USGS CMG Menlo Park Fine-Particle Size Analysis Lab--M3009A

Room	Fine-Particle Size Analysis Lab
Location	Rm. M3009a, 3rd floor, Bldg. 15 (McKelvey Bldg.)
Size
Contact	Michael E. Torresan
Description	This laboratory houses two state-of-the-art fine-particle size analysis equipment used to determine the particle-size distribution of silt- and clay- size material, defined as particles finer than 0.063 mm.
Specialized Equipment	Two automated, state-of-the-art, computer controlled particle size characterization systems and a constant temperature bath, as follows: A Micromeritics 5100 Sedigraph that employs x-ray attenuation technology to determine the particle-size distribution of a aqueous suspension of material finer than 0.063 mm. System employs sedimentation analysis and Stoke's Law for particle settling. Particle size analysis is determined in terms of equivalent spherical settling velocity. A Coulter LS100Q laser particle size analyzer that employs a laser beam and light scattering theories to determine the particle size distribution of samples having particles that range from 1000 - 0.004 mm in size. Unlike the Sedigraph this system does not perform sedimentation analysis, but rather is a sophisticated particle counter that uses light scattering to determine the particle-size distribution of a sample. Constant temperature bath.
Equipment and Technique	The fine particle analysis lab employs two techniques, one modern and one classical. A Micromeretics SediGraph that employs x-ray attenuation in a homogeneous silt and clay particle suspension is the modern technique. The SediGraph method assumes that the particles are dispersed in a fluid and settle in accordance to Stokes' Law. The SediGraph monitors the rate at which particles settle, and it determines the concentration of particles remaining at specific depths and times in a suspension-filled cell. The SediGraph measures the amount of x-ray attenuation that occurs in clear water and compares that to the x-ray attenuation in a sediment suspension. An accurate measure of the cumulative size distribution of the settling particles is determined. The SediGraph can determine a full range of particle sizes in any predetermined size-class interval from as small as 0.1 microns to 300 microns. Most any type of sediment can be analyzed, but samples rich in magnetic minerals and organics (carbonate) may give problems. Like all size-analysis techniques samples must be pre-treated to remove flocculating agents, cements, organic matter, solubles (salt), etc., to obtain an accurate reading of the grain size distribution. The pipette method is the classical sedimentation analysis technique employed in the fine particle size analysis laboratory. Pipette analysis is conducted when particle concentration is too low for the SediGraph technique. The pipette method is, like the SediGraph technique, based on Stokes' Law for settling. The pipette method involves the withdrawal of aliquots from a sediment suspension, and the aliquots are withdrawn at precalculated times and depths as determined by Stokes' law. Conducted properly, a pipette analysis though effective and reasonably accurate, has the disadvantage of being highly labor intensive and time consuming, taking hours and days for routine analysis, and a greater chance for operator error and sample disturbance. In the next few years the sediment laboratory hopes to expand its capabilities to include a laser-based particle analysis system for low concentrations of sediment that are not applicable to the SediGraph or the pipette method. The laser method is based on the scattering of light by suspensions.