Van Deemter equation — What key operating parameter can be identified from a Van Deemter plot (plate height H versus linear velocity u)?

Introduction / Context:
The Van Deemter equation relates plate height (H) to linear velocity (u) via contributions from eddy diffusion (A), longitudinal diffusion (B/u), and mass-transfer resistance (Cu). Plotting H against u reveals where a column operates most efficiently.

Given Data / Assumptions:

• H = A + B/u + Cu describes band broadening mechanisms.
• The minimum of the H–u curve corresponds to maximum efficiency (highest N).
• Flow rate is proportional to linear velocity for a given column geometry and gas.

Concept / Approach:
By scanning linear velocity and measuring peak widths, one obtains a Van Deemter curve. The minimum in this plot indicates the optimum linear velocity (and thus flow) that gives the smallest H (largest N) under the tested conditions.

Step-by-Step Solution:

Verification / Alternative check:
Method robustness studies show narrower peaks and better resolution near the H minimum; deviating to very high or very low u worsens peak broadening as predicted.

Why Other Options Are Wrong:

• Selectivity (alpha) depends on phase chemistry and temperature, not directly on H–u plots.
• Oven temperature and column length are optimized by other considerations.
• Distribution constants require retention data, not just H vs u.

Common Pitfalls:
Assuming the minimum is identical for all analytes; multicomponent mixtures may favor a compromise u near, but not exactly at, the global minimum.

Final Answer:
Optimum mobile-phase linear velocity (flow rate) that minimizes H.