Exam (elaborations) analytical chemistry 3 (SCHA032)

Analytical Chemistry 3 (SCHA032):
Module 1 - Chromatography
This module covers the core principles of chromatography, focusing on efficiency, the Van
Deemter equation, and resolution.


1. The Van Deemter Equation
The Van Deemter equation describes the relationship between the plate height (H) and the
linear velocity (u) of the mobile phase:

H = A + B/u + Cu

Breakdown of terms:

• A (Eddy Diffusion): Caused by multiple paths through the stationary phase.
Independent of flow rate.
• B (Longitudinal Diffusion): Caused by diffusion of analyte in the direction of flow.
Inversely proportional to flow rate.
• C (Mass Transfer Resistance): Caused by the time taken for the analyte to equilibrate
between stationary and mobile phases. Proportional to flow rate.

2. Effect of Parameters
Temperature: Increasing temperature increases the diffusion coefficients. This generally
improves mass transfer (decreasing C) but can increase longitudinal diffusion (B).

Flow Rate (u): The optimal flow rate is found at the minimum of the H vs. u curve.


3. Practice Problem: Resolution
Resolution (Rs) is defined as:

Rs = 2 * (tR2 - tR1) / (w1 + w2)

Problem: Two peaks are observed in a chromatogram. Peak 1 has a retention time of 10.0
min and a base width of 0.50 min. Peak 2 has a retention time of 12.0 min and a base width
of 0.60 min. Calculate the resolution (Rs).

Solution:
tR1 = 10.0, w1 = 0.50

tR2 = 12.0, w2 = 0.60