Chromotography

Question 1

Describe the 3 types of chromatography

Answer 1

Thin layer chromatography (TLC) - a plate is coated with a solid and a solvent moves up the plate
Column chromatography (CC) - a column is packed with a solid and a solvent moves down the column
Gas chromatography (GC) - a column is packed with a solid or with a solid solid coated by a liquid, and a gas is passed through the column under pressure and high temperature

Question 2

What is chromatography used for ?

Answer 2

To separate and identify the components in a mixture

Question 3

What does separation depend on?

Answer 3

The balance between solubility in the moving phase and retention by the stationary phase

Question 4

What is used to identify different substances

Answer 4

Retention times
Rf values

Question 5

use of mass spectrometery in chromatography

Answer 5

To analyse the components separated by GC

Question 6

What is the stationary phase

Answer 6

A solid substance which the mixture passes over in order to be separated

Question 7

What is the mobile phase

Answer 7

A liquid or gas which carries the mixture over the solid material

Question 8

How is the mixture separated in the mobile phase

Answer 8

If a component of the mixture is highly soluble in the mobile phase it will travel along quickly with solvent
If low solubility then will travel slowly with the solvent

Question 9

How is a mixture separated in stationary phase

Answer 9

If a component of the mixture has a high affinity to the stationery phase it will travel slowly
If a component of the mixture has a low affinity to the stationary phase it will travel quickly

Question 10

What is the TLC plate

Answer 10

Thin piece of aluminium or glass which is coated in silica gel or aluminia which acts as stationary phase

Question 11

How TLC chronographs my works

Answer 11

Samples of the mixture to be separated are dropped on a line on the bottom of the TLC plate
The TLC plate is suspended in a small amount of solvent (Eluant) which soaked up by the plates. The solvent is the mobile phase
Once dry the plate is examined under UV light or developed using chemical reagents to make spots visible

Question 12

How can you determine number of components present in the mixture

Answer 12

The number of spots on the finished chromatogram

Question 13

What does an Rf value show

Answer 13

How far a component has moved compared to solvent front

Question 14

How to calculate Rf value

Answer 14

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

Question 15

Advantages of TLC

Answer 15

Faster than paper chromatography
Will work on very small samples
Can be used to determine when a chemical reaction is complete

Question 16

Limitations of TLC

Answer 16

Similar compounds may have similar Rf values
Conditions must be kept the came in order to fairly compare Rf value
New and unknown compounds have no reference Rf values
Can be difficult to find a solvent which separates all components in a mixture
Cannot be used to separate large quantities

Question 17

Stationary phase in column chromatography

Answer 17

Colomn is packed with a powder such as silica or aluminia

Question 18

Describe colomn chromatography

Answer 18

Uses a narrow glass tube with a spout and tap (this is the colomn)
The colomn is packed with powder such as silica or aluminia to act as a stationary phase.
The mixture to be separated is loaded onto the top of the colomn and a solvent is poured onto the top and soaks down through the stationary phase (solvent is the mobile phase)
Different components of the mixture travel down through solid phase at different rates. One will reach the bottom and can be collected into a beaker first and then the next one collected into a separate beaker later

Question 19

Advantages of colomn chromatography

Answer 19

Larger quantities can be separated

Question 20

Why is gas-liquid chromatography used

Answer 20

To separate complex mixtures of volatile components. Widely used in organic chemistry

Question 21

Mobile phase in GC

Answer 21

a gas known as the carrier gas, typically unreactive has such as nitrogen or helium

Question 22

Stationary phase in GC

Answer 22

A column made of glass or metal with a thin coating of liquid or a solid which acts as stationary phase

Question 23

Describe GC

Answer 23

The sample to be separated is injected into the colomn, the carrier has transports the components of the mixture along the colomn at different rates causing them to separate out. NOT DUE TO SOLUBITLY AS NO SOLVENT separate purely sue to affinity to the stationary phase.

Question 24

What is retention time

Answer 24

Used in gas chromatography- the time taken from the point of injection for the component to the detector. When the components leave the colomn and reach the detector an electrical signal is produced

Question 25

Gas chromatogram evaluating

Answer 25

The area of the peak in a gas chromatogram is proportional to the amount of component in a mixture. This enables the relative amounts of components in a mixture to be determined by comparing peak areas

Question 26

Limitations of GC

Answer 26

When anyalsing a complex mixture the chromatogram will contain many peaks
Potentially 1000’s of chemicals have simailr retention times and peak shapes and detector responses so compounds cannot be positively identified as they held in a simple way to the stationary phase
Not all substances in a sample many be separated and detected/ peaks may be hidden behind other
Unknown compounds have no reference retention times

Question 27

How to overcome limitations of GC

Answer 27

By lining a gas chromatograph with a mass spectrometer called GC-MS.
GC can separate conponents in a mixture but cannot identify them conclusively
MS cannot separate components in a mixture but can identify them

Question 28

Advantages of GC-MS

Answer 28

enables the separation of complex mixtures and identification of each component in that mixture
Quick
Only requires a very small amount of sample

Question 29

Uses of GC-MS

Answer 29

Forensics - minute particles at the scene of a crime can be identified often providing critical evidence in court during a trial
Environmental analysis- used to identify organic pollutants like waste water and drinking water quality
Airport security- used to detect traces of explosives in luggage and on humans
Space probes - spectrometers have been sent to mars and Venus to collect and analyse material and their atmospheres

Question 30

Predicting how components will interact with the mobile and stationary phases

Answer 30

Consider there polarity
Mobile phase : like dissolves like
eg if a polar solvent is used and component polar it will dissolve well therefore move up TLC more or be first to be collect in GC
Stationary phase : like sticks with like
eg if polar stationary phase used and polar molecule, the molecule will have higher affinity to stationary phase so doesn’t move far in TLC and takes longer to collect in GC