To test the method, we start with a solution of two dye molecules, Rhodamine B and Methylene Blue (chemical structures shown in Fig. ), which have distinctly different UV/Vis absorption spectra, with maxima at 555 nm and 665 nm respectively. The spectrum of a mixture of these dyes (11 µM Rhodamine B, 30 µM Methylene Blue) is shown in the top panel of Fig. . We inject the mixed solution in the lower part of the channel and water in the top part, stop the flow at t = 0 and then record consecutive UV/Vis spectra at the top of the water-filled part. The result is shown in Fig. . Initially, the absorption is zero everywhere, but with increasing time the absorption spectra of Rhodamine B and Methylene Blue appear as these molecules diffuse into the water-filled part of the cell. The Methylene Blue peak (555 nm) appears faster than the Rhodamine B peak (665 nm), see Figure . This difference is due to the lower diffusion coefficient of Rhodamine B compared to Methylene Blue, which is a consequence of the larger size of the Rhodamine B molecules. A size difference translates directly into a difference in diffusion coefficient D through the Stokes-Einstein equation: