An elemental species is defined as a particular binding form or oxidation state of the element. The toxicity of a chemical element like Hg, Se or As is not dependent on the total elemental concentration, but rather on the concentration of its respective toxic species. In order to access species information with high sensitivity and selectivity, hyphenated techniques with chromatographic separation and mass spectrometric detection are applied. The concept of speciation analysis is presented for the example of gadolinium-based contrast agents for magnetic resonance imaging, which are discharged in the aquatic environment via the wastewater. In this case, hydrophilic interaction liquid chromatography (HILIC) is combined with electrospray mass spectrometry (ESI-MS) for species identification and inductively