A method, instrument, and computer program software product for characterizing a sample with respect to the presence of a specified element, either as a constituent of a surface layer or of the bulk of the sample. Intensities of fluorescent emission at two characteristic emission lines are compared to establish whether the specified element is disposed above the bulk of the sample. In the case where the specified element is disposed above the bulk of the sample, an areal density of the specified element is determined, whereas in the case where the specified element is disposed within the bulk of the sample, a volumetric concentration of the specified element within the sample is determined.