Asymmetrical flow field-flow fractionation (AF4) has been used to characterise the size and organic carbon concentration of colloids in environmental samples, without extensive pre-treatment. The addition of a true online organic carbon detector (OCD) to the combination of UV, fluorescence and multi-angle light scattering detectors provides a unique tool for a better understanding of the composition of dissolved and particulate organic matter (OM) present in environmental samples. Polystyrene sulfonate sodium salt polymers were used as standards. Colloidal material was characterised in surface water, road runoff from a residential area and the effluent from a recirculating aquaculture system. All samples contained organic or inorganic colloids associated with organic carbon to various degrees and with various sizes. The OCD clearly added another layer of information, allowing for detection of organic colloids independent of their ability to scatter light or absorb UV-light. The recovery of carbon varied between 10% and 100% depending on the nature of the sample.

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