Study of the application of air-water flow injection inductively coupled plasma mass spectrometry for the determination of calcium in steels

Abstract

This paper describes a methodology for the determination of Ca, an important element in the process of steel making. Determination of Ca is typically problematic when using ICP-MS because of the ⁴⁰Ar interference on the most abundant Ca isotope. In the present study the use of the ⁴⁴Ca isotope for ICP-MS measurement was considered. To dissolve the samples, a microwave digestion system was applied to guarantee the total dissolution of the Ca compounds, thus minimizing the sample preparation time, the amounts of reagents used and the risk of contamination. Carbon and Si, normally present in steel samples, were eliminated by treatment with H₂SO₄ and HF to avoid ¹²C¹⁶O₂ and ²⁸Si¹⁶O isobaric interferences on the ⁴⁴Ca isotope. After matrix removal by the use of mercury cathode electrolysis, air–water FI-ICP-MS was used for the determination of Ca. Scandium, with an ionization potential and mass number close to those of Ca, was used as an internal standard for quantitative measurements. The operating parameters for air-water FI-ICP-MS were optimized with respect to signal peak height intensities, stability and matrix effect. The detection limit obtained was 0.6 µg g^–1 and the RSD value, calculated from ten replicates of a 200 µl injection of a 50 µg l^–1 Ca solution spiked with 2 µg l^–1 of Sc, was 0.7%. Data are presented for four certified reference steel materials: JSS 169–5 and 171–5, from Japanese Standards of Iron and Steel; and ECRMs 195–1 (Cr–Ni–steel) and 096 (low-S, low-Ca steel), from the Bureau of Analysed Samples.