Crude oil is commonly found in reservoirs associated with sedimentary rocks. It is a hydrocarbon-based matrix, containing some elements, such as sulfur, nitrogen, oxygen and metals (e.g, Ni, V, Na, Ca and others). Regarding to the metals presence in crude oil, it is well known that their concentration depends on the type of geological formation, location as well as age and depth of reservoir.1 The presence of metals in crude oil may cause poisoning, corrosion, and affect equipment during crude oil refining process, even at low concentration.1 However, taking into account the difficulties for crude oil digestion and the low levels of same contaminants typically found in crude oil, as well as the low carbon content and low acidity required in digests to use plasma-based techniques, the development of an accurate sample preparation is necessary. In this work, crude oil decomposition was performed using microwave-assisted wet digestion in a pressurized digestion cavity (MAWD-PDC)2 for the determination of Ba, Ca, Fe, K, Li, Mg, Na, Ni, Sr and V by inductively coupled plasma optical emission spectrometry (ICP-OES). A high-pressure microwave-assisted digestion system (Multiwave 7000, Anton Paar, Austria) equipped with five quartz vessels (80 mL), with maximum temperature and pressure of 270 ºC and 160 bar, respectively, was used. The metals and carbon determination were performed by an ICP-OES equipment (Optima 4300DV, Perkin Elmer, USA). The determination of the residual acidity was performed by titration (836, Metrohm, Switzerland). Operational conditions, such as the sample mass (0.5 to 1.2 g), the volume of HNO3 (6, 8, and 10 mL), the maximum temperature (220, 250, and 270 ºC), and microwave irradiation time (5, 10, 15, and 20 min) were investigated. Using the optimized conditions (8 mL of 14.4 mol L-1 HNO3, 270 °C and 20 min), the MAWD-PDC method allowed the digestion of 1.2 g of crude oil, resulting in a very low limit of quantification (from 0.10 to 0.52 µg g-1). The proposed method was applied to four oil samples and the carbon content and residual acidity in digests were about 3.5 g L-1 and 10%, respectively. No statistical difference was observed between the results obtained by ICP-OES after MAWD-PDC and the certified values of certified reference material (NIST 1634c, fuel oil). The proposed method was suitable for the determination of Ba, Ca, Fe, K, Li, Mg, Na, Ni, Sr and V at ultra-trace levels in crude oil and it can to be used for routine analysis in the petroleum industry.

1. J.G. Speight. Handbook of Petroleum Analysis, John Wiley and Sons Inc, New Jersey, 2001, 519 pp.

2. Anton Paar, Instruction Manual and Safety information, Multiwave 7000, 43 pp.