Determination of iron, nickel, and vanadium in crude oil by inductively coupled plasma optical emission spectrometry following microwave-assisted wet digestion

Abstract

The complexity of crude oil samples is the primary difficulty in reliable quantitation of their metal contents. However, substantial advances in sample digestion procedures have been presented as a result of the utilization of microwave radiation. Herein, a new method based on microwave-assisted digestion (MW-AD) for the decomposition of crude oil samples has been developed for efficient determination of iron, nickel, and vanadium by inductively coupled plasma optical emission spectrometry (ICP OES). Yttrium was used as an internal standard to achieve as accurately as possible quantitation. The digestion efficiency was optimized using various HNO3 concentrations (14.5, 9.1, 6.5, and 4 mol L−1) and different volumes of H2O2 by determining the residual carbon content (RCC) and residual acidity (RA) of the final digests. The obtained results revealed that 9.1 mol L−1 HNO3, 2 mL H2O2, and sample mass of 300 mg were the appropriate compromise conditions for efficient digestion and suitable final digests since RCC values and RA were below 10% and 35%, respectively. The MW-AD method was successfully validated by the analysis of samples spiked with the target metals at different concentration levels. The results demonstrated a satisfactory recovery in the range of 95–104.2% with intra-day and inter-day relative standard deviation of less than 10%. The method was also applied to the determination of the analyte metals in crude oil samples, and the results were compared with those obtained by the application of the ASTM D5708 method as a reference method.