- What is meant by Interference in ISE measurements?
Unfortunately, most ion-selective membranes are not entirely specific for the designated ion but also allow the passage of some other ions to a greater or lesser extent. This increases the charge on the membrane above that which is due solely to the detected ion and causes a spuriously high measurement. This phenomenon is known as interference and the extent of this interference is expressed as the Selectivity Coefficient.
The Selectivity Coefficient can be determined experimentally to give some idea of the order of magnitude, but it is not a constant factor since it depends on several parameters including the concentration of both ions, the presence of other ions in solution, and the temperature. In the past this phenomenon has limited the use of ion-selective electrodes to only those applications where the interfering ion is known to be absent or only in a relatively small concentration compared to the detected ion. Ion-selective electrodes are not completely ion-specific.
A value of 0.1 of Selectivity Coefficient indicates that the electrode is ten times more sensitive to the primary ion than to the interfering ion. A value of 1 indicates equal sensitivity to both ions.
- What can I do about a noisy electrode signal?
If the signal is very erratic and jumps by tens or even hundreds of millivolts then this is probably due to minute bubbles in the reference electrode electrolyte. These can develop during transport or prolonged storage. This can normally be cured by holding the electrode firmly with the active tip pointing downwards and shaking down several times with a flick of the wrist.
- What precision can I expect from an ISE sensor?
Precision is the reproducibility of repeat measurements of the same sample. This is different from accuracy which is how near is the result to the true value. Ideally, results of any experimental measurement should give values that are accurate within the precision limit but this is not always true for ISE measurements.
Assuming that there are no variable systematic errors from interference or activity coefficient effects (which also affect the accuracy), the precision depends on the error in the measurement of the electrode potential (mV) and the slope of the calibration line (mV/decade of concentration).
These errors are unlikely to be better than about ± 1 mV and are equivalent to ~4% error in the concentration for monovalent ions (slope ~54) and ~8% error for divalent ions (slope~27), when working in the normal linear range of the electrodes. However, if there are added problems of interfering ions or high ionic strength or variable temperature or variable sample motion (as may be encountered in direct field measurements in natural samples) then the errors may be higher and a value of ±10 or 15% may be more realistic.
- How should I store the electrodes between analytical sessions?
In general, for overnight or longer storage, ISEs should always be rinsed with de-ionised water and gently dabbed dry with a low-lint tissue, and the black plastic cap should be replaced to protect the membrane from atmospheric oxidation/corrosion. Reference electrodes must be prevented from desiccation whenever they are not in use by replacing the protective cap containing a small quantity of the appropriate outer filling solution.
- What is the shelf life and operating life of an ISE?
The maximum usable life of Ion Selective electrodes depends very much on whether they are used for continuous monitoring or intermittent measurement of individual samples (in this case also on the frequency of use) and also on the nature of the samples. Prolonged exposure to solutions outside the optimum pH range for the electrode or to pure water or to solutions which do not contain any of the target ion will shorten the life.