Feasibility of wear metal analysis in oils with parts per million and sub-parts per million sensitivities using laser-induced breakdown spectroscopy of thin oil layer on metallic target

Abstract

The performance of laser-induced breakdown spectroscopy for quantitative analysis of wear metals in oils has been evaluated in this work with a specific ablation configuration of a thin layer of oil covering the polished surface of a pure aluminum target. A set of reference samples containing 12 metallic elements (Ag, Al, Cr, Cu, Fe, Mg, Na, Ni, Pb, Si, Sn, and Ti) was prepared by dilution with a 75cSt hydrocarbon base oil for the concentration range from 20 to 500μg/g. Calibration curves were therefore established for these elements with an echelle spectrometer and then with a Czerny–Turner spectrometer. The obtained results show first high linearity of the calibration curves established with the two types of spectrometer, the determination coefficient, R2, of all the calibration curves are superior to 0.99. The limits of detection (LoDs) with an echelle spectrometer vary in the range from 0.29 to about 10μg/g with an average value of 6.02μg/g for the 9 elements (Al, Na and Pb excluded). The use of a Czerny–Turner spectrometer reduces furthermore the LoDs to the range from 0.24 to about 10μg/g with an average value over the 10 elements (Al and Na excluded) of 4.04μg/g. In addition, the LoDs for the 3 elements (Mg, Cu and Ag) are determined lower than μg/g, i.e. in the sub-ppm level. Comparison with the previously published data shows moreover the efficiency of the introduced ablation configuration as one of the most suitable methods for highly sensitive and precise wear metal analysis in oils.