DETERMINATION OF TRACE ELEMENTS IN COAL BY SOLID SAMPLING ELECTROTHERMAL VAPORIZATION-INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY

Coal combustion is a potential source of trace element emission to the atmosphere, including As, Sb, Se and Te. The determination of these elements in low concentrations is required and inductively coupled plasma mass spectrometry (ICP-MS) is commonly used. However, a sample preparation step is required to bring the analytes into a solution and assure a quantitative determination. Nevertheless, the difficulty to decompose coal and the low concentration of those analytes make this step a challenge. To overcome limitations of sample preparation for hard matrices as coal, electrothermal vaporization (ETV) has been proposed for sample introduction in ICP-MS. Several advantages can be highlighted, such as minimum sample pretreatment, separation between analytes and sample matrix and high sample introduction efficiency. In this work, a method for the determination of As, Sb, Se and Te in coal by ETV-ICP-MS is proposed. Samples were vaporized using an electrothermal vaporization system (ETV-4000c, Spectral Systems, Germany) which was coupled to an inductively coupled plasma mass spectrometer (Elan DRC II, Perkin-Elmer Sciex, Canada) through a PTFE tube (1 m long, 6 mm inner diameter). The furnace temperature program, especially the pyrolysis temperature, in the presence or not of oxygen was evaluated. Moreover, the evaluation of modifiers such as Pd and Ir was performed. The determination of As, Sb, Se and Te in coal was performed by weighing the solid sample on the graphite platform of the ETV system. When reference solutions or modifier solutions were used, they were added and dried on the platform to avoid water vapor condensation inside the ETV system. Then, the graphite platform was introduced into the ETV furnace with a pair of tweezers and the heating program was initiated. Calibration was possible using aqueous standard solutions. An oxygen flow-rate was used during the pyrolysis step in order to remove coal organic matrix prior to the vaporization step. By using Ir the pyrolysis and vaporization temperatures were established at 1000 °C and 2300 °C, respectively. Under the optimum operating conditions, the LOD for As, Sb, Se and Te was between 0.001 to 0.07 µg/g. Accuracy was evaluated by using spike recoveries, certified reference materials and by comparison with results obtained by ICP-MS after MAWD. The proposed method does not require sample pretreatment, avoiding the use of large amounts of concentrated acids and reducing the analysis time.