Application of ICP-MS and ICP-OES on the Determination of Nickel, Vanadium, Iron, and Calcium in Petroleum Crude Oils via Direct Dilution

The direct introduction of crude oil samples diluted in organic solvents to be analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) is an attractive way to perform elemental analysis, particularly in high sample volume facilities, where turnaround time is critical. Here, it is presented results obtained by performing direct injection of a broad set of petroleum crude oils diluted in o-xylene and analyzed by ICP-OES/MS. By utilizing proper sample preparation and instrument tuning parameters, we set out to assess data accuracy, standard deviation, and recovery, particularly in the region of mg kg–1 to μg kg–1. A multielement Conostan standard (10 elements at 2 mg kg–1 in crude oil) as well as NIST 1634c CRM were selected for this purpose. In general, the data from both instruments were comparable regarding precision, relative standard deviation, and recoveries for metals evaluated at a concentration range of interest. The potential effect of crude oil viscosity on a sample introduction system was assessed using three crude oil samples representative of the total group, diluted in o-xylene diluent. In general, the viscosity values of the final solutions do not impact the quality of the results obtained. Furthermore, 18 petroleum crude oil samples with API values spanning from 9.5° up to 51° were analyzed. In general, there is good agreement between the two techniques as seen from ICP-OES/ICP-MS ratio values for the elements spanning in most cases from 0.80 to 1.20. Finally, V/(V + Ni) ratios were successfully determined for all samples and the ICP-OES/ICP-MS ratios calculated span from 0.96 up to 1.05, which provides insight into the high precision that was achieved by the direct dilution method for both instruments.