Determination of non-protein-bound zinc in human serum using ultrafiltration and atomic absorption spectrometry with electrothermal atomisation

Abstract

Ultrafiltration through cellulose acetate membranes has been combined with electrothermal atomic absorption spectrometry to measure non-protein-bound zinc in serum. The retention of zinc-binding proteins by ultrafiltration membranes is virtually complete. The effects of the matrix on the sensitivity of the method for zinc are overcome by the use of a matrix modifying reagent and by the addition of amino acids to the calibrating standards. To control the problems of contamination that are associated with the measurement of the metal at nanomolar concentrations, all equipment is washed with dilute hydrochloric acid before use and serum ultrafiltrates are analysed for zinc without preparative handling. The procedure is simple and requires only 400 µl of serum for duplicate analyses. The relative standard deviation for the determination of an ultrafiltrable serum zinc concentration of 90 nmol l^–1 is 10.8%. The concentration of non-protein-bound zinc measured in sera from 17 healthy volunteers is 81 ± 16 nmol l^–1 with a range of 62–112 nmol l^–1. The effect of CO₂ loss from samples has been investigated and inaccuracies resulting from the use of heparin-treated plasma samples are described.