Yergey A L, Vieira N E, Covell D G
Laboratory of Theoretical and Physical Biology, NICHD, Bethesda, Maryland 20892.
Biomed Environ Mass Spectrom. 1987 Nov;14(11):603-7. doi: 10.1002/bms.1200141105.
Fractional dietary Ca absorption, 'a', is measured by determining the ratio of two stable isotopic tracers, one of them orally (44Ca + 0.2-0.5 mg/kg) and the other intravenously (42Ca / 0.02-0.1 mg/kg). Thermal ionization mass spectometry (TIMS) is used to measure the perturbation of natural abundance isotope ratios (delta % excess). Typical sensitivity of the TIMS permits detection of a 2.5 delta % excess change from the natural Ca isotope ratio with relative standard deviations of about 0.5%. At sufficiently long times absorption becomes constant so that 'a' is determined by a product of constants and a measured ratio.
