Before venturing off and wasting time trying to do the impossible just check that your sample meets the following criteria:
Ionic strength is the total concentration of all ions in your sample. The greater the number of ions in solution the less active the individual ions are. The outcome of this is that e.g. you will get a higher result for say 100 ppm nitrate sample in spring water than 100 ppm nitrate sample in sea water using the same standards and no ionic strength adjustment.
We can eliminate the problem completely making the total ionic strength of all samples and standards the same. We can do this by adding a small volume of a concentrated inert solution to each standard and sample. This is usually 2 ml of ISA (Ionic Strength Adjuster) to 50 ml of standard and sample.
Ion selective electrode measurements are also susceptible to interfering ions that will falsify a correct reading. It might be required to eliminate those interfering ions when their level is too high. This is specific for each type of ISE, see their manuals.