X-rays are made to impinge on the surface of of a sample at glancing incidence such that total reflection occurs. The X-rays excite atoms in the top layers of the material and the fluorescence is detected by a Si(Li) detector (see EDX) placed above the sample. The technique is sensitive to very dilute quantities of material (a few parts in 10⁹), but does require very flat samples. The X-ray source can be an X-ray tube, or a synchrotron.