Rare earth element determination in heavy crude oil by USN-ICP-MS after digestion using a microwave-assisted single reaction chamber

Abstract

In this work a method for rare earth element (REE) determination by inductively coupled plasma mass spectrometry (ICP-MS) with an ultrasonic nebulizer (USN) was proposed after heavy crude oil digestion by microwave-assisted wet digestion (MAWD) using a single reaction chamber (SRC) system. Operational conditions of the MAWD-SRC method, such as sample mass (from 250 to 1000 mg), type and volume of digestion solutions, temperature achieved during digestion (from 200 to 270 °C) and microwave irradiation time (35, 40, 45 and 50 min) were investigated. Using optimized digestion conditions, the carbon concentration and residual acidity in digests were 2345 mg L⁻¹ and 14.6%, respectively. Since the acidity was low and dilution was not required after digestion by the MAWD-SRC method, lower limits of quantification (LOQs) were obtained (0.1 up to 2 ng g⁻¹) and the determination of some analytes present in very low concentration was possible. It was possible to digest a relatively high crude oil mass (up to 1 g) using 8 mL of 14.4 mol L⁻¹ HNO₃ in just 40 min, which can be considered as an important aspect taking into account the difficulties involved in heavy crude oil digestion for further REE determination by ICP-MS. The accuracy was evaluated by analyte spike and also by comparison of results obtained by MAWD-SRC with those using conventional MAWD and also by microwave-induced combustion. Suitable recoveries were obtained for all analytes (94 to 110%) and no statistical difference was observed between the results obtained by MAWD-SRC and those using other methods.