Interferent ion effects on the Orion SS-20 Calcium Ion Analyser

Selectivity studies were carried out on bis-dialkyl phosphate and neutral carrier sensors available with the Orion SS-20 Calcium Analyser. Neutral carrier sensors showed better selectivity; changes of H+, Na+, K+ and Mg2+ within the physiological range produced no significant error in Ca2+ measurement. The Nicolsky equation, used as the basis of these studies, did not fully account for electrode response to mixed electrolytes. The importance of a check on the selectivity of sensors is emphasised.