The time-dependent spectral data of Figure can be converted to a two-dimensional spectrum that has wavelength on one axis and diffusion coefficient on the other. To obtain such a DOSY spectrum from the time-and frequency-resolved data, we quantitatively analyze the data using the diffusion equation. It is easily derived that the time-dependence of the concentration at the top of the channel is determined only by the channel height and the diffusion coefficient; and by solving the diffusion equation (see Materials and Methods) one finds that the normalized timedependent concentration at that position is given by a universal function C(Dt/L 2 ) where t is time, D the diffusion coefficient, L the channel height (see eq. 2 above for the explicit expression of this function C). If there are N species present in the solution, with absorption spectra A i (λ) (with i the species number and λ the wavelength), then the total absorption at wavelength λ and time t is given by