The present invention includes a method and apparatus for measuring the transport of analytes from living, biological cells through a cell barrier, which includes, but is not limited to, steps of providing the living, biological cells loaded with analytes, unloading at least a portion of said analytes from the cells through ion channels, removing unloaded analyte remaining in the cells, and measuring the analytes using x-ray fluorescence, specifically wherein the measurement uses an energy dispersive x-ray fluorescence spectrometer equipped with a microfocus x-ray tube. These steps may be repeated so that multiple measurements can be obtained over a period of time.