Quantitative analysis of chlorine in cement pastes based on collinear dual-pulse laser-induced breakdown spectroscopy

On-line detection of chlorine in cement is the key to evaluate the corrosion of the reinforced concrete inside the structure. A collinear dual-pulse LIBS system based on two nanosecond lasers with a total energy of 30 mJ is developed to detect the chlorine in cement. The key parameters of the dual-pulse LIBS system are optimized to improve the detection sensitivity of trace chlorine element in cement pastes, and the optimal values of the parameters are obtained to be: 4 L/min helium gas flow rate, 2000 ns inter-pulse delay, 800 ns gate delay, 19 mJ/11 mJ pulse energy ratio, and 42.8 mm lens-to-sample distance. After the optimization of the dual-pulse system, the signal-to-noise ratio of the trace chlorine emission line at 837.6 nm has been improved from 1.75 to 2.68 for a sample containing 0.706 wt% chlorine. The temperatures of plasma are obtained based on Saha-Boltzmann plot for exploring the plasma radiation features in the dual-pulse system. The results show that the plasma temperature which is influenced by laser irradiance and parameters of the double pulse configuration, is closely related to the signal-to-noise ratio of chlorine spectrum line. Sixteen standard cement pastes made from a series of sodium chloride solutions with various concentration are used for LIBS calibration. Two calibration methods including internal standardization and partial least squares regression are adopted for determining chlorine concentrations within a series of standard cement pastes, and the limit of detection based on internal standardization model is calculated to be 103.4 ppm. The prediction performance of IS and PLSR is evaluated by Leave-One-Out Cross-Validation with the root mean square error of calibration of 0.0910 and 0.0859, respectively.