PLASMA KINETICS OF MAGNESIUM AND CALCIUM STABLE-ISOTOPE TRACERS IN A HUMAN SUBJECT AFTER SIMULTANEOUS ORAL INGESTION OF MG-25 AND CA-44 DETERMINED BY THERMAL IONIZATION MASS-SPECTROMETRY

The plasma concentrations of stable isotope tracers of magnesium (Mg-2 5) and calcium (Ca-44) were determined during a 50 h period, after sim ultaneous oral tracer ingestion in an adult man. The measurements were performed by magnetic sector field mass spectrometry using surface io nization ion sources. The data were used to evaluate the potential of this method in long-term kinetic studies. It is concluded that under t he experimental conditions specified, the isotopic tracers will be det ectable in blood plasma with sufficient precision and accuracy for app roximately 600 h after oral ingestion. The differences in the plasma c oncentration curves of Mg-25 and Ca-44 were analysed by means of the c onvolution integral method. These differences indicate inhomogeneities in the distribution of absorption activity relative to magnesium and calcium within the upper small intestine.