Abstract: A novel group of nuclear magnetic resonance shift reagents is disclosed. This group comprises the anionic chelates of dysprosium and thulium wherein the chelate moiety has a charge of at least -3. These reagents cause a shift in the nuclear magnetic resonance spectrum of aqueous solutions of alkali and alkaline earth metal ions. By providing shift reagents to one side of a membrane capable of permitting the transport of alkali or alkaline earth metal ions therethrough and having such ions on one or the other side of said membrane and providing an environment in which said ions can pass through said membrane, by measuring the NMR spectrum of the total environment, the area of the peak of the absorpiton due to a predetermined ionic species caused by the shifted ions is compared to that due to the unshifted ions, the rate of transport through the membrane may be determined.