Evaluation of spark discharge as strategy to improve sensitivity in Laser Induced Breakdown Spectroscopy

Analysis by laser-induced breakdown spectroscopy (LIBS) may be considered rapid, direct and minimally destructive. The low sensitivity of the technique may be circumvented by adopting different strategies, but most of them restrict the portability1. A simple and low-cost alternative tool consists in using an electric spark discharge (SD) between two conductive electrodes. The SD increases the temperature and electron density of the plasma immediately after sample ablation by the laser pulse, which improve the emission intensity and sensitivity1. This work deals with the development and evaluation of a SD LIBS coupling. Two W cylinder electrodes (diameter 2.6 mm, length 60 mm) separated by a 3-mm gap and positioned 2-mm above the sample holder were connected to the high-voltage (~ 2 kV) output terminal of a circuit from a commercial bug zapper racket. The SD LIBS coupling operated with a Nd: YAG laser at 50 mJ maximum energy and four spectrometers, covering the spectral range from 200 to 620 nm (0.2 nm resolution). The SD LIBS system was applied to blood deposited over a Whatman 903TM paper discs (Dried Blood Spot, DBS), orange leaves, polyethylene terephthalate (PET), and liver and muscle of bovine. Except for DBS, pellets of samples (12 mm diameter) were made using a 10 ton mechanical press. The emission spectra were obtained using delay and integration times fixed at 2 and 1 μs, respectively. Thirty spectra of each sample were acquired and used to calculate a mean spectrum. Emissions of some elements were identified and signal-to-noise (S/N) ratios were calculated by using the mean spectrum. Results showed the SD improved the emission lines of several elements. As example, S/N ratios for Ca I (422.7 nm) and Al I (309.5 nm) in orange leaves obtained with SD LIBS were 2.3 and 3.1 times higher than those observed without SD. The PET, DBS, liver and muscle did not presented recordable emission spectrum with conventional LIBS. On the other hand, the SD LIBS system allowed favorable S/N ratios (Table 1) for analytical purposes. These findings show that SD LIBS is feasible to increase sensitivity and enlarge analytical applications. In addition, the SD tool is simple, portable and uses low-cost components available readily in the market.