-2670; 1873-4324

This article describes the development and validation of an analytical procedure for the matrix separation, preconcentration and determination of sub-ng kg-1 levels 232Thin 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 < 2 seawater samples were mixed online with the ammonium acetate buffer (pH of 6.0 ± 0.2) before loading on the column containing resin with iminodiacetic and ethylenediaminetriacetic functional groups. At this pH, 232Th was quantitatively retained on the resin, demonstrating a good affinity (selectivity) for Thin 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. 1SO/1EC17025 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 232Thwas 0.005 ng kg-1. The developed method was successfully applied to the determination of 232Thmass 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.