The distance between the 84 and 16 per cent levels of the depth profile of an element in a perfect sandwich sample with an infinitesimally small overlap of the components. These limits correspond to the 2σ-value of the Gaussian distribution of the signal at the interface.
Source:
PAC, 1979, 51, 2243. 'General aspects of trace analytical methods—IV. Recommendations for nomenclature, standard procedures and reporting of experimental data for surface analysis techniques' on page 2247 (https://doi.org/10.1351/pac197951112243)