Influences on Concentration Measurements of Liquid Aerosols by Laser-Induced Breakdown Spectroscopy

Abstract

Results are presented for laser-induced breakdown spectroscopy (LIBS) as a method for measuring salt concentrations in seawater aerosol droplets. An excimer laser, operating at high pulse energy with KrF gas (λ = 248 nm) produces laser-induced breakdown plasmas in an aerosol spray. Emission lines of Na and H are monitored with an optical multichannel analyzer to characterize the plasma spatially and temporally. Studies of temporally resolved atomic line emissions from the plasma determine the optimum time for gating of the detector to be 2–4 μs after the excimer laser pulse arrives in the probe volume. Spatially resolved measurements of atomic emission line intensities are studied by positioning a stream of monodisperse droplets at various locations relative to the measurement probe volume. The electron temperature of the plasma is estimated to be 12,600 ± 4600 K, averaged over 1700 measurements during a 100-ns interval 2 μs after breakdown. Calibration curves are presented relating the Na(I) 589-nm to H_α 656.3-nm intensity ratio as a function of Na concentration, ranging from 100 to 10,000 ppm. Limits of detection for Na by the current method under the experimental conditions are estimated to be approximately 165 ppm for monodisperse sprays and 925 ppm for one case involving a polydisperse spray. Droplet diameter strongly influences the observed emission intensity ratio.