The ability to quantify the amount of metals ions that are present as macromolecular, nanoparticulate, or colloid phases is critical for understanding bioavailability and transport as well as performing risk assessments and remediation strategies. Flow field-flow fractionation-inductively coupled plasma mass spectrometry (FI FFF-ICP-MS) is a powerful separation tool that has been previously used to characterize colloidal metals in environmental samples. In this study we examine the degree to which FI FFF-ICP-MS provides quantitative data on uranium speciation by comparing the results to centrifugation followed by filtration. Sorption of uranium to nanoparticulate hematite (approximately 60 nm) was examined over the pH range of 3 to 6. Close agreement was found between the two approaches over the pH range.