Comparison of ionic, atomic and molecular emissions in underwater laser-induced breakdown spectroscopy for quantitative analysis of Ca and Sr

Abstract

Various studies have been conducted on the quantification of alkaline earth elements, such as Ca and Sr, using underwater laser-induced breakdown spectroscopy (LIBS). However, the used spectral lines are either ionic or atomic lines, while less attention has been paid to molecular emissions. In this study, we compared the ionic, atomic and molecular emissions for the quantitative analysis of Ca and Sr in underwater LIBS. The emission spectra of Ca II, Sr II, Ca I, Sr I, CaOH, and SrOH were collected at different concentrations of Ca and Sr dissolved in water. Both a quartz cuvette and a high-pressure chamber were used to establish the calibration curves for Ca and Sr under atmospheric pressure and high pressure conditions. This shows that the molecular emissions of CaOH and SrOH account for a significant portion of the total plasma emissions within the period from 200 ns to 2 µs due to the rapid recombination process of Ca and Sr atoms with OH radicals. Although the LoDs of CaOH and SrOH are slightly higher than those of their atomic or ionic emissions, they exhibit unique features in reducing the influence of self-absorption on the calibration curves. Moreover, the CaOH and SrOH molecular emissions have good pulse-to-pulse stabilities and suffer less interference from varying ambient pressures, which can clearly improve the accuracy and precision of the calibration curves of Ca and Sr, especially under high pressure conditions. This study provides insights into improving the quantitative performance of Ca and Sr in underwater LIBS using molecular emissions instead of the commonly used ionic or atomic emissions.