In chromatography theory, what does an eluotropic series describe when selecting solvents for a given adsorbent or stationary phase?

Introduction / Context:
An eluotropic series is a practical guide used by chromatographers to choose mobile phases. It orders solvents by their eluting power on a particular stationary phase, which is crucial for method development and troubleshooting.

Given Data / Assumptions:

• The stationary phase (e.g., silica) is fixed.
• Solvents can be ordered from weak (low elution strength) to strong (high elution strength).
• The series is adsorbent-dependent (ranking changes with the stationary phase chemistry).

Concept / Approach:
Eluting power reflects how effectively a solvent competes with analyte–adsorbent interactions. A stronger solvent displaces analytes more readily, reducing retention and speeding elution. The eluotropic series captures this in a ranked list (e.g., for silica, hexane < toluene < dichloromethane < ethyl acetate < acetone < acetonitrile < methanol < water).

Step-by-Step Solution:
Identify what is being ranked: solvents, not the column material.Define the criterion: relative ability to displace solutes from the stationary phase.Select the option that explicitly states this ranking role.

Verification / Alternative check:
Method development guides show solvent switches along an eluotropic series to tune retention times and resolution. Empirical retention changes confirm the rankings.

Why Other Options Are Wrong:

• Ranking column packing (b) confuses stationary phase selection with solvent choice.
• “Measure of solvent adsorption energy” (c) is not the standard definition; the series is empirical and contextual.
• None of the above (d) is incorrect because (a) defines it correctly.

Common Pitfalls:
Assuming one universal series; rankings depend on the stationary phase and sometimes on temperature or additive content.

Final Answer:
ranks solvents by their relative abilities to displace solutes from a given absorbent