Laser-induced breakdown spectroscopy of liquid droplets:: correlation analysis with plasma-induced current versus continuum background

We have developed a current normalization method for laser-induced breakdown spectroscopy (LIBS) for analyzing liquid droplets. An electrospray ionization device is employed to generate a stream of microdroplets. The spray needle serves as the anode, through which the analyte solution is spread toward the other metal base which is the cathode. Upon laser irradiation at the liquid droplets, the time-resolved laser-induced breakdown (LIB) emission and plasma-induced current signals are acquired concurrently on a single-shot basis. The plot of LIB emission intensity against the current intensity yields a straight line. The slopes in the correlation plots increase with the sample concentration, whereby the calibration curve is obtained. The resultant limit of detection (LOD) of Na sample may reach 0.6 +/- 0.1 mg l(-1), about 20 times better than that obtained by LIB/background normalization. The impinging laser energy dependence of both normalization methods is also investigated. The correlation linearity for the background normalization is found to be restricted within a small range of laser energy. When a two-line ratio is involved to account for the plasma temperature, its linear feature is markedly improved. According to the Boltzmann equation, the plasma temperature as a function of delay time relative to the onset of continuum background is determined to gain insight into the temperature effect on the LIB/background method. In contrast, the LIB/current method, which has taken into account the ablated amount of microdroplets and the plasma excitation temperature, retains correlation linearity over a much wider range of laser energy.