Experimental evidence of laser diffraction accuracy for particle size analysis

Laser diffraction analysis is a fast, reliable and automated method that provides detailed and highly resolved soil and sediment particle size distribution. In recent studies, the methods were compared against independent methods based on direct observation of particles by digital imaging. The data showed that laser diffraction results were in better agreement with the digital imaging independent method than with sedimentation-based methods. However, analysis was performed over a limited number of samples. In this study, 47 soil samples with a wide range of textural properties were analyzed with Laser Diffraction, Pipette, Sieving, Sedigraph and Digital Imaging methods. Detailed statistical analysis using Altman plots and Honest Significant Difference tests demonstrated (at 95% significance) that the five methods do not show statistically significant differences for grain sizes above 100 µm. However, in the lower end of the size range, i.e. less or equal to 50 µm, Laser Diffraction showed much better agreement with the reference method selected for comparison, which was Digital Imaging. New regression equations were derived with slope coefficients for linear regressions, Based on regression and equivalent diameters, Laser Diffraction data were represented on textural triangles for classification, allowing for employing Laser Diffraction for soil texture classification. Two alternative for representing the Laser Diffraction data in textural triangles were employed: (1) using regression equations to convert data to be represented on the standard triangles and (2) modify the upper limit for the clay range, from 2 to 8 μm. Finally, based on the additional evidence presented in this research, demonstrating that the Laser Diffraction was in better agreement with the optical method with respect to traditional sedimentation methods, it is suggested that the standards for particle size analysis be changed from sedimentation to Laser Diffraction methodologies.

激光衍射分析（Laser Diffraction）是一种快速、可靠且自动化的分析方法，可提供精细且高分辨率的土壤与沉积物粒径分布数据。过往研究中，该方法曾与基于数字成像（Digital Imaging）直接观测颗粒的独立方法开展对比，数据显示激光衍射分析的结果相较于基于沉降法的测试方案，与数字成像独立方法的吻合度更高。但过往分析仅针对有限数量的样本展开。本研究针对47种具备广泛质地属性的土壤样本，分别采用激光衍射法（Laser Diffraction）、移液管法（Pipette）、筛分法（Sieving）、沉降粒度仪法（Sedigraph）与数字成像法（Digital Imaging）进行分析。通过阿尔特曼图（Altman plots）与诚实显著差异检验（Honest Significant Difference tests）开展的详细统计分析表明（显著性水平设置为95%），对于粒径大于100 µm的颗粒，五种方法未呈现统计学意义上的显著差异。但在粒径区间的下限，即粒径小于或等于50 µm的范围内，激光衍射法与本次研究选定的对照参考方法——数字成像法的吻合度显著更高。本研究推导了带有斜率系数的线性回归新方程；基于回归结果与等效粒径，将激光衍射法的数据投射至质地三角图中完成分类，从而可利用激光衍射法开展土壤质地分类。本次研究采用了两种在质地三角图中展示激光衍射数据的替代方案：（1）通过回归方程将数据转换为可在标准质地三角图中展示的形式；（2）将黏土粒级的上限从2 µm调整至8 µm。最后，基于本研究提供的额外证据——相较于传统沉降法，激光衍射法与光学成像方法的吻合度更优——本研究建议将粒径分析的标准方法从沉降法调整为激光衍射法。