Examining Figure 2, we see two resonances. Perfect, right? Well, you have to be careful and not jump to conclusions too quickly! There are a few points to discuss here: The first is the concept of solvent residual peaks. But wait a minute, we used D2O to dissolve the CH3COOH. Why would we still see an H2O peak? Well, when you buy D2O from an isotope laboratory, 99.9% of the H2O has been converted to D2O. But, the process isn’t perfect, so there is still some H2O in there, which is why you see a bit of a residual solvent in the spectrum. It is commonly marked with an asterisk (*), and labeled as a solvent residual peak, as shown in Figure 3. Solvent residuals, as well as common impurities, are compiled in an awesome article from my favourite ACS Journal, Organometallics, and it is a great resource when you’re trying to nail down peaks.[4]