The practical implementation of optical DOSY is shown in Figure . We use a liquid-sample cell suitable for transmission spectroscopy, combined with a standard double syringe pump to inject the solution and solvent. The cell has two entrance holes and one exit hole, and we inject the sample solution into one entrance, and pure solvent into the other at identical flow rates. After the pumping is stopped (at t = 0), the dissolved compounds diffuse into the solvent-filled half of the cell, at a rate that depends on their diffusion coefficient, and hence on their size. The cell is placed in a conventional UV/Vis spectrometer to record the time-dependent absorption spectrum in the portion that is initially filled with solvent. The light beam in a standard UV/Vis spectrometer typically has a diameter of several millimeters, which is too large to selectively sample the relevant part of the sample volume. To ensure that we measure the absorption spectrum of a specific region of the volume we mount a optical slit onto the sample cell (indicated by the green rectangle in Fig. ).