Abstract：Laser induced breakdown spectroscopy with long-pulse laser(500 μs) 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 (402~409 and 420~436 nm) and the long-pulse laser induced plasma had a longer overall life time (about 220~270 μs), which was different from the dynamic characteristics using nanosecond laser and ultra-short pulse laser. Besides, the spectral lines of Pb Ⅰ405.78 nm and Cr Ⅰ425.43 nm appeared at about 210 and 190 μs. Intensity of Pb Ⅰ405.78 nm and Cr Ⅰ425.43 nm increased as time passed by, reaching to its maximum at 320 and 350 μs, 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%~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 3.7×1017 cm-3 in the condition of long-pulse laser. Experimental results showed that it was in local thermodynamic equilibrium.
Key words：Long-pulse laser induced plasmas; Life-time of plasma; Relative standard deviation; Limit of detection
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