Deuterium exchange and a deuterium kinetic isotope effect can help distinguish among E1cBrev, E1cBanion, and E1cBirr. If the solvent is protic and contains deuterium in place of hydrogen (e.g., CH3OD), then the exchange of protons into the starting material can be monitored. If the recovered starting material contains deuterium, then the reaction is most likely undergoing an E1cBrev type mechanism. Recall, in this mechanism protonation of the carbanion (either by the conjugate acid or by solvent) is faster than loss of the leaving group. This means after the carbanion is formed, it will quickly remove a proton from the solvent to form the starting material.