Retention Factor (k), Void Time (t0) & Selectivity (α) Calculator

Estimate t0 from column size and flow, then check k and α for two peaks.

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Column length mm

Column ID mm

Flow mL/min

Measured t0 (optional) min

Peak 1 tR min

Peak 2 tR min

Void volume estimated as Vm ≈ 0.5 · L · d² for fully porous particles. Enter a measured t0 (uracil, thiourea) to override.

t0–

k peak 1–

k peak 2–

α–

k = (tR − t0)/t0 · α = k₂/k₁ · t0 = Vm/F, Vm ≈ 0.5 · L · d²

How it works

Retention factor k expresses how long a compound stays on the column relative to an unretained marker. Values between about 1 and 10 give a good balance of resolution and run time; below 1, peaks crowd the solvent front where matrix and system peaks interfere.

Selectivity α = k₂/k₁ is the lever that moves resolution most. The resolution equation, Rs = (√N/4)·((α − 1)/α)·(k/(1 + k)), shows why: doubling the plate count improves Rs by only 41%, while a small change in α can double it.