Solid and liquid state speciation of chromium of relevance for health

The chemical form (chemical speciation) of chromium (Cr) is important for human health. Hexavalent Cr (Cr VI ), present as oxyanions in water, is of great concern even at trace levels. Here, we briefly describe and discuss common and additional liquid state (often standardized) and solid state Cr speciation methods for typical samples of health concerns. This review covers common standardized, extraction-based liquid state methods and various solid state Cr speciation methods. Liquid state methods include chromatography, colorimetric methods, mass spectrometry, and electrochemical methods, with widely varying detection limit ranges to accommodate all sample needs. The most sensitive liquid state method can detect trace amounts of Cr VI in the nanograms per litre range. Colorimetric methods can be used both for the liquid and solid state and are the simplest methods without the need for a laboratory or equipment. Other solid state methods include vibrational spectroscopy, electrochemical methods, and various laboratory- or synchrotron-based methods: X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and X-ray diffraction. No method is perfect on its own, and we therefore recommend best practices, the investigation of potential interfering agents, and validating the method with another method. However, the largest threat to accurate Cr speciation-based hazard assessments is the dynamic change of Cr speciation in a potential exposure scenario or during sample preparation for the analytical method. To avoid wrong conclusions, we recommend considering the Cr chemistry, the sample chemistry, and the method-specific interferences and detection limits.