Study of optically thin condition for quantification of trace quantity of boron in aqueous sample using LIBS assisted by acoustic levitation

Abstract

LIBS analysis of liquids has faced challenges such as splashing and weak signals due to laser energy absorption by numerous liquid molecules. The analysis of evaporable liquids has been improved using surface-assisted/enhanced methods; however, these approaches have disadvantages, such as undesired ablation of the substrate material and uneven analyte distribution during preconcentration on the substrate surface, also known as the coffee ring effect. To address these issues, a novel technique has been introduced, utilizing an acoustic levitator to suspend liquid droplets without contact with any material. During preconcentration, the water content of the droplet is evaporated using a CW laser, leaving behind the mineral residue for LIBS analysis. Quantifying boron is particularly challenging by the conventional LIBS technique in the standard UV-NIR spectral range due to the presence of only one detectable doublet at 249.7 nm, which is often subject to self-absorption. Optimization of the delay and gate times of the iCCD camera was performed using the branching ratio technique to evaluate the optically thin condition. A calibration curve was constructed by preparing solutions at different concentrations of H₃BO₃ in an aqueous solution and was represented as HBO₂, the LOD was found to be 28.7 mg/L (ppm) of HBO₂. Finally, trace amounts of boron in bottled mineral water were successfully determined as 34.7 mg/L of HBO₂ with a 12 % error margin.