Enhancing laser-induced breakdown spectroscopy stability in liquid via high-quality focusing

When applying Laser-induced Breakdown Spectroscopy to liquid media, the instability of the breakdown process within the bulk liquid significantly degrades detection sensitivity and reproducibility. Current improvement methods, such as altering sample states or adding auxiliary equipment, increase experimental complexity and undermine the core advantages of LIBS. To address this, a customized three-element air-spaced lens group was designed in this study. Combining optical simulations with experimental validation, the focusing characteristics of the proposed lens group were systematically compared with those of a standard aspheric lens. The results indicate that the Strehl ratio of the proposed lens group is 1.43 times that of the aspheric lens, achieving a highly concentrated energy distribution. This optimization reduced the breakdown threshold from 15.34 mJ to 9.16 mJ and extended the plasma lifetime. Furthermore, the high-quality focusing significantly reduced fluctuations in plasma position and area. Consequently, stronger and more stable spectral signals were obtained, with the relative standard deviation of characteristic spectral line intensities decreasing from over 17% to below 8% across various energy levels. This study presents a solution to mitigate detection instabilities and enhance the analytical performance of LIBS in liquid samples.