Optimization of Chromium, Nickel and Vanadium analysis in crude oil using graphite furnace atomic absorption spectroscopy

Abstract

Graphite Furnace Atomic Absorption Spectroscopy (GF-AAS) technique had been widely used for the determination of trace metals because of its high sensitivity. However, when this technique is applied for metals in organic samples such as crude oil, inconsistent reading were obtained due to smoke produced during the heating program which in turn interfere light absorption by the analyte. As such, organic components of the crude must first be removed to obtain a more accurate analysis of the metal analyte. This study evaluates the use of different acid combinations such as H2SO4/HNO3/H2O2, HCl/HNO3, HNO3/H2O2, H2SO4/HNO3 and H2SO4/HNO3/HCl for the decomposition of crude oil for the determination of nickel, chromium and vanadium. First, the temperature for GF-AAS analysis was optimized. The optimum drying, pyrolysis and atomization temperature for Ni were 110, 1000, and 2600 oC, while for vanadium were 110, 1100 and 2650 oC, and for Cr were 110, 1650 and 2500 oC respectively. The results show that vanadium concentrations in crude oil were 10.60±4.28 µg/g, 32.95±10.00 µg/g and 26.31±10.90 µg/g after being digested using H2SO4/HNO3/H2O2, H2SO4/HNO3/HCl and H2SO4/HNO3 respectively.. The concentration of nickel using the acid combination of H2SO4/HNO3/HCl, H2SO4/HNO3 and HNO3/H2O2 are 35.27±2.40 µg/g, 12.35±7.19 µg/g and 37.61±0.85 µg/g respectively. The chromium concentrations are 47.26±3.38 µg/g, 4.46±4.76 µg/g and 6.04±4.73 µg/g for the acid combination of H2SO4/HNO3/HCl, H2SO4/HNO3 and HNO3/H2O2, Good recovery was obtained only for vanadium digested with H2SO4/HNO3/H2O2 with an average of 94.95±13.73 %.