A single-step microwave-assisted acid extraction of total sulphur in coal samples followed by ICP-OES determination

A rapid microwave-assisted acid extraction (MW-AAE) procedure using a dilute [HNO₃-H₂O₂] mixture followed by inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis has been developed for the determination of total sulphur in coal samples. Parameters affecting the MW-AAE such as extractant concentration, microwave temperature, extraction time, coal amount and HNO₃:H₂O₂ volume ratio were optimized by using a certified reference material (SARM 20) in order to attain total sulphur quantitative recoveries. The optimum conditions for the extraction procedure were found to be 180 °C, 5 min, 3 mol L^-1, 3 mol L^-1, 0.05 g and 2:1 for microwave temperature, extraction time, [HNO₃], [H₂O₂], coal amount and HNO₃:H₂O₂ volume ratio, respectively. Under optimum conditions, the accuracy of the MW-AAE method was examined by analyzing three coal certified reference materials (SARM 18, 19 and 20) and quantitative recoveries (83-102%) were achieved. Additionally, the precision of the proposed method, expressed in terms of relative standard deviation (n = 15), was satisfactory (≤3%). The method limit of detection (0.026 μg g^-1) and method limit of quantification (0.087 μg g^-1) for sulphur analysis were relatively good as compared to work reported in the literature.