Study on the method to improve the stability of LIBS spectral signals by out-of-focus large spot breakdowns

Poor stability of spectral signals has been a key issue limiting the development of laser-induced breakdown spectroscopy (LIBS) technology. Unlike traditional LIBS where the sample is punctured at the focal point, this study proposes a method using an out-of-focus large spot to excite the sample. A Cu-Zn alloy target was selected as the experimental object, and the obtained spectra were pre-processed. By calculating the relative standard deviation (RSD) and signal-to-background ratio (SBR) of the spectra for Cu (514.325 nm) and Zn (468.010 nm) elements in the Cu-Zn alloy target, it was found that as the sample moved back and forth along the optical axis with the laser focal point as the center, the spot became larger and the spectral signals became more stable. The RSD showed a tendency of decreasing and then increasing, while the SBR showed a tendency of increasing and then decreasing. At a distance of 2-3mm from the focal point, the spot size became about 2 times the focal point, and the RSD was the lowest at this point. The RSD of the spectral signal for the Zn (468.010 nm) element in the pre-focal area decreased to 1.5%, which was 1/12 of that at the focal point, while the RSD of the Cu (514.325 nm) element decreases to 0.6%. Mathematically, the 2-3mm range from the focus represented the inflection point of energy density change and had the highest electron density and lowest electron temperature, resulting in the best stability. Compared with other methods to improve the spectral stability of LIBS, the proposed method of outof-focus large spot excitation is simple, convenient, easy to operate, and has good practical value for wider application and dissemination.