为改善激光诱导击穿光谱的质量，提高激光诱导击穿光谱技术对土壤样品的检测能力，研究了圆形碳片距离样品表面的高度变化对土壤等离子体辐射强度的影响，并通过Boltzmann图方法和光谱线Stark展宽法测量了等离子体的电子温度和电子密度。实验结果表明，当有碳片从轴向约束等离子体时，等离子体辐射强度比没有碳片约束时的明显增强；随着碳片距离样品表面高度的加大，等离子体辐射强度逐渐升高并在11 mm处达到最强，随后减弱。计算可知，样品中元素Fe、Mn、K和Ti在碳片距离靶面11 mm处的谱线强度要比无碳片约束时的分别提高179.88%、117.02%、123.21%和91.24%；光谱信噪比分别提高107.30%、92.26%、68.48%和67.66%；等离子体的电子温度升高2800 K，电子密度升高2.16×1016 cm-3。研究结果为土壤中痕量元素的检测提供了一种简单、易行的方法。

To improve the quality and the detection capability on soil samples of laser induced breakdown spectroscopy, the effect of different heights of carbon sheets from the target surface on the soil plasma radiation intensity is investigated. Moreover, the electron temperature and electron density of plasmas are measured through the Boltzmann graph method and Stark broadening. Experimental results indicate that the plasma radiation intensity is significantly enhanced when the plasma is confined by carbon sheets, compared with no carbon sheets confinement. As the height of carbon sheets from the sample surface increases, the plasma radiation intensity gradually increases, peaks at 11 mm and then decreases. The calculated results show that the spectral line intensity of elements Fe, Mn, K and Ti with carbon sheets confinement at 11 mm from the target surface are enhanced by about 179.88%, 117.02%, 123.21% and 91.24%, respectively, than those without carbon sheets confinement; spectral signal-to-noise ratios are increased by about 107.30%, 92.26%, 68.48% and 67.66%, respectively; the electronic temperature and electronic density of plasmas are increased by 2800 K and 2.16×1016 cm-3, respectively. It provides a simple and easy method for the detection of trace elements in soil.