Chromatography

Question 1

What is chromatography used for?

Answer 1

It is used to seperate individual components from a mixture of substances

Question 2

What do all forms of chromatography have?

Answer 2

• stationary phase
• mobile phase

Question 3

What is a stationary phase?

Answer 3

It does not move and is normally a liquid or a solid supported on a solid

Question 4

What is the mobile phase?

Answer 4

It does move and is normally a gas or liquid

Question 5

Describe TLC.

Answer 5

• it is a quick and inexpensive analytical technique that indicates how many components are in a mixture
• the technique uses a TLC plate usually made of plastic coated by a thin layer of solid adsorbent which is usually silica

Question 6

What is the stationary phase in TLC?

Answer 6

• The absorbent on plastic/glass (usually silica)
• different components in the mixture have different affinities to the adsorbent and bind with differing strengths to its surface

Question 7

What is adsorption?

Answer 7

The process by which solid silica holds the different substances in the mixture to its surface.
Seperations are achieved by the relative adsorptions with the stationary phase (solvent)

Question 8

What is the mobile phase in TLC?

Answer 8

• the organic solvent that moves up the plate by capillary forces

Question 9

What is one way you can interpret a TLC plate?

Answer 9

• by comparing the rf value of a component to known values recorded by the same solvent system and adsorbent

Question 10

How do you calculate RF values?

Answer 10

Rf = distance moved by component/distance moved by the solvent front

Question 11

What is gas chromatography used for?

Answer 11

It is used for seperating and identifying volatile (easily evapourated) organic compounds in a mixture

Question 12

What is the stationary phase in gas chromatography?

Answer 12

• A high boiling point liquid adsorbed onto an inert solid support

Question 13

What is the mobile phase in gas chromatography?

Answer 13

• An inert carrier gas such as helium and neon that carries components through the capillary column

Question 14

What is the process of gas chromatography?

Answer 14

• A small amount of volatile mixture is injected into the apparatus called a gas chromatograph
• the mobile carrier carried the components through the capillary column.
• The more soluble the components in the liquid stationary phase, the slower it moves through the capillary column
• The components are therefore seperated based on thier solubility in the liquid stationary phase
• the components in the mixture reach the detector at different times depending on their interactions with the stationary phase
• The compound retained in the column for the shortest time has the lowest retention time and is detected first

Question 15

What is retention time?

Answer 15

time taken for each component to travel through the column

Question 16

How do you interpret a gas chromatogram?

Answer 16

• retention time = can be used to identify components by comparison with known retention times
• peak integrations = area under each peak which can be used to determine the concentrations of each component in the sample

Question 17

How can you determine the concentration of components using a gas chromatogram?

Answer 17

1.prepare a standard solution of known concentrations of the compounds being investigated
2. obtain a gas chromatogram for each concentration
3. plot a calibration curve of peak area agaisnt concentration. This is called external calibration
4. obtain a gas chromatogram of the compound being investigated under the same conditions
5. use the calibration curve to measure the concentration of the compound