The rate of transport across the barrier is measured, not the amount present in the matrix. Sorption measures the amount of transport from one phase to the other, but no definition is made of the initial state of the migrant in the matrix, which is usually considered as zero concentration. Desorption, however, depends not only upon the initial concentration in the matrix, but also on the thermodynaincs of transport, since the distribution of binding energies in the active sites of the matrix can become significant at sufficiently low initial concentration, with zero levels in the external phase.