To improve performance first make sure the cyrstal surface is dry, then use a very fine emery cloth or sand paper to rub off the oxidised layer. Be very gentle and rinse thoroughly afterwards to remove any loose particles. This should return the electrode performance.
Ensure you do not use this process to clean other types of ISE such as PVC electrodes. If you are unsure what type of electrode you have please get in touch with one of our technical team on +44 (0)1954233 144 or email us firstname.lastname@example.org