Overcoming the problem of non-ideal liquid ion exchanger selectivity in microelectrode ion flux measurements

Ion-selective microelectrodes are a powerful tool in studies on various aspects of cell membrane biology in both animal and plant tissues. Further application of this technique is, however, limited to a large extent by the problem of non-ideal selectivity of the liquid ion exchanger used in the preparation of microelectrodes for ion flux measurements. Because of this problem, which is persistent in many commercial liquid ion exchangers, the microelectrode does not discriminate between the ion of interest and other interfering ions (for example, Mg2+ and Ca2+; Na+ and K+), thereby leading to inaccurate concentration readings and, consequently, inaccurate flux calculations. In this work we show that the existing analytical procedure to overcome this problem, using the inverted Nicolsky-Eisenman equation, is inadequate, and suggest an alternative analytical procedure that can be applied directly to the data obtained with commercially available liquid ion exchangers. We show that this alternative procedure allows accurate measurement of ionic concentrations with non-ideal ion-selective microelectrodes in the presence of interfering ions, and illustrate the method by direct experiment using Ca2+ and Mg2+ as a "case study". Several more examples are given, further illustrating practical applications of the method for study of plant responses to salinity, osmotic and reactive oxygen species stresses.