Large molecules/particles. A different issue arises if the species size is large (≫ 10 nm). In that case the diffusion coefficient is so low that diffusion over mm distances would take prohibitively long. This can be remedied in two ways. If the sample contains only one single prohibitively large species, then a simple approach is to measuring the time-dependent absorption spectrum in the solution-filled half of the channel, thus tracking the depletion of all the other, smaller solute molecules due to their diffusion. In this way all the species spectra can obtained, as well as the diffusion coefficients except the one of the largest species. A more generally applicable method for studying larger compounds is to reduce the distance over which they have to diffuse in the experiment. The characteristic time for the diffusion to spread a molecule over the entire channel width is τ c = L 2 /π 2 D. This means that using a narrower channel can decrease the waiting time, and using a 10 times narrower channel reduces the measurement time by a factor of 100. With such a narrower channel, the spatial selection of the optical probing can no longer be done with a slit because the transmission would be too low (leading to unworkable signal-to-noise levels).