Combination of ultrasonic nebulization and thermal drying to assist LIBS (UNTD-LIBS): a sensitive online method for heavy metal element analysis in liquid samples

Abstract

Online and highly sensitive detection technology for heavy metal elements in water is of great significance for preventing water pollution and enforcing relevant environmental protection standards. Based on laser-induced breakdown spectroscopy (LIBS) technology, a novel LIBS detection method combining ultrasonic nebulization technology and thermal drying technology (UNTD-LIBS for short) was proposed in this paper. In this method, an ultrasonic nebulizer is used to convert liquid samples into massive micro-droplets, and a resistive electric heating coil in conjunction with a condenser tube is subsequently employed to transform the liquid droplet aerosol into an aerosol composed of quasi-solid particles, thereby improving the signal quality of LIBS detection. Using a newly established integrated online system equipped with nebulization, thermal drying, and LIBS analysis capabilities, a series of experimental studies were conducted targeting six heavy metal elements: zinc (Zn), cadmium (Cd), chromium (Cr), nickel (Ni), manganese (Mn), and copper (Cu). Experimental results demonstrate that after optimizing parameters such as heating temperature, sample injection rate, and laser energy, the absolute intensity, signal-to-background ratio (SBR), and signal-to-noise ratio (SNR) of the six elements are significantly improved. Compared with liquid droplet aerosol samples without thermal drying, the average SNR of spectral peaks for the six elements increases by 4.20 times, with a maximum increase of 14.39 times. The detection limits of the six elements all reach the order of approximately 0.1 mg L⁻¹, showing an advantage of 1–2 orders of magnitude compared with other online-capable LIBS technologies and basically meeting the quantitative limit requirements of relevant standards. Results of plasma temperature and density indicate that thermal drying facilitates the formation of plasma with similar temperature and higher density, thereby promoting signal improvement. Quantitative inversion experiments of six target heavy metal elements in six samples with different concentrations show that within the concentration range of 15–65 mg L⁻¹, the method achieves an average inversion error of 7.62% and a minimum error of only 0.67% for heavy metal concentrations. Considering that this method retains the continuous online operation capability inherent to LIBS technology while significantly improving sensitivity, it holds great potential for development into an on-site, real-time, and highly sensitive detection technology for trace heavy metal elements in water in the field of water environmental monitoring.