The lucigenin assay can be employed to study the effect of different metal cations on the transport process, which can aid to elucidate possible co-transport of M + and Cl -by the studied anionophore. The assay can be adapted by replacing the NaNO3 solution for LiNO3, KNO3, RbNO3 or CsNO3 (and by using the corresponding MCl salts). A clear change of the rate of Cl -transport indicates that the cation plays a role in the transport process. On the other hand, similar transport rates in all salt solutions (see for instance Figure ), suggest that the tested cations are not co-transported with chloride or that the transporter is not selective to any of them.