Modern Analytical Techniques II

Question 1

Chromatography

Answer 1

Used for the separation and identification of compounds

All types of chromatography have a stationary phase and a mobile phase

• mobile phase - the substance (e.g. solvent) that moves the substance during the experiment
• stationary phase - the material that does not move

Each substance will prefer either the stationary phase or the mobile phase and will be in dynamic equilibrium between the two phases

It depends on the relative solubility of the components in the mixture as to how quickly it moves through the stationary phase

• if a compound is more strongly attracted to the stationary phase, it will move more slowly during separation and so moves less distance
• if it is more strongly attracted to the mobile phase, it will move much faster and further

Question 2

Paper Chromatography

Answer 2

Stationary phase: paper

Mobile phase: solvent

• used to separate a mixture of coloured dyes

1. samples of ink are spotted onto pencil line (base line)
2. the paper is suspended in shallow layer of solvent
3. solvent must be below base line

• pencil used to not contaminate ink
• if the ink is colourless, ninhydrin is used to get the spots to show up

Factors determining Solubility

The interaction of molecules in the sample with water molecules in the chromatography paper is the most important factor.

The distance travelled depends on the samples relative attraction to the water molecules in the paper compared to the water molecules in the solvent

• Polar molecules are attracted to the water in the paper.
• if a non-polar solvent is used, the sample of polar molecules will interact with the stationary phase more than the mobile phase

Question 3

Thin Layer Chromatography

Answer 3

Stationary Phase: Silica Gel

Mobile Phase: solvent

• the silica gel is coated onto a piece of glass, metal or rigid plastic
• carried out in the same way as paper chromatography

Factors affecting Solubility

The surface of the silica gel is very polar and because of the -OH bond it forms hydrogen bonds with compounds around it. It also has van der waal dispersion forces and dipole-dipole interactions

The compound which forms polar hydrogen bonds will stick to the surface of the silica gel more firmly than the other one, so it is adsorbed more strongly and will travel less far

Identifying the Compound

• use potassium permanganate
• use ninhydrin
• use a thin layer plate with a fluorescent dye incorporated into the gel
  
  • the fluorescence will glow when exposed to UV light and the spots will show up as dark patches

Question 4

Retention Factor

Answer 4

R_f = Dist. travelled by compound/Dist. travelled by solvent

Question 5

Column Chromatography

Answer 5

Used to separate mixtures on a larger scale, there are two different types - HPLC and Gas

• the silica gel is packed into a column rather than spread thinly
• the gel must be saturated with a solvent
  
  • the solvent is applied at the top of the column and allowed to run through before the sample is added
  • it moves slowly downwards through the gel
  • the solvent must be continuosly added to continue to move down the column carrying the sample with it

Stationary phase: silica gel

Mobile phase: solvent

The sample can be washed off the column - Eluted (collected for further analysis)

If the sample is made of a mixture of compounds, they will elute at different times so can be collected separately

Factors affecting Time taken

• strength of intermolecular forces affects retention time

Question 6

High Performance Liquid Chromatography

Answer 6

Instead of the solvent dripping through under gravity, it is injected into the column and forced through with the solvent under high pressures of 400 atmospheres, making it faster

It also uses a much smaller particle size for the packing material, which increases the surface area for interactions between the stationary phase and molecules, so there is better separation

The whole process is automated and further analysis takes place as the compounds are eluted

Time Taken for Elution:

• solvent of solvent mix used
• pressure used
• temperature inside column

Question 7

Gas Chromatography

Answer 7

The sample is vaporised and then carried into the column as a gas by a carrier gas (e.g. helium or nitrogen) which is the mobile phase

The stationary phase is a porous material which is coated in a high boiling point polymer, which is a waxy substance

The stationary phase is packed into a long and thin column which is in an oven of temperatue 50°C-250°C

Factors affecting Retention Time

• boiling point - higher than column temperature, high retention time
• solubility in liquid stationary phase - high solubility means higher retention time
• temperature of column - higher temperature reduces retention times as they are more likely to become gaseous