Q1(e). Chromatographic zone broadening

Answer in brief

Molecules experience distributions of path lengths, diffusion distances and phase-transfer times.

Key points

• Eddy dispersion in packed columns gives different flow paths. Longitudinal molecular diffusion spreads a zone along the column.
• Finite mass transfer prevents instantaneous equilibration between mobile and stationary phases, especially at high flow.
• The van Deemter form is H=A+B/u+Cu. Reducing one term by changing flow can increase another.
• Appropriate packing, small particles, temperature and flow optimisation minimise broadening but do not eliminate these stochastic processes.