Measurement of intracellular potassium ion concentrations by n.m.r.

39K n.m.r. was used to detect and quantify K+ within human erythrocytes. A shift reagent consisting of an anionic complex of dysprosium(III) with sodium tripolyphosphate permitted a distinction to be made between K+ inside and outside erythrocytes. Intracellular K+ concentrations determined by this method were similar to values obtained by flame photometry.