Size, morphology and composition of particulates in aquatic ecosystems: solving speciation problems by correlative electron microscopy

Abstract

Particulates can impact directly on aquatic ecosystems by determining the availability and mode of dispersion of both contaminants and nutrients. An understanding of the mechanisms of such particle-associated phenomena is being augmented by particle analysis technology. In this context, microscopic and spectroscopic techniques, devised for problem solving, are being applied to frequently encountered sub-micrometre particulates which are ‘unstable’ with respect to methods of sample preparation and storage used routinely for particulates prior to analysis. These unstable aquatic particulates include ‘species’ sensitive to dehydration and to artificial aggregation induced by surfaces within a fractionation apparatus. These species, as defined broadly, include polysaccharide gels, hydrated humic substances, iron oxyhydroxides, viruses, the smallest micro-organisms and decomposing parts of cells. To develop predictive models of their roles as dispersing agents for contaminants, and to speciate such associations, it is necessary to characterize them in a state as close to the natural as possible. This critical review presents the state-of-the-art in the realistic characterization of hydrated sub-micrometre particulates by correlative electron microscopy (EM) used in conjunction with spectroscopy and minimally perturbing preparatory techniques. Correlative EM is a strategy for using several different kinds of microscopes and accessory techniques in a multi-method context to analyse a given specimen for different kinds of information, including relationships in three dimensions within colloid systems. Sizing, morphology and gross composition are determind on a ‘per particle’ basis by transmission EM used in conjuction with energy-dispersive spectroscopy, electron diffraction and molecule-specific stains. Heterogeneity measures and positional information are refined and quantified by correlating observations form transmission and scanning EM with those from optical microscopy. In a systematic, manner, artifacts of perturbation can be readily detected, assessed and minimized. Artifacts of shrinkage, swelling and redistribution of materials can be measured against a vitrified standard.