Why Do Bands Spread

An infinitely narrow band of solute that is stationary inside the column slowly broadens because solute molecules diffuse away from the center of the band in both directions. This inescapable process, called longitudinal diffusion, begins at the moment solute is 

The faster the flow rate, the less time a band spends in the column and the less time there is for diffusion to occur. The faster the flow, the sharper the peaks. Broadening by longitudinal diffusion is inversely proportional to flow rate  

Solute Requires Time to Equilibrate Between Phases 

Broadening by finite rate of mass transfer broadening (21-6) 

---