Using laser diffraction data to obtain accurate particle size distributions: the role of particle composition

Abstract

Suspended particle composition has a large influence on light scattering by small (<20 µm) particles. Inorganic particles and organic phytoplankton cells, both common in natural waters, have very different indices of refraction. This can affect results from laser diffraction particle size analyzers, which measure light scattering and assume an index of refraction to calculate the particle size distribution. Here we examine the effect of the assumed index of refraction on laser diffraction data. A Laser In‐situ Scattering and Transmissometry (LISST) 100X instrument is used to record scattered light distributions from glass microspheres, phytoplankton monocultures, and natural particle samples of mixed composition. The scattering distributions are processed with kernel matrices, derived using Mie theory, assuming either organic or inorganic compositions. Resulting particle size distributions are compared against microscope images. Processing with an assumed inorganic index of refraction was found to produce the most accurate results over the majority of the samples tested. The effect of scattering by particles having diameters outside the instrument inversion range was also assessed. The presence of significant concentrations of particles exceeding the upper limit of the LISST range (>250 µm) produced little effect; however, particles smaller than the lower limit (<1.25 µm) influenced concentrations over the entire LISST size range and produced counterintuitive effects, such as increased concentrations in the largest size bins.