Ultra-Low Energy LIBS for Chlorine Detection in Cement Pastes via Microwave Enhancement Using 0.05 mJ Laser Energy

This study investigates chlorine detection in cement pastes using microwave-enhanced laser-induced breakdown spectroscopy (MWE-LIBS) under ultra-low laser energy conditions. The motivation is to enable in situ analysis inside core-drilled cement structures, where laser energy must be minimized to reduce fume generation and avoid structural damage. A pulsed nanosecond laser with energy as low as 0.05 mJ was able to generate the initial plasma; however, this energy was insufficient to produce measurable spectral emissions without microwave (MW) assistance. When MW was applied, plasma formation was dramatically enhanced without the additional surface ablation, with the plasma volume increasing by over 2,200 times. This expansion potentially reduces fume generation and enabled multifiber collection, thereby capturing emissions from a larger plasma region. The MW pulse width and power directly influence Cl I peak values at 837.5 nm. Under maximum MW conditions, a linear calibration curve for Cl concentration from 0.11 to 3.0 wt % yielded a limit of detection (LOD) of 0.204 wt % and a limit of quantification (LOQ) of 0.679 wt %. The relative standard deviation (RSD) dropped below 10 % at concentrations above 1 wt %, demonstrating acceptable analytical precision.