An ion exchange atomic absorption method for the determination of ionized calcium at millimolar levels

Abstract

A quantitative ion exchange/atomic absorption method is described for measuring the fraction of free calcium in solution at millimolar levels. Sample solutions are equilibrated with a micro-column of strong-acid type cation-exchange resin under electrolyte concentrations (0.75 mol/L NaNO3) high enough to provide conditions where the calcium sorbed on the resin is proportional to the free calcium in solution. After a water wash, the sorbed calcium is eluted from the resin with nitric acid directly into an atomic absorption spectophotometer. Free calcium levels were estimated using calibration curves obtained over the range 0–5 mM calcium with a precision of ± 0.05 mM. The effect of magnesium, at concentrations twice that of calcium, on measured calcium levels is negligible. The method is pH insensitive over the range 5–7. Selectivity for free calcium in the presence of complexing ligands such as citrate and phosphate is high. The method shows promise for the determination of ionized calcium in biological systems. Key words: ion exchange, atomic absorption, free calcium determination, ion speciation.