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

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 degrees 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 tow 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.