Calibration solutions for the simultaneous potentiometric measurement of sodium, potassium and calcium in blood plasma: examination of the electrochemical factors affecting precision and accuracy in direct potentiometric clinical analysers

Abstract

A series of well defined calibration solutions is proposed to eliminate some of the discrepancies in the measurement of sodium, potassium and calcium in blood plasma by ion-selective electrode potentiometry. The calibration matrix has been chosen to resemble the electrolyte composition of blood plasma in terms of ionic composition and ionic strength. This reduces liquid-junction and activity coefficient bias. A flow-through test rig was constructed from sodium, potassium and calcium ion-selective electrode modules from commercial instruments and a single ceramic plug KCl reference electrode. These modules were calibrated with the proposed solutions and used in blood plasma. Preliminary experiments were performed to evaluate residual liquid-junction potentials and the effects of heparin and carbon dioxide.