EVALUATION AND VALIDATION OF INSTRUMENTAL PROCEDURES FOR THE DETERMINATION OF NICKEL AND VANADIUM IN FUEL OILS

A direct method for the determination of Ni and V in fuel oils by flame atomic absorption spectrometry has been developed which is sufficiently rapid, accurate and precise for monitoring batches of fuel oil. Fifty-four European laboratories participated in the round-robin test (RRT), which was carried out on six fuel oil samples in order to evaluate the precision of the method. Various procedures and instrumental techniques such as electrothermal atomic absorption spectrometry, inductively coupled plasma atomic emission spectrometry, X-ray fluorescence, inductively coupled plasma mass spectrometry and neutron activation analysis were employed to control the accuracy of the results. The participating laboratories provided 1879 results for Ni and 1945 results for V. The precision of the flame direct method (FDM) was calculated according to the ISO 5725 method. The repeatability and reproducibility data for the FDM showed definite improvements on comparison with the precision data given by the Institute of Petroleum (IP) 288 method. The limit of determination (six times the interlaboratory standard deviation of the FO01NV sample) was 5.6 mug g-1 for Ni and 4.2 mug g-1 for V. In the concentration range investigated (1-70 mug g-1 of Ni and 2-180 mug g-1 of V) the repeatability (r) and reproducibility (R) showed a linear relationship with both element contents. The r values were 3.4 mug g-1 for an Ni content of 30 g g-1 and 3.8 mug g-1 for a V content of 50 mug g-1, while the R values for the same element contents, were, respectively, 8.1 mug g-1 for Ni and 11.1 mug g-1 for V. Evaluation of the accuracy of the method was also performed using standard reference materials. The instrumental techniques showed the results to be in excellent agreement. The control of accuracy and the comparison of results obtained by the independent techniques allowed validation of the proposed method. Therefore, it was possible to advance consensus values for the Ni and V content in the six fuel oil samples examined in the RRT.