What does the van Deemter plot illustrate?

What does the van Deemter plot illustrate?

• A. The relationship between velocity and viscosity in a fluid.
• B. The effect of velocity on plate height in chromatography.
• C. The correlation between temperature and solubility in a liquid.
• D. The rate of diffusion in a gas.

Answer: (B)

In chromatography, the van Deemter plot shows how column efficiency, quantified by plate height (H), changes as you vary the linear velocity of the mobile phase. The relationship is captured by the van Deemter equation: H = A + B/u + C u, where u is the mobile phase velocity. The terms correspond to different broadening mechanisms: A comes from eddy diffusion (multiple paths through the packed bed) and is essentially a constant with velocity; B/u reflects longitudinal or axial diffusion (molecules spreading along the column, which becomes more significant at lower speeds); and C u represents mass transfer resistance (how quickly solute moves in and out of the stationary phase, which becomes more limiting at higher speeds). As you increase velocity, the B/u term shrinks, reducing H initially, but the Cu term grows, increasing H at higher speeds. This creates a minimum in H at an optimal velocity, where the overall column efficiency is highest. Practically, engineers aim to operate near this minimum to maximize the number of theoretical plates (N ≈ L/H) and achieve better separations. Other options don’t fit because this plot is not about how velocity affects viscosity, solubility, or diffusion rates in a gas; it specifically links flow velocity to chromatographic efficiency through the plate height concept.

In chromatography, the van Deemter plot shows how column efficiency, quantified by plate height (H), changes as you vary the linear velocity of the mobile phase. The relationship is captured by the van Deemter equation: H = A + B/u + C u, where u is the mobile phase velocity. The terms correspond to different broadening mechanisms: A comes from eddy diffusion (multiple paths through the packed bed) and is essentially a constant with velocity; B/u reflects longitudinal or axial diffusion (molecules spreading along the column, which becomes more significant at lower speeds); and C u represents mass transfer resistance (how quickly solute moves in and out of the stationary phase, which becomes more limiting at higher speeds).

As you increase velocity, the B/u term shrinks, reducing H initially, but the Cu term grows, increasing H at higher speeds. This creates a minimum in H at an optimal velocity, where the overall column efficiency is highest. Practically, engineers aim to operate near this minimum to maximize the number of theoretical plates (N ≈ L/H) and achieve better separations.

Other options don’t fit because this plot is not about how velocity affects viscosity, solubility, or diffusion rates in a gas; it specifically links flow velocity to chromatographic efficiency through the plate height concept.