Chromatography fundamentals — In liquid or gas chromatography, what does the retention (capacity) factor k' describe and quantify in practical terms?

Introduction / Context:
The retention (capacity) factor, usually written as k' (or k), is one of the most important dimensionless descriptors in chromatography. It expresses how strongly an analyte is retained on the stationary phase relative to its time traveling with the mobile phase, allowing comparisons across instruments and methods.

Given Data / Assumptions:

• t_R = retention time of the analyte peak apex.
• t_M (or t_0) = dead time/void time for an unretained species.
• k' = (t_R − t_M) / t_M by definition.
• Distribution ratio K = C_s / C_m relates to k' through the phase ratio (beta).

Concept / Approach:
Although k' is calculated from times, it fundamentally reflects how the analyte partitions between stationary and mobile phases. Higher k' means greater time spent in the stationary phase (stronger retention). Appropriate design targets often keep k' in roughly 1–10 for good resolution and run time.

Step-by-Step Solution:

Verification / Alternative check:
As solvent strength increases (e.g., higher % organic in reversed-phase LC), k' drops because the analyte spends less time in the stationary phase. Observed shorter t_R confirms the distribution-based interpretation.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing k' (dimensionless) with absolute times; forgetting k' compares retained time to dead time, not to total run time alone.

Final Answer:
The distribution of an analyte between the stationary and the mobile phase.