Development of an Accurate Method based on MAD-SRC for the Determination of Metals and Sulfur in Crude Oil Atmospheric and Vacuum Residues by ICP OES

The present study proposes for the first time a microwave-assisted digestion in an ultra-high pressure single reaction chamber (MAD-SRC) method for the digestion of atmospheric residue (AR) and vacuum residue (VR) from crude oil distillation. The determination of metals and sulfur was performed by inductively coupled plasma optical emission spectrometry (ICP OES). The type of digestion solution (HNO3 or HNO3 + H2O2), the volume of digestion solution (6 to 10 mL), maximum temperature (200 ºC and 270 ºC), and sample mass (up to 1000 mg) were evaluated. Using 1000 mg, 8 mL of HNO3, and 270 °C the final digests presented carbon concentrations lower than 2500 mg L-1 for both crude oil AR and VR. The accuracy of MAD-SRC was evaluated by using a certified reference material, recovery tests, and comparison with results obtained by using microwave-induced combustion (MIC). No statistical difference was observed between the results obtained after the proposed MAD-SRC method and the certified values and recoveries higher than 95% were obtained for all analytes. Comparing the results obtained by the MAD-SRC method with those obtained by MIC, an agreement higher than 95% was achieved for all analytes. The relative standard deviation was always lower than 7%. The quantification limits ranged from 0.06 µg g-1 for Ba to 3.13 µg g-1 for S enabling metals and sulfur determination in crude oil distillation residues virtually free of interferences. The main advantages of the proposed method is the possibility to digest up to 1000 mg of crude oil AR and VR without the use of relatively long processing time and with lower risks of contamination and volatilization of analytes when compared to an official method using open vessels (about 240 min). In addition, it was possible to obtain digests fully compatible with the determination technique making this method a suitable option for the routine quality control of crude oil AR and VR.