Chromatography

Question 1

What is chromatography used for?

Answer 1

separate components in a mixture

Question 2

What does the mobile phase do?

Answer 2

flows past the stationary phase carrying the components of the mixture with it

Question 3

What does the stationary phase do?

Answer 3

slows components down to a varying extent allowing them to separate

Question 4

What is thin layer chromotography used for?

Answer 4

to assess the progress of reactions and purify products

Question 5

What is the stationary phase of a TLC?

Answer 5

a thin layer of solid eg silica gel or SiO2 mounted on a piece of glass, metal or plastic plate

Question 6

What is the mobile phase of a TLC?

Answer 6

an organic solvent

Question 7

How does the solvent move up the TLC plate?

Answer 7

by capillary action

Question 8

How do the components of a mixture move up the TLC plate?

Answer 8

they dissolve in the solvent (mobile phase) and move up the plate with the solvent
but they are held back to varying degrees by adsorption to the plate

Question 9

What does the speed of the component moving up the TLC plate depend on?

Answer 9

how strongly the component is adsorbed to the stationary phase
stronger adsorbtion = slower migration up

Question 10

What are the 2 things the strength of adsorption depend on?

Answer 10

the attractive forces b/w the component and stationary phase
the solubility of the component in the mobile phase

Question 11

How does the strength of adsorption depend on the attractive forces b/w the components and the stationary phase?

Answer 11

the stronger these attractions, the more difficult it will be for the solvent to dissolve the component and carry it up the plate = slower migration

Question 12

How does the strength of adsorption depend on the solubility of the component in the mobile phase?

Answer 12

the more soluble the component is, the easier it is for the solvent to overcome the adsorption and free the component from the stationary phase = migrates faster up the plate

Question 13

What does the components solubility in the mobile phase depend on?

Answer 13

solubility depends on the attractive intermolecular forces b/w each component and the solvent

Question 14

What is the Rf value definition?

Answer 14

Rf is a measure of the distance migrated by an individual substance, relative to the distance moved by the solvent front

Question 15

if the stationary phase is highly polar, what will the Rf value be like for polar substances?

Answer 15

lower Rf value = polar substances will adsorb strongly to it

Question 16

What are the advantages of TLC?

Answer 16

quick and equipment is inexpensive

Question 17

What are the disadvantages to TLC?

Answer 17

limited resolution - compounds with similar Rf values will not be clearly separated

Question 18

What types of molecules may be difficult to separate?

Answer 18

molecules with similar sizes
molecules with similar functional groups

Question 19

What is the stationary phase in gas chromotography (GC)?

Answer 19

a liquid adsorbed onto an inert solid support

Question 20

What is the mobile phase in GC?

Answer 20

a gas that is unreactive and mostly helium

Question 21

What is retention time?

Answer 21

the time taken for each component of the sample to travel through the capillary tube in GC

Question 22

What causes a shorter retention time?

Answer 22

the more time a component spends moving with the gas rather than stuck in the stationary phase = shorter retention time

Question 23

What increases retention time in the stationary phase?

Answer 23

stronger intermolecular forces b/w substances and stationary phase = greater solubility - will take longer to get through column

Question 24

What causes polar substances to have a long retention time in polar stationary phases?

Answer 24

they can form permenant d-d forces whuch are stronger than induced d-d

Question 25

What causes non-polar substances to have a long retention time in a non-polar stationary phase?

Answer 25

the molecules with a bigger surface area for contact with other substances will have longer retention times = they have more induced d-d to be broken to vapourise

Question 26

What molecules will have similar retention times?

Answer 26

compounds with similar functional groups will tend to have similar interactions in the column = similar retention times

Question 27

What does the area under the peak on a gas chromatogram show?

Answer 27

it is proportional to the amount of each compound

Question 28

How to you find the actual concentration of a substance from gas chromatography?

Answer 28

using the peak area from a gas chromatogram, you need to use a calibration curve that is proportional to different concentrations

Question 29

What other spectrum is GC combined with sometimes?

Answer 29

mass spectrums

Question 30

What are the advantages of GC-MS?

Answer 30

• very good resolution - resolve complex mixtures
• use tiny amounts of sample mixture
• large database of mass spectra available = positive identification

Question 31

What are the limitations of GC-MS?

Answer 31

• similar compounds have similar retention times = difficult to separate and identify
• unknown compounds will have no reference for mass spectra
• equipment is sophisticated and expensive

Question 32

What are the uses of GC-MS?

Answer 32

• forensic science - traces from crime scenes and illegal drugs
• airport security - detecting explosives
• space missions - substances that could indicate life traces

Question 33

What is the purpose of a cover on TLC?

Answer 33

prevent evapouration of the solvent

Question 34

Why is the GC column inside an oven?

Answer 34

the sample needs to be in gas phases

Question 35

Why is helium used instead of air in GC?

Answer 35

helium is unreactive
air contains O2 which can oxidise the sample reactants

Question 36

How can a calibration curve be made to give a more accurate conc of sample?

Answer 36

use a molecule that is similar in shape to the sample to form calibration - it will have a similar retention time

Question 37

How can you make colourless compounds visible on the TLC plate?

Answer 37

expose the plate to Iodine vapour = iodine adsorbs to the compound spots and it will appear brown