Development of a New Instrument for Measurement of Particle Size Distribution and Optical Properties from Underway Flow-through Systems

Wayne H Slade, Thomas Leeuw, David R Dana, Chuck Pottsmith and Yogesh C Agrawal, Sequoia Scientific, Inc., Bellevue, WA, United States
Particle size distributions (PSD) define the relationship between particle concentration and size. For ocean bio-optical and biogeochemical studies, the relevant particle size range covers roughly 0.2 µm to a few mm, including plankton, minerals and detritus, zooplankton, and aggregates. Most particle sizing methods do not cover a wide size range; or are not conducive to routine or unattended measurement such as deployment in underway flow-through systems. Alternatively, multi-angle light scattering (MALS) is a bulk optical measurement that can analyze large volumes of sample water continuously in an underway flow-through system, covering a wide dynamic range of sizes and concentrations. In MALS, particle size is inferred (using a suitable inversion technique) from the scattering measurements based on the principle that larger particles scatter light more near-forward than smaller particles.

The LISST-Horizon is a self-contained MALS instrument intended for bench-top deployment in a research vessel laboratory, plumbed to continuous underway uncontaminated seawater. LISST-Horizon uses a sample cell with windows for measuring scattering with near-forward and side scatter detector arrays, across an angular range from approx. 0.04° to 150°, covering a size range of approx. 0.2 µm to 500 μm. For automated deployment, a manifold of valves allows seawater and other fluids (rinsing, cleaner, scattering standard) to enter the cell. Additionally, an integrated pump allows the sample to be recirculated through a particle filter to make periodic filtered seawater measurements as blanks. Here we present an overview of the technology and instrument design; as well as results from simulations, standard particle measurements, and long-term testing.