Heterogeneous particles collected from seawater were analysed for physical dimensions and elemental composition using a computer-controlled image and chemical analysis system based on a scanning electron microscope and energy dispersive X-ray spectrometer. Staining the particles with the heavy metal ruthenium largely overcame the difficulties of forming an adequate image of organic particles that usually limits such automated analyses. Ruthenium staining provides an image comparable to that achieved with metal coatings, improves the contrast of organic particles, and provides a more uniform background grey level. In addition, this treatment simplifies sample preparation and improves the X-ray count rate compared with metal-coated specimens. The major X-ray energy peak of ruthenium interferes only with that of chlorine, which is usually removed during the sample preparation procedures. In its present configuration, the system can perform analyses of particles from 0.5 to 100 μm in one pass for abundance, size, shape, and elemental composition at a rate of about 500 particles per hour. A generalized particle classification scheme based on elemental proportions and ratios is presented for suspended marine particles. The scheme is applied to samples from three depths in the photic zone for a station in the Sargasso Sea. As illustrative data, particle abundance, volumes, and size-volume distributions are presented vs chemical class for the major types of particles found, and a bulk refractive index is calculated. © 1992.
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