Activity and interference effects in measurement of ionized calcium with ion-selective electrodes

We assessed the magnitude of the errors in ionized-calcium measurements resulting from changes in electrolyte composition for three different ion-selective electrodes: the Orion Model 93-20, the Radiometer F2112, and the Simon neutral carrier membrane electrode. We attempted to distinguish between errors arising from changes in calcium ion activity and those due to interferences in the electrode response. Variation in sodium ion concentration over the range 100--180 mmol/liter produces changes in apparent ionized-calcium concentration that are largely attributable to activity effects for the Radiometer and Simon electrodes. The Orion electrode is subject to an additional sodium-interference effect. Apparent ionized calcium concentration measurements are independent of pH for the Radiometer electrode but not for the Orion electrode; the Simon electrode exhibits intermediate pH response, which is probably clinically negligible. Magnesium and potassium ions have little effect on ionized calcium concentration measurements, particularly when these ions are incorporated into calibration standards.