Sensitive detection of chlorine in iron oxide by single pulse and dual pulse laser-induced breakdown spectroscopy

Abstract

The halogen chlorine is hard to detect in laser-induced breakdown spectroscopy (LIBS) mainly due to its high excited state energies of 9.2 and 10.4eV for the most intense emission lines at 134.72nm and 837.59nm, respectively. We report on sensitive detection of Cl in industrial iron oxide Fe2O3 powder by single-pulse (SP) and dual-pulse (DP) LIBS measurements in the near infrared range in air. In compacted powder measured by SP excitation (Nd:YAG laser, 532nm) Cl was detected with limit of detection LOD=440ppm and limit of quantitation LOQ=720ppm. Orthogonal DP LIBS was studied on pressed Fe2O3 pellets and Fe3O4 ceramics. The transmission of laser-induced plasma for orthogonal Nd:YAG 1064nm and ArF 193nm laser pulses showed a significant dependence on interpulse delay time (ipd) and laser wavelength (λL). The UV pulses (λL=193nm) were moderately absorbed in the plasma and the Cl I emission line intensity was enhanced while IR pulses (λL=1064nm) were not absorbed and Cl signals were not enhanced at ipd=3μs. The UV laser enhancement of Cl signals is attributed to the much higher signal/background ratio for orthogonal DP excitation compared to SP excitation and to the increased plasma temperature and electron number density. This enabled measurement at a very short delay time of td≥0.1μs with respect to the re-excitation pulse and detection of the very rapidly decaying Cl emission with higher efficiency.