where N is the number of species in the mixture (in this case 2), D i is the diffusion coefficient of species i, and L is the channel width. Since the two dyes have distinct absorption peaks, we can determine their diffusion coefficients in straightforward manner, by least-squares fitting the expression for the time-dependent absorption of a single compound (eq. 2) to the time-dependent absorptions at 550 nm and 667 nm. In this way we obtain accurate estimates for D 1 and D 2 , and subsequently fitting equation 3 (with N = 2) to the entire time-and wavelength dependent data set (keeping D 1 , D 2 fixed), we obtain the spectra A i (λ) associated with the two diffusion coefficients. From this combined data, we obtain the optical-DOSY spectrum S(λ, D) by multiplying the spectral amplitude A i (λ) with the appropriate probability distribution for D i : 13