The Effects of Sample Matrix and Laser Pulse Energy on Laser-induced Breakdown Spectroscopy of Liquid Droplets

1 We have investigated the influence of matrix salts on the liquid analysis by laser-induced breakdown spectroscopy (LIBS). An electrospray ionization technique coupled with LIBS is employed to generate the microdroplets of the Na sample solution with various matrix salts added. A sequence of single-shot time-resolved LIB emission signals is detected. The LIB signal intensity integrated within a gate linearly correlates with the plasma-induced current response obtained simultaneously on a single-shot basis. The slopes in the correlation plots increase with the sample concentration. A matrix-free LIB measurement may be achieved with the treatment of the current normalization. The ionization interference caused by the matrix doesn't significantly enhance the LIB emission of Na sample, which is surrounded in an electron-rich atmosphere. In addition, the LIB emission signals are weakly affected by the chemical interference, since the plasma temperature should be high enough to decompose most stable salts. Finally, the current normalization and continuum emission normalization methods are compared. Given a laser radiation emitting at 355 nm with the energy fixed at 23 mJ, the former one may achieve a limit of detection of 1.0 mg/L for the Na analysis, while the latter can reach 12 mg/L. The current normalization has taken into account the ablated amount of the sample and the electron temperature such that the obtained linearity of the correlation plot may extend to a much wider range of laser energy. In contrast, the continuum emission normalization is sensitive to the electron temperature, yielding a correlation linearity limited in a small range of laser energy