Investigation on Emission Spectra of Microsecond Laser-Induced Soil Plasmas

Laser induced breakdown spectroscopy with long-pulse laser(500 mu s) was used to generate plasma of soil sample in air. The spectroscopy emission characteristic of soil plasma was investigated under the low power-density conditions. Intense continuum background could not be detected (402 similar to 409 and 420 similar to 436 nm) and the long-pulse laser induced plasma had a longer overall life time (about 220 similar to 270 mu s), which was different from the dynamic characteristics using nanosecond laser and ultra short pulse laser. Besides, the spectral lines of Pb I 405. 78 nm and Cr I 425. 43 nm appeared at about 210 and 190 mu s. Intensity of Pb 1405. 78 nm and Cr I 425. 43 nm increased as time passed by, reaching to its maximum at 320 and 350 mu s, respectively. The study results showed that increased interaction time between laser and sample contributed to the formation of "quasi-stable state plasma". The relative standard deviation was 2. 21%similar to 6. 35% concluded by 8 times repeated experiments, which showed a better stability of soil plasma by using a long-pulse laser. The detection limits of Pb and Cr were 34. 7 and 40. 0 mg . kg(-1), respectively, which was below the trace element thresholds for Class 1 soil used in the environmental quality standard in China. Parameters characterizing a laser-induced plasma were obtained with the temperature of 6612 K and electron density of 3. 7 X 10(17) cm(-3) in the condition of long-pulse laser. Experimental results showed that it was in local thermodynamic equilibrium.