Determining Size Distributions and Composition of Particles Suspended in Water: A New SEM-EDS Protocol with Validation and Comparison to Other Methods

Knowledge of particle size distributions (PSDs)in seawater is importantfor understanding several facets of marine science, such as the behavior of light scattering in seawater, phytoplankton dynamics, and biogeochemical cycling. Here, a method has been developed to quantify the size distribution of particle suspensions and characterize their chemical composition utilizing a scanning electron microscope (SEM) coupled with an energy dispersive spectrometer (EDS) and applying image analysis techniques, including automatic thresholding. The method was validated by verifying the PSD and chemical composition of the Arizona Test Dust (ATD), which has a well-documented size distribution and chemical composition. Size distributions of ATD particles containing specific elements important in the marine environment, such as silicon, iron, calcium, aluminum,andpotassium, were quantified. PSDs determinedwith thetechnique infield samples from coastal Long Island Sound and the remote South Pacific were compared with other sizing methods, including electroresistivity and laser diffractometry. Most accurate results for PSD determinations occurred when the particle mass loading on the filter was between 0.04 and 0.1 mg cm 22 . With this in mind, immediate feedback in the field can be provided to prepare appropriate filtration sample volumes due to alinearrelationshipbetweenthebeamattenuationcoefficientat650 nm(c650)andthetotalsuspendedmatter (TSM).Overall,themethodpresentstwodefiningadvantagesin 1)minimizinguserbias,becausethemajority of the analysis is automated, and 2) providing an elemental distribution in the context of a particle size distribution.