Chromatography Fundamentals—Meaning of the Selectivity Factor (α) In chromatographic method development, what does the selectivity factor describe for a pair of analytes?

Introduction / Context:
The selectivity factor, commonly denoted α, is a central parameter in chromatographic resolution. Along with efficiency (plate number, N) and retention, α governs how distinctly two analytes are separated by a given stationary/mobile phase combination.

Given Data / Assumptions:

• Two analytes elute with different retention factors (k values).
• Peak broadening is comparable between the two analytes.
• Isocratic or gradient elution context, definition of α remains the ratio of adjusted retentions.

Concept / Approach:
Selectivity describes how differently two analytes interact with the chromatographic system. Formally, α = k2 / k1 (with k2 > k1), or equivalently the ratio of adjusted retention times. Larger α means greater relative separation for the same efficiency and capacity, directly improving resolution without changing peak widths.

Step-by-Step Solution:

Verification / Alternative check:
Resolution equation shows Rs increases with α for fixed N and k-averages, highlighting the importance of selectivity tuning (e.g., mobile-phase composition, temperature, stationary phase chemistry).

Why Other Options Are Wrong:

• Peak width differences relate to efficiency/tailing, not selectivity.
• “Maximum number of species” concerns peak capacity—distinct from α.
• Detector linear range is unrelated to selectivity.

Common Pitfalls:
Confusing selectivity (α) with resolution (Rs) or retention (k); each contributes differently to separation performance.

Final Answer:
The relative separation between two species (e.g., ratio of their retention factors)