Determination of Cl, Br and I in soils by ICP-MS: microwave-assisted wet partial digestion using H2O2 in an ultra-high pressure system

Abstract

In this study, we proposed a method for halogen determination using inductively coupled plasma mass spectrometry (ICP-MS) after partial digestion of soils using only H₂O₂ as a reagent for organic matrix decomposition. It was performed by microwave-assisted wet digestion (MAWD) using a high-pressure single reaction chamber (SRC) system and found to be feasible for the digestion of soils with intermediate or high organic matter (OM) content. The following parameters were evaluated for the MAWD-SRC method: the OM content in soils (50 and 98%), temperature of microwave heating (from 200 to 270 °C), and volume of H₂O₂ (5 or 10 mL). Initially, soils were digested by the MIC method to obtain the reference values. Using the optimized conditions, 50 and 25 mg of soils containing 50 and 98% of OM, respectively, were efficiently digested using only 5 mL of 50% H₂O₂ solution and a maximum temperature of 250 °C. For I, results were affected when digestion temperatures lower than 250 °C were used probably due to the formation of I₂ during nebulization by ICP-MS. The agreement between the results was in the range from 95 to 106% for Cl, Br, and I after digestion of both types of soils. Relatively low limits of quantification (LOQs), especially for Br and I, were obtained, i.e. 132 and 264 μg g⁻¹ for Cl, 0.06 and 0.13 μg g⁻¹ for Br, and 0.03 and 0.05 μg g⁻¹ for I, considering 50 and 25 mg of soil, respectively. Accuracy was evaluated by comparing the results with those obtained via combustion followed by the pyrohydrolysis method and by the analysis of a certified reference material (CRM). No statistical difference was observed between the results obtained using both methods and the values of CRM. The MAWD-SRC method that uses only H₂O₂ has been considered as an alternative for the effective digestion of OM in soils with an intermediate or high OM for the further determination of Cl, Br, and I by ICP-MS.