We have air with constant benzene concentration 0.02628 mol/m3 connected via the capillary (0.31 mm i.d., 48 mm length) to the flow path of air flowing at the rate of 100 mL/min (velocity = 2.36 mm/s, diameter of a tube 30 mm). According to the Fick's law of diffusion (Flux = D C / Z), flux of benzene (D = 1.04E-5 m2/s) via the capillary is 5.96E-6 mol/(m2 s) meaning that 4.29E-13 mol of benzene is supplied via the capillary cross section (area = 7.55E-8 m2) to the flowing air per second (or 2.57E-11 mol/min). If we divide this rate of benzene supply by the flow rate of air, we should get the concentration of benzene in the flowing air of 2.57E-13 mol/mL or 20 ug/m3. In practice, we measure around 80-120 ug/m3 at these conditions. We could not find the reason of the problem and checked using Comsol, which gave us the same flux as calculated above, but concentration was 281 ug/m3, not as calculated, which seems against the mass conservation law. Did we forget something important? Thanks in advance