Investigating trace element determination in rocks and taking into account possible instrumental interferences, it is shown that the general relationship between I (net peak intensity) and c (fluorescent element concentration) is (Z - Z,)M = Ac + B where M is the matrix effect correction factor and A is a constant proportional to the efficiency of excitation; B and f,, correspond to the intensities due to, respectively, the scattering (especially, by the sample) of any instrumental interfering radiation, and a possible stray emission by the instrument itself. Using this equation under optimized conditions allows interferences as high as 200 ppm (if expressed in equivalent concentration) to be compensated without any significant decrease regarding the general accuracy of determinations