由于自由定标法-激光诱导击穿光谱(CF-LIBS)需要元素归一化，多元素同时参与计算，土壤中微量元素谱线较弱，计算Saha-Boltzmann斜线困难，因而采用元素粒子比方法对多种国家标准土壤样品和实地采集的土壤样品中Cr的含量进行了预测。通过Saha-Boltzmann方程计算了等离子体中Cr、Si、Fe 3种元素的等离子体温度，以Al I 309.284 nm特征谱线的Stark展宽计算了等离子体电子密度，验证了实验条件下等离子体处于局部热平衡(LTE)状态。结合Saha-Boltzmann方程和Saha方程，计算土壤样品等离子体中Cr、Si元素的粒子比，从而计算出Cr元素在土壤样品中的含量。实验对国家标准土壤样品中Cr的预测相对误差在7%以内，对实地采集土壤样品中Cr的预测相对误差最大值为16.438%。研究结果表明元素粒子比方法可以用于土壤中Cr含量的快速分析，提高了LIBS技术用于土壤元素含量快速探测的优势。

Calibration- free laser induced breakdown spectroscopy (CF- LIBS) requires that the elements be normalized, various elements are involved in calculation. However, the spectral line of microelements in the soil is weak, and it is difficult to draw the Saha-Boltzmann plot. The element particle ratio method is used to calibrate the concentration of Cr in soils from a variety of national standard samples as well as local acquisition. The temperature of the plasma is calibrated from Saha-Boltzmann equation and the electron density is determined from the line width of Al I 309.284 nm. The assumption of the local thermal equilibrium (LTE) is validated by the McWhirter criterion. The particle ratio of Cr and Si is calculated by Saha-Boltzmann plot and Saha equation. The concentration of Cr can be calculated. The relative error for the national standard soil samples is less than 7%, and is less than 16.438% for the field collected samples. The results indicate that the element particle ratio method can be used in the concentration analysis of Cr in soils to improve performance of the LIBS technique in rapid detection of soil element content.