However, the rate-limiting step in this process may not be chloride transport as the anion transporters of different classes might show selectivity for transporting chloride over protons or hydroxide. Therefore, a measurement of the HCl transport activity of a particular anionophore may underestimate its true ability to facilitate chloride transport. Additionally, it should be noted that commercial lipids such as POPC or EYPC contain free fatty acid impurities, which are incorporated into the vesicular membranes prepared from those lipids. In particular, a difference in the amount of fatty acid impurities between batches of commercial lipids can result in variable proton transport rates mediated via a fatty acid flip-flop mechanism. This problem may lead to batch-dependent data inconsistency; however, this can be mitigated by adding free fatty acids (1-2 mol% with respect to the lipid concentration) to the vesicles to maximise the fatty acid-dependent H + transport rate.