1. What is Longitudinal Diffusion in the Van Deemter Equation?

Definition: Longitudinal diffusion is the contribution to band broadening in chromatography that results from the diffusion of analyte molecules along the column axis.

Purpose: This calculator helps determine the longitudinal diffusion coefficient (B) using the Van Deemter equation parameters.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( B \) — Longitudinal diffusion coefficient (m²/s)
• \( H \) — Plate height (m)
• \( A \) — Eddy diffusion term (m)
• \( C \) — Resistance to mass transfer coefficient (s)
• \( u \) — Average mobile phase velocity (m/s)

Explanation: The formula isolates the longitudinal diffusion component from the Van Deemter equation by subtracting the eddy diffusion and mass transfer resistance terms from the total plate height.

3. Importance of Longitudinal Diffusion Calculation

Details: Understanding longitudinal diffusion helps in optimizing chromatographic separation conditions and predicting column efficiency.

4. Using the Calculator

Tips: Enter all parameters from the Van Deemter equation. Plate height and velocity must be > 0, while eddy diffusion and resistance can be ≥ 0.

5. Frequently Asked Questions (FAQ)

Q1: What is the Van Deemter equation?
A: The Van Deemter equation describes the relationship between plate height (H) and mobile phase velocity (u) in chromatography.

Q2: Why is longitudinal diffusion important?
A: It's one of three main factors affecting band broadening in chromatography, particularly at low flow rates.

Q3: What are typical values for these parameters?
A: Plate height is typically 10⁻⁴-10⁻³ m, eddy diffusion 10⁻⁵-10⁻⁴ m, resistance 10⁻³-10⁻¹ s, and velocity 10⁻³-10⁻¹ m/s.

Q4: How does mobile phase velocity affect B?
A: Longitudinal diffusion becomes less significant at higher velocities.

Q5: Can B be negative?
A: No, if you get a negative result, check your input values as this suggests an impossible combination.