Lecture on Gas Chromatography: Gas-Solid vs. Gas-Liquid

Overview

Discussed Gas Chromatography (GC) focused on Gas-Liquid Chromatography (GLC) and now shifting to Gas-Solid Chromatography (GSC).

Basis: Adsorption of gaseous molecules on a solid surface.
Distribution Coefficients (K): Higher in GSC than in GLC, leading to greater separation efficiency.
Applications:
- Useful for components not retained by GLC, including:
  - Air components, Hydrogen sulfide, Carbon disulfide, Nitrogen oxides, Carbon monoxide, Carbon dioxide, and rare gases.
Columns Used:
- Both packed and open tubular columns.
- Open tubular columns have a thin layer of adsorbent on the inner wall.

**Molecular Sieves:"
- Composed of aluminum silicate ion exchangers.
- Molecules as small balls, facilitating ion exchange between mobile and stationary phases.
- Pore sizes depend on present cation: Typically 4, 5, 10, or 13 angstroms.
- Smaller molecules penetrate particles' interior for adsorption.
Porous Polymers:
- Made from styrene cross-linked with divinyl benzene.
- Uniform pore size controlled by cross-linking.
- Useful for separating gaseous polar species such as Hydrogen sulfide, Nitrogen oxides, Water, Carbon dioxide, Methanol, and Vinyl chloride.**

Columns Compared:
- 5 x 1/8 micro molecular sieve column vs. plot column.
Gas Liquid Column:
- Retains only Carbon dioxide (CO2), other gases pass through.
- CO2 is identified as 'b' in exhaust mixtures.
- Need to avoid permanent absorption of CO2 on sieves by switching direct flow upon CO2 elution.
- Involves coordination between ordinary gas-liquid column and molecular sieve column.

Switching Mechanism:
- When CO2 elutes, the flow is switched to avoid absorption.
- After CO2 signal returns to zero, flow switches back to separate and evolve remaining sample components.
- Coordination helps prevent CO2 retention on sieves and prolong column utility.

Open Tubular Column:
- Lined with porous polymer.
- Demonstrates effective separation using the plot column.

These notes provide an overview of Gas-Solid Chromatography, focusing on its principles, adsorbents, applications, and practical column operations.