Laser Diffraction Particle Size Analysis Contact: World Agroforestry Centre (ICRAF), P.O. Box Nairobi, Kenya. Tel: Soil particle size distribution is a key determinant of many soil functions including soil fertility, and hydraulic and mechanical behaviour. Soil particle size analysis using traditional hydrometer or pipette sedimentation methods is tedious and error-prone. Laser diffraction particle size analysis is a rapid low cost technology for measuring particle sizes using light diffraction patterns. Particle size distribution can be measured in narrow size classes in dry soils, or in water using different dispersion treatments. Features of modern laser diffraction systems for particle size analysis: High-output centrifugal pump, capable of suspending even the largest dense particles. Powerful in-line ultrasonic probe for dispersion of agglomerates. Dry and wet analysis modules. Allows complete sample dispersion and analysis sequence to be handled without the need for external sample preparation. Automation ensures speed, reproducibility and ease of use. Complete sample analysis sequence completed in 60 seconds, including fill, alignment, blank, measurement, drain and rinse cycles. Introduction High throughput, fine resolution analysis of soil particle size distribution as input to soil pedotransfer functions. Consistent comparative indices of soil microaggregation and stability based on dry and wet dispersion treatments. Diagnosis of soil physical problems: Soil erodibility and erosion hazard Hardsetting soils Dispersive soils Tunnelling and piping susceptibility Soil suitability for irrigation Applications Fine resolution particle size distribution using small samples Wide measurement range (0.01 µ m – 3 mm) High repeatability Accuracy guaranteed to within 0.6% Precision guaranteed to within 0.1% Fast sample-to-sample measurement time - 60 Seconds Dry powder or liquid suspensions analyzed Key advantages Sample dispersion systems Laser diffraction particle size analysis relies on the fact that particles passing through a laser beam will scatter light at an angle that is directly related to their size. As particle size decreases, the observed scattering angle increases logarithmically. Scattering intensity is also dependent on particle size, diminishing with particle volume. Large particles scatter light at narrow angles with high intensity whereas small particles scatter at wider angles but with low intensity. A typical laser diffraction system (Fig. 1) consists of: a laser - a source of coherent, intense light of fixed wavelength; a series of detectors - to measure the light pattern produced over a wide range of angles; and a sample presentation system to ensure that material under test passes through the laser beam as a homogeneous stream of particles in a reproducible state of dispersion. Light of smaller wavelengths (blue laser) provides improved sensitivity to sub-micron particles, whereas larger wavelengths (red laser) are used to measure larger particles. Although diffraction theory assumes particles are spherical, consistent comparative measures of soil particle sizes can be obtained (Fig. 2). Working principles Fig 2: Particle distribution of a soil sample; histogram and cumulative curve Fig 1: Red and blue light lasers