Development of dual-pulse laser-induced breakdown spectroscopy for trace elements in water

Abstract

A study has been made to characterize the shielding effect and its influence on the analyte emission signals in laser-induced breakdown spectroscopy. For the purpose of sensitively analyzing trace elements in water, dual-pulse excitation system has been applied and the parameter influence such as interpulse delay, laser wavelengths and ambient gases on the plasma characteristics has been examined in detail. Firstly, characteristics of the plasma shielding effect was investigated through simultaneous measurement of time evolution of the nitrogen emission in ambient air and analyte-specific emission in a glass matrix with varying lens-to-sample distance (LTSD), laser pulse energy, and repetition rate. Even under the threshold energy of air breakdown, strong nitrogen-emission lines could be measured by laser-induced breakdown spectroscopy (LIBS) for the glass samples in air at atmospheric pressure. The time evolution of the nitrogen emission was correlated reversely with a behavior of various analyte emissions in the glass samples. Based on the reverse relation between the intensities of nitrogen and calcium emission intensities, the corrected values of the calcium emission line were calculated. This methodology shows more robust and consistent results independent of experimental conditions such as different LTSDs, laser energies, and repetition rates. Secondly, dual-pulse laser-induced breakdown spectroscopy (DP-LIBS) has been developed to sensitively detect concentrations of boron and lithium in aqueous solution. Sequential laser pulses from two separate Q-switched Nd:YAG lasers at 532 nm wavelength have been employed to generate laser-induced plasma on a water jet. For achieving sensitive elemental detection, the optimal timing between two laser pulses was investigated. The optimum time delay between two laser pulses for the B atomic emission lines was found to be less than 3 μs and approximately 10 μs for the Li atomic emission line. Under these optimized con...