Determinação de elementos do grupo da platina em carvão com alto teor de cinzas por ICP-MS

Abstract

In this work a method for the digestion of high ash content coal (>30%) using a high pressure asher (HPA) system for further determination of platinum group elements (PGEs) by inductively coupled plasma mass spectrometry (ICP-MS) was developed. The efficiency of coal digestion with was evaluated by the residual carbon content (RCC), being evaluated solutions of HNO3 + HCl (3+1, v/v) and (1+1, v/v). About 500 mg of coal were digested in HPA system using 6 mL of HNO3 + HCl (3+1, v/v) and time of heating of 20, 40 and 60 min at 320 ºC. Due to the very low concentrations of these elements in the coal sample used in this work, the coal sample was enriched with PGEs and digested at the same conditions. To evaluate the accuracy, coal was digested by microwave induced combustion (MIC) and also by microwave-assisted wet digestion (MAWD). The MIC procedure was evaluated using 6 mL of HNO3 + HCl (3+1, v/v), (1+1, v/v) and (1+3, v/v) and pellets with 500 mg of coal. The addition of 300 mg of NH4Cl to the coal pellets was also evaluated by the MIC. For the MAWD were used 150 mg of coal and 4.5 mL of 14.4 mol L-1 HNO3, 1.0 mL of 12 mol L-1 HCl and 1.5 mL of 23 mol L-1 HF. During the development of this work, a study of interferences for PGEs (Cu, Hf, Rb, Sr, Y and Zn, possible interferences for 193Ir, 105Pd, 195Pt, 103Rh and 101Ru) was performed. The determination of Os was difficult due to the formation of OsO4 during the digestion process, remaining in the nebulizer chamber of the ICP-MS. Generally, the digestion of enriched coal in HPA system presented satisfactory results when compared with MIC and MAWD. It was possible to digest up to 500 mg of coal with 6 mL of HNO3 + HCl (3+1, v/v) in HPA system. Recoveries between 100 and 127% were obtained for PGEs, except for Os, where errors over 350% were observed. RCC values in coal digested by HPA system were less than 40%. Limits of quantification were between 0.19 and 3.91 ng g-1.