Q1(b). Partition ratio k′ and relative retention α

Answer in brief

k′ = (tR − tM)/tM: how many times longer a solute stays in the stationary phase than in the mobile phase. α = k′₂/k′₁ (≥ 1): relative retention, a measure of column selectivity.

Key points

• Partition ratio k′ (capacity factor; IUPAC retention factor k): amount of solute in the stationary phase divided by the amount in the mobile phase; k′ = K·VS/VM.
• From the chromatogram: k′ = (tR − tM)/tM = t′R/tM, where tM is the time of an unretained species.
• Best range k′ = 1 to 5; below 1 separation is poor, above 10 the analysis is long and peaks broad.
• Relative retention α = k′₂/k′₁ = t′R2/t′R1 = K₂/K₁; the later peak is in the numerator, so α ≥ 1.
• α = 1 means no separation, whatever the plate number; α depends on the two phases and the temperature.