Laser Diffraction Particle Size Analysis
• 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.
• Laser diffraction particle size analysis relies on the fact that particles
• 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.
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.
• 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
Fig 1: Red and blue light lasers
• Although diffraction theory assumes particles are spherical, consistent
comparative measures of soil particle sizes can be obtained (Fig. 2).
• 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
Fig 2: Particle distribution of a soil sample; histogram and cumulative curve
• Soil suitability for irrigation
Contact: World Agroforestry Centre (ICRAF), P.O. Box 30677-00100 Nairobi, Kenya. Tel: +254 020 722 4000. www.worldagroforestry.org