The tendency of molecules to diffuse away from a source of high concentration contributes to longitudinal diffusion. This diffusion, which can occur in both the mobile and stationary phases, naturally broadens solute bands. Larger molecules generally diffuse much slower than smaller molecules and tend to exhibit less band broadening. This is particularly important in gas chromatography because the diffusion rates in gases are much higher than in liquids. Resistance to mass transfer, the C term in the equation, is the third contributor to band spreading.
Mass transfer is directly proportional to flow rate.
Often, the flow rate of the mobile phase is so rapid that a true equilibrium between the phases cannot be reached. Since neither the rate of transfer nor equilibrium is instantaneous, both the beginning and the end of the zone are broadened. Stationary phases may have rough surfaces in which pockets of mobile phase enter and sit stagnant because they are removed from the rest of the flow. These types of mass transfer problems are a predominant cause of bandspreading in liquid chromatography and are made even worse with the use of viscous liquids. Illustration of mobile phase molecules can become stagnant as they are trapped in the rough surface of stationary phase particles.