Decolorization of Ionic Liquids for Spectroscopy

It has been widely recognized that although ionic liquids should be colorless, they are frequently not. Colored samples appear to be pure by most analytical techniques (e.g., NMR spectroscopy, mass spectrometry, HPLC, and ion chromatography), and there have been many attempts to identify the source of color in our own laboratories and others-after 20 years the best that can be said is that the impurities are at a very low level (probably parts per billion) with very high molar extinction coefficients. In this paper, we do not identify these impurities but describe a practical method for removing them for spectrochemical applications. We clearly note that the method is not "green", but we anticipate that it will only be applied to the small volumes of ionic liquids required for fundamental spectroscopic studies in academia but not in industrial processes.