We present x-ray fluorescence (XRF) results from studies of metal impurities in silicon. A synchrotron-based XRF microprobe, with μm spatial resolution, was used to detect and map the impurities. The sensitivity of the XRF microprobe was determined for copper and iron in silicon using well-characterized standard samples. We have concluded the system can detect one iron or copper precipitate in silicon with a radius of ≈14nm. This sensitivity pertains to other relevant impurities in silicon, such as, chromium, manganese, cobalt, nickel and gold. Furthermore, we have detected and spatially mapped metal impurity precipitates in silicon, which are undetectable by Energy Dispersive Spectroscopy in a Scanning Electron Microscope. These results exhibit the extraordinary sensitivity of the XRF microprobe for metal impurities in semiconductors.