[en] The use of high-intensity, tunable monochromatic x-rays for the quantitative analysis of biological and geochemical specimens at the 10^-8 g level is described. Incident x-rays were obtained from the new LBL-EXXON permanent magnet wiggler beamline at the Stanford Synchrotron Radiation Laboratory. The sample detector geometry was designed to make optimal use of polarization advantages for background reduction. Questions regarding the sensitivity and accuracy of the measurements were studied with particular emphasis on the advantages of tuning the x-ray energies for optimum excitation for specific elements. The implications of these measurements with respect to the use of x-ray microprobe beams will be discussed