Quantitative determination of chlorides by molecular laser-induced breakdown spectroscopy

Abstract

The presented work reports on an improved analysis of chlorides in cement and concrete using molecular LIBS. The quantitative determination of the atomic chlorine emission requires sophisticated setups under helium atmosphere and specialized spectrometers. Molecular LIBS can be performed with standard spectrometers and without additional buffer gas. In a LIBS plasma of chloride contaminated cement or concrete atomic chlorine and calcium form calcium-mono-chloride (CaCl) radicals which will be used for the quantification of chlorides. By calculating the intensity ratio of chlorine dependent and independent molecular band emission the standard deviation of the intensity ratio compared to the absolute species emission can be reduced by a factor of 5. Air purging further reduces the total standard deviation by two-thirds compared to measurements in ambient air without purging. A linear calibration with a limit of detection and limit of quantification complying with the threshold of a critical chloride contamination in concrete is determined. Spatially and temporally resolved measurements of different fixed concentrations of chloride are shown to proof the homogeneity of reference samples. In order to determine the diffusion process of the chloride contamination measurements on a drilling core are presented. As supplemental method for micro analysis, spatially resolved energy dispersive X-ray (EDX) measurements are performed. EDX can help to reveal the atomic distribution in a hydrated sample on a scale much smaller than the spot size of the laser. Thereby, relevant reaction partners for the molecular formation could be identified.