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; α, β,… are rate constants; and t is time.