Described herein are devices, systems, and methods for determining the composition of fluids, and particularly for describing the identity and concentration of one or more components of a medical fluid such as intravenous fluid. These devices, systems and methods take multiple complex admittance measurements from a fluid sample in order to identify the identity and the concentration of components of the fluid. The identity and concentration of all of the components of the solution may be simultaneously and rapidly determined. In some variations, additional measurement or sensing modalities may be used in addition to admittance spectroscopy, including optical, thermal, chemical, etc.
