1. State Key Laboratory of Applied Optics, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China
2. Key Laboratory of Optical System Advanced Manufacturing Technology, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China
3. University of Chinese Academy of Sciences, Beijing 100049, China
4. Department of Mechanical and Electrical Engineering, Changchun University of Technology, Changchun 130012, China
5. Department of Electronics and Electrical Engineering, Changchun University of Technology，Changchun 130012, China
Abstract：To improve the detection effect of laser-induced breakdown spectroscopy on the metal elements in lubricating oil and avoid the problems of plasma quenching, oil splashing, and low spectral intensity when laser-induced breakdown spectroscopy is applied to the detection of lubricating oil, beeswax was used as the matrix to convert the lubricating oil sample from the liquid phase to the solid phase. The Mg element and Ca element in the lubricating oil were quantitatively analyzed. Firstly, a scheme was proposed to prepare lubricating oil test samples using beeswax. Secondly, the experimental parameters such as pulsed laser energy, acquisition delay, and laser focus position were optimized. The laser energy was adjusted from 30 to 120 mJ. Each time, it was increased by 10 mJ, and the effect of laser energy on the spectral line intensity and signal-to-back ratio was compared and analyzed. The results showed that the experimental results were the best when the laser energy was 90 mJ. The best experimental results can be obtained with an acquisition delay of 2.5 μs, choosing different acquisition delays, varying by 0.5 μs each time, and comparing the experimental results with acquisition delays ranging from 1 to 5 μs. The influence of the laser focus position on the spectral signal was compared and analyzed, and the laser focus position was adjusted from 0.5 mm above the sample surface to 5 mm below the sample surface. By moving 1 mm each time, it was concluded that when the laser focus position was 2 mm below the sample surface, the target element's spectral signal intensity and signal-to-background ratio were the best. Then, Mg(Ⅱ) 279.552 8 nm and Ca(Ⅱ) 393.366 nm were selected as the analytical lines of Mg and Ca. Under the best experimental conditions, seven lubricating oil samples prepared with different concentrations of beeswax were spectra collected, and the calibration curves of Mg and Ca were established. The linear correlation coefficient of the calibration curve of Mg and Ca reached 0.996 1 and 0.995 8, and the detection limits of Mg and Ca were 4.08 and 6.11 μg·g-1. Finally, based on the established calibration curves, the concentrations of Mg and Ca in four other lubricant samples with different concentrations were detected, and the recoveries of Mg and Ca were 92.67%~106.15%. The recoveries of Ca were 95.88%~108.57%. The results show that the use of beeswax as a matrix to prepare samples for the detection of metal elements in lubricating oil solves the problems of low spectral intensity and oil splashing when laser-induced breakdown spectroscopy is used for lubricating oil detection and realizes the detection of metal elements in lubricating oil on the order of μg·g-1.The proposed method is of great scientific significance for detecting metal elements in lubricating oil by laser-induced breakdown spectroscopy.
Key words：Laser-induced breakdown spectroscopy;Wear elements in lubricating oil;Beeswax;Quantitative analysis;Limit of detection
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