什么是 Mass-dependent and mass-independent fractionation？

An element with three isotopes or more has a mass-dependent fractionation when the fractionations of its isotope ratios are in the ratio of their mass differences, as set by the masses; otherwise it has a mass-independent fractionation, measured by the departure from the mass-dependent line, such as $\Delta^{33}\mathrm S = \delta^{33}\mathrm S - 0.515\,\delta^{34}\mathrm S$.