Mathematical process leading to a model of transport of a substance in terms of compartments and rate constants, usually taking the form \[C=A\ \rm{e}^{-\alpha\ t}+B\ \rm{e}^{-\beta \ t}\rm{...}\] where each exponential term represents one experiment. \(C\) is the substance concentration; \(A\), \(B\),… are proportionality constants; \(\alpha \), \(\beta \),… are rate constants; and \(t\) is time.