A NEW METHOD FOR Ca, P, Zn, and S DETERMINATION IN LUBRICATING OILS BY TOTAL REFLECTION X-RAY FLUORESCENCE

The knowledge about the concentration of different elements in lubricating oils is an economic and environmentally strategic task in the petrochemical industries, because indicates if the sample has high levels of additives or metals due to wearing of mechanical parts. The elemental determination using X-ray fluorescence techniques in lubricating oils has already been consolidated by Wavelength-Dispersive X-ray Fluorescence (WDXRF) and Energy-Dipersive X-ray Fluorescence (EDXRF). A specific configuration of EDXRF spectrometry, Total Reflection X-ray Fluorescence (TXRF) spectrometry has the ability to improve sensitivity and to maximize the signal-background ratio, without requiring calibration curve or cooling gases. In this work, a new method was developed for Ca, P, Zn, and S determination in lubricating oil samples by TXRF. The lubricating oil was simply diluted with organic solvents and Y was used as internal standard. After evaporation of the solvent, TXRF measurements took it place. Firstly, the method was optimized using an interlaboratorial reference material and xylene as solvent. The studied experimental parameters were the sample volume (50 or 100 L), measurement time (250 or 500 s) and volume deposited on the quartz glass sample carrier (5 or 10 L). Optimized conditions with xylene were used to study other organic solvents (kerosene or hexane). The analytical figures of merit (accuracy, precision, LOD and LOQ) were used to evaluate the performance of the analytical method, for all solvents. The recoveries varied from 89 to 111% and the relative standard deviation remained between 2.3% and 10.4%. For all elements, the results obtained applying the new method were in agreement with the certified value. After the validation step, the method was applied for quantification of Ca, P, Zn, and S in eight new and four used lubricating oil samples, for all solvents. The concentration of the studied elements in the new and used lubricating oils varied in a range of 1620 – 3711 mg kg-1 for Ca; 704 – 1277 mg kg-1 for P; 898 – 1593 mg kg-1 for Zn and 2027 – 9147 mg kg-1 for S. The proposed method is a simple, accurate and efficient alternative for Ca, P, Zn, and S determination in lubricating oils. The methodology described is suitable for routine analysis of lubricating oil for monitoring relevant elements in the petrochemical industry.