Determination of Se and Te in coal at ultra-trace levels by ICP-MS after microwave-induced combustion

Abstract

A method for ultra-trace determination of Se and Te in coal by inductively coupled plasma mass spectrometry (ICP-MS) was developed. Samples (up to 500 mg) were prepared by microwave-induced combustion (MIC) in a closed system pressurized with 20 bar of oxygen. Inorganic acids or their mixtures were evaluated as absorbing solution. Using HNO₃ + HCl (1 + 1), results obtained for Se and Te after sample preparation by MIC agreed with those obtained by reference methods, microwave-assisted wet digestion (MAWD) using concentrated HNO₃ and HF, and solid sampling electrothermal vaporization inductively coupled plasma mass spectrometry (SS-ETV-ICP-MS), and also with a certified reference material of coal (NIST 1632c) for Se. Very low limits of quantification (LOQ) were obtained: 0.002 and 0.007 mg kg⁻¹ for Se and Te, respectively. Major elements were determined in final solutions obtained by MAWD and MIC, and their concentration in solutions of MIC was negligible (<10 μg L⁻¹), minimizing the risk of interference in ICP-MS analysis. The proposed method was applied to samples of coal from the United Kingdom, with a wide range of inorganic composition (from 3 to 41% ash content), showing its robustness and feasibility for routine analysis. Se concentration ranged from 0.291 to 11.5 mg kg⁻¹, and Te was mainly found at ultra-trace levels (<0.007 to 0.11 mg kg⁻¹). The proposed method presents several advantages over the reference methods, such as the use of a higher sample mass, low LOQs and, an important aspect, no need of using HF.