Determination of S, Ca, Fe, Ni and V in crude oil by energy dispersive X-ray fluorescence spectrometry using direct sampling on paper substrate

Crude oil samples contain several types of contaminants including metalorganic compounds, sulfur containing compounds and sediments. Energy dispersive X-ray fluorescence spectrometry was used for the direct determination of S, Ca, Fe, Ni and V in crude oils, enabling a fast and reliable method that avoids complex sample preparation. Crude oil samples were homogenized and 20 μL of a diluted sample (1/1 v/v oil/toluene) were placed in the center of a 10 mm diameter filter paper that was sandwiched between two polypropylene film foils and attached to the instrument cell for measurements. For the purpose of matrix corrections, fixed amounts of H, N and O were introduced in the calibration model with C (achieved through elemental analysis) used to balance the matrix content. Quantification was made by fundamental parameters method using ASTM proficiency test samples to perform mathematical adjustment of the analytical model. The results for metals were statistically comparable to the ones obtained by flame atomic absorption spectrometry while the ones for S were in accordance to the results achieved in ASTM proficiency programs and achieved by elemental analysis. Ca, Fe, Ni and V were directly determined in the oil at concentrations as low as 1 μg g−1 and for S, the value was 0.002% w/w, showing the potential of the proposed method to screen for samples with higher amounts of these critical contaminants.