Instrumentation: Laser Particle Size Analysis
The Saturn Digisizer
The laser particle size analysis (LPSA) procedure is used to determine the percentage of size-class fractions in soil or sediment samples. The procedure is based on ASTM C 1070–01 to determine particle size distribution analysis (PSDA) of alumina and quartz powders by laser light scatter (ASTM, 2000). Laser-light scattering is based on the Mie theory of light scattering by a spherical particle using the Micromeretics Saturn DigiSizer 5200®. The DigiSizer is capable of analyzer particles in the range of 0.05 to 1000 micrometers.
The sample is internally dispersed using ultra-sonication in an aqueous medium of 0.005% surfactant (sodium?metaphosphate) and circulated through the path of the laser light beam. As the particles pass through the laser beam, the light scatters at angles inversely proportional to particle size and with intensity directly proportional to particle size. A 45° rotational charge-coupled device collects the scattered light intensity, which is converted to electrical signals and analyzed in a microprocessor. Data reduction consists of a mathematical convolution based on scattering model sets, each calculated from the general Mie theory for narrow distributions of isotropic spheres having a specific index of refraction and suspended in a liquid having a specific index of refraction. Data reported by the Saturn DigiSizer relates directly to an equivalent Mie sphere. The Mie theory consists of a "real" refractive index (1.550 for soils) and an "imaginary" refractive index (0.100 for soils) determined by Micromeretics Laboratories. The predictive model error (weighted residual) is proportional to the measure of the calculated Mie theory model to predictions of the observed laser-light scattering pattern.
Prior to LPSA, samples may be either 1) externally dispersed, sieved to remove the entire sand fractions (> 62.5 micrometers) and analyzing the fine fraction (typical for soils), or 2) dry sieved to remove the coarse sand particles >1 mm and internally dispersed with ultra-sonication. Method 1 has the advantage of added resolution to the fine end by removing larger sand-sized particles, thereby reducing multiple light scatter. Method 2 has the advantage of the entire sample being presented enabling the ability to determine a mean, mode and kurtosis of entire particle size distribution. However, coarse sand particles >1 mm must be sieved to assure the sample falls within instrument size range.
Below: Typical laser particle size output for a well-sorted channel sand and a fine-grained floodplain deposit from the Las Vegas Wash.
Below: Laser particle size analysis is correlated to more traditional methods of PSDA (sieve/pipette or hydrometer). However, the relationship is not necessarily one to one as indicated below for distributions found in suspended sediment samples from Los Alamos National Laboratory.
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