In the industrial and mechanical fields, lubricating oil is essential for ensuring the stable operation of equipment. Therefore, accurate and rapid prediction of trace metal elements in lubricating oil is of great significance for oil production and application. Currently, detection methods such as spectrophotometry and inductively coupled plasma mass/optical emission spectrometry are accurate and sensitive, but they require complex sample pretreatment, making them difficult to meet the requirements of online rapid detection. To achieve rapid and accurate quantification of trace metal elements in lubricating oil, this study employed indirect ablation laser-induced breakdown spectroscopy (LIBS) technology combined with the standard addition method, providing fast and reliable technical support for the manufacturing, production, and condition monitoring of lubricating oil.

First, standard samples containing 400×10⁻⁶ kg/L each of Cr, Al, and Ca were dripped into circular holes of adhesive tape on the surface of a Zn substrate. After an oil film was formed, a Nd:YAG laser beam was focused through a lens onto the oil film surface to generate plasma (Fig.1). Subsequently, the analytical lines were determined as Cr Ⅰ 425.433 nm, Al Ⅰ 396.152 nm, and Ca Ⅰ 422.673 nm (Fig.2). The quantitative analysis performance of the basic calibration method, conventional standard addition (SA) method, one-point gravimetric standard addition (OPGSA) method, and multi-energy calibration (MEC) method for Cr, Al, and Ca in standard lubricating oil samples was systematically compared. To further evaluate the effect of the MEC method on correcting matrix effects of actual lubricating oil samples, MEC was applied to the determination of Ca in two commercial lubricating oils (Shell 5W-40 and Mobil 0W-40). The spectral lines selected were Ca 393.366 nm, 396.847 nm, and 422.673 nm, and the coefficients of determination (R²) of calibration curves were all 0.99999. The results were compared with reference values measured by inductively coupled plasma optical emission spectrometry (ICP-OES).

The results showed that the basic calibration method exhibited good linearity (Fig.3), with an average relative error of 6.35% for Cr, Al, and Ca. The average relative errors obtained by the conventional SA, OPGSA, and MEC were 7.89%, 10.00%, and 7.40%, respectively (Table 1). Among these methods, MEC required only two sample groups (blank and standard), adopted multiple characteristic spectral lines for calibration, and achieved accuracy comparable to that of the basic calibration method. After the MEC method was applied, the relative errors of the basic calibration method were significantly reduced from 21.20% and 20.97% to 10.96% and 4.68%, respectively (Table 2).

This study demonstrates that the MEC method can effectively improve the accuracy of LIBS for quantitative analysis of trace elements in lubricating oil, and is expected to provide a fast and reliable technical solution for lubricant manufacturing, production, and monitoring of mechanical lubrication conditions. The proposed LIBS technology combined with the SA method systematically compares the element quantitative performance of multiple methods applied to lubricating oil, clarifies the superiority of the MEC method, and achieves rapid and accurate quantification of trace metal elements in lubricating oil. Further application verification can be carried out in the detection of wear metals.