Variables
Description
What is this formula?
The selectivity factor measures how differently two compounds interact with the chromatographic system.
It is one of the three fundamental parameters that determine chromatographic separation quality.
When to use it
Use this formula when evaluating separation performance, comparing stationary phases, or optimizing chromatographic methods.
Example
Given:
k'₁ = 2.0
k'₂ = 5.0
Formula:
α = k'₂/k'₁
Substitution:
α = 5.0/2.0
Result:
α = 2.5
The second compound is retained 2.5 times more strongly than the first compound.
Applications
HPLC
GC
Method development
Column selection
Analytical chemistry
Note
The selectivity factor is always greater than or equal to 1.
Interpretation:
α = 1 → no separation
α ≈ 1.05–1.20 → difficult separation
α > 1.2 → useful separation
Increasing selectivity is often the most effective way to improve chromatographic resolution.