1. A method of in vivo measurement of the concentration of a specified trace element in bone, comprising steps of: irradiating a portion of a bone with x-rays having energies suitable for causing the trace element to fluoresce, the irradiated portion of the bone being covered by overlying tissue; measuring intensities of first and second fluoresced x-ray peaks characteristic of the trace element; and calculating the concentration of the trace element based on the intensity of a selected one of the first and second fluoresced x-ray peaks and a function having as an argument the intensity ratio of the intensities of the first and second fluoresced x-ray peaks, at least one parameter of the function being empirically determined by measurement of samples having different thicknesses of overlying tissue or a surrogate thereof.