Laser-Induced Breakdown Spectroscopy Combined With Spatial Confinement of Plasmas and Laser-Induced Fluoresence for Trace-Materials Detection

Abstract

Laser-induced breakdown spectroscopy (LIBS) with spatial confinement effects and LIBS combined with laser-induced fluorescence (LIBS-LIF) have been investigated to improve the detection sensitivity and element-selectivity of LIBS. An obvious enhancement in the emission intensity of aluminum (Al) atomic lines was observed when a cylindrical wall was placed to spatially confine the plasma plumes. The maximum enhance factor for the emission intensity of Al atomic lines was measured to be around 10. Assuming local thermodynamic equilibrium conditions, the plasma temperatures are estimated to be in a range from 4,000 to 5,800 K. It shows that the plasma temperature increased by around 1,000 K when the cylindrical confinement was applied. Fast images of the laser-induced Al plasmas show that the plasmas were compressed into a smaller volume with a pipe presented. LIBS-LIF has been investigated to overcome the matrix effects of LIBS for the detection of trace uranium (U) in solids. An optical parametric oscillator wavelength-tunable laser was used to resonantly excite the uranium atoms and ions within the plasma plumes generated by a Q-switched Nd:YAG laser. Both atomic and ionic lines can be selected to detect their fluorescence lines. A U concentration of 462 ppm in a glass sample can be detected using this technique at an excitation wavelength of 385.96 nm for resonant excitation of U II and a fluorescence line wavelength of 409.01 nm from U II. The mechanism of spatial confinement effects and the influence of relevant operational parameters of LIBS-LIF are discussed. In this work, detection in open air of trace phosphorus (P) in steels using LIBS-LIF has also been investigated. The optical parametric oscillator laser was used to resonantly excite the P atoms within plasma plumes generated by the Q-switched Nd:YAG laser. A set of steel samples with P concentrations from 3.9 to 720 ppm were analyzed using LIBS-LIF at wavelengths of 253.40 and 253.56 nm for resonant excitation of P atoms and fluorescence lines at wavelengths of 213.55 and 213.62 nm. The calibration curves were measured to determine the limit of detection for P in steels, which is estimated to be around 0.7 ppm.