Application of asymmetric flow field-flow fractionation to the study of aquatic systems: Coupled methods, challenges, and future needs

•The theoretical background and limitations of AF4 techniques are outlined.•Hyphenated AF4 methods for analyzing natural and manufactured nanoparticles are reviewed.•Advances in investigating colloidal behavior, and colloid-metal interactions are reviewed.•Future needs for methodological advances are outlined. Aquatic ecosystems are facing multiple direct and indirect stressors from anthropogenic activities. Aquatic colloids, including natural and manufactured nanoparticles, have received global concern owing to their ability to transport metals, nutrients and pollutants, and their potential to pose ecological risks for wildlife and human health. Asymmetric flow-field flow fractionation (AF4) has become one of the most promising and powerful approaches to characterize colloidal particles over the last ten years. This review overviews the advances and applications of AF4 for the study of aquatic systems over the last decade. The analytical challenge of AF4 and known deviation from theoretical assumptions are also critically discussed, such as sample losses, membrane-analyte interactions, and overloading. Future needs for methodological advances are addressed.