Q1(b). van Deemter equation and its significance

Answer in brief

H = A + B/u + Cu. It links plate height to mobile-phase velocity and gives the optimum velocity, uopt = √(B/C), at which H is least and the column most efficient.

Key points

• H = plate height (HETP), u = linear velocity of the mobile phase; N = L/H: smaller H, more plates, sharper peaks.
• A: eddy diffusion (multiple paths), A = 2λdp; independent of u, small for small, uniform, well-packed particles.
• B/u: longitudinal diffusion, B = 2γDm; dominant at low u, important in GC, almost negligible in LC.
• Cu: resistance to mass transfer between stationary and mobile phases (C = CS + CM); dominant at high u.
• It fixes the optimum flow rate; small uniform particles, thin film, capillary columns (A = 0) raise efficiency; sub-2 µm particles are the basis of UHPLC.