Chromatography

Graph

Chromatography is a laboratory technique for separation and identification.

Types of Chromatography include:

In all examples of chromatography:

There exists a stationary and mobile phase.
Separation of substances relies on solubiilty.
- In general, if a substance is more soluble in the mobile phase in comparison to the substance's desire to adsorb (bind to a surface) to the stationary phase.

Paper Chromatography

Is used to separate mixtures, especially dyes or pigments.
Procedure (dyes in ink mixture):
- We put the mixture on a line of origin
  - Note that we do not place the line of origin within the solvent, as the mixture would simply dissolve in the solvent and not move up the stationary phase.
- Dots of single dyes are placed alongside a dot of the unknown mixture.
- As the solvent moves up the paper, the pattern of the single dyes can be compared to that of the mixture's pattern.
The solvent(mobile phase) is drawn up the paper by capillary action.
In general, paper chromatography uses polarity of the substances.
- The chromatography paper (stat. phase) is usually very polar.
- Polar substances in a mixture will be more attracted to the stationary phase and thus will adsorb and not move with the mobile phase as much.
- Non-polar substances are less attracted, and thus do not adsorb well and can be dissolved in the mobile phase, and move greatly along with the mobile phase.
This form of chromatography is most useful if the substances involved have distinct colours.

Thin-Layered Chromatography

In TLC, the stationary phase is a layer of silica gel fixed onto a glass plate.
The mobile phase is a solvent which travels up the plate, carrying the substances.
Has the same principles as paper chromatography.
TLC involves a stable silica gel, but paper in paper chromatography is flexible and might affect results of chromatography. In TLC, the glass plate does not bend, and thus you will have better results.
Capillary action still draws the solvent up the matrix; however while the molecules in paper chromatography are separated based on mass, in TLC, separation often depends upon solubility or charge, due to the interaction of solute and matrix.
A dry sample is placed in the silica gel matrix. As the solvent front moves up the gel, it dissolves the sample and caries it up the matrix with it.
Some of the particles in the sample stick more strongly to he silica gel than others, so they lag behind the solvent
Eventually the different substances in the sample separate out, with similar molecules travelling a similar distance.

Gas Chromatography:

Used for identifying substances and their concentrations in stable mixtures (ones that do not decompose)
- Mobile phase: inert carrier gas
  - Helium
  - Nitrogen
  - Hydrogen
- Stationary phase: High BP viscous liquid (that stays liquid(non-volatile))
Retention time is based on volatility of analytes in the mixture.
- More volatile analytes prefer to remain as gas (vapour), and thus are carried by the carrier gas and leave the chromatography chamber easily.
- The opposite applies for less volatile analytes
- Note that volatility is highly related to boiling-point, which is in turn related to intermolecular forces.
  - Stronger intermolecular forces leads to a higher boiling point. Substances that have a high boiling point are less likely to remain in the gaseous state, and thus have a low volatility.
- Retention time is also impacted by the column's temperature, as high temperature tends to increase volatility and thus retention time decreases.
As analytes leave the column, they are analysed by a detector.
When all analytes have left the chamber, data collated by the detector can be analysed to produce a chromatogram.
When used with Mass spectrometry, gas chromatography becomes highly accurate identifier of compounds.
Flame ionization detector (FID) is a commonly used detector that give retention time.

HPLC:

HPLC is for mixtures that easily decompose, or have extremely low volatility (prefer to be liquid/solid rather than gas)
Mobile phase: liquid solvent (usually non-polar in normal)
Stationary phase: fine particles of $SiO_2$.