Mathematical process leading to a model of transport of a substance in terms of compartments and rate constants, usually taking the form $$C=A{\text{e}}^{-\alpha t}+B{\text{e}}^{-\beta t}\text{...}$$ 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.