Needle tip-enhanced laser-induced breakdown spectroscopy for nebulized aqueous solution analysis

Abstract

In the detection of aqueous solutions through Laser-induced Breakdown Spectroscopy (LIBS), the implementation of nebulization is instrumental in mitigating liquid splashing and preventing plasma quenching. However, gas dilution during nebulization would greatly undermine the detection limits. This study proposes a needle tip-enhancement method that excites the metal tip to produce a high-temperature plasma before ablating the nebulized solution. The optimization results for needle tip materials reveal that among the seven materials Zn, Ag, Cu, Mo, Ni, Ti, and W, Zn, Ag, and Cu exhibit minimal interference with the target elements Mn and Cr, with Zn showing the highest spectral enhancement effect. The average Pearson correlation between the enhancement effect and the melting and boiling points of the materials exceeds 0.90. Further optimization of the needle tip diameter revealed that a diameter that is too thin can cause material deformation, while a diameter that is too thick would lead to liquid adsorption and deep pit formation. Therefore, a diameter of 0.5 mm was chosen as the optimal under experimental conditions. Additionally, the optimization results of the relative position between the aerosol plume and the needle tip indicate that the best enhancement effect occurs when the aerosol plume passes through the core of the plasma. Using a needle tip-enhancement method, we detected Cu, Mn, and Cr in water samples, applying machine learning for quantitative analysis. The Gaussian Process Regression (GPR) model yielded the highest accuracy at 0.99. The spectral intensities of Cu, Mn, and Cr were enhanced by 7.20, 8.57, and 11.49 times, respectively. Detection limits of Cu, Mn, and Cr with needle tip-enhancement reached 42.80 mg L⁻¹, 51.25 mg L⁻¹, and 58.40 mg L⁻¹. This study has the potential to facilitate real-time online monitoring of water solutions.