Chromatography

The Key Ideas:

Mobile Phase: Used to describe what carries the components of the mixture to be separated at different rates over the stationary phase Stationary Phase: Used to describe the static medium which allows the mobile phase to move the mixture along its surface.

• Separation Techniques
  1. Mixture is dissolved (carried) in mobile phase
  2. Then the mobile phase moves through the stationary phase
  3. Components of the mixture experience attraction to the mobile phase and to the stationary phase via intermolecular forces
     • This is a “competition of forces”
       • Intermolecular forces of component & stationary VS component vs mobile

We use these terms to describe what happens to the components during chromatography:

• “Adsorb” into stationary phase
  • Not to be confused with absorb which relates to the volume while adsorb is the term coined for the formation of liquid or gas on a surface.
• “Resorb” into mobile phase
  • Resorption and reabsorption mean the same thing; to take back something that was previously secreted/emitted. Resorption is just the process of re-adsorbing something.

The Types of Chromatography

Paper Chromatography

• The solvent moves up the paper through capillary action.
  1. Adhesion: Solvent and Paper attraction
  2. Cohesion: Solvent and Solvent attraction

$R_f$ = Retention Factor

$R_f$ = $d_{blue}/d$ where d is the total distance the solvent has travelled over a specific period of time and $d_{blue}$ is the distance where the blue ink (or whatever component you are wanting to measure the retention factor of) has travelled.

Thin Layer Chromatography

• This is similar to the normal paper chromatography but uses a thin layer of silica gel on glass instead

Gas Chromatography

• The mobile phase is a gas (as the name suggests) - mostly helium but it can be other gases too
• Stationary phase is a column packed with solid substrate

HPLC - High Pressure Liquid Chromatography

• Pressurized liquid is used as the mobile phase
• Solid substrate is used as the stationary phase