Chromatography

Question 1

what is chromatography?

Answer 1

a series of separation techniques

Question 2

what is the mobile phase?

Answer 2

the phase that carries the soluble components of the mixture with it

Question 3

the more soluble the component in the mobile phase = ?

Answer 3

the faster it moves

Question 4

what is the stationary phase?

Answer 4

the phase over which the mobile phase passes - it does not move with the sample

Question 5

the more affinity the component has for the stationary phase = ?

Answer 5

the slower it moves with the solvent

Question 6

what are the 3 types of chromatography?

Answer 6

thin-layer chromatography (TLC)
column chromatography
gas-liquid chromatography (GC)

Question 7

what is the eluent?

Answer 7

the solvent in the mobile phase

Question 8

what must all Rf values be less than?

Answer 8

1

Question 9

Rf = ?

Answer 9

distance moved by spot / distance moved by solvent

Question 10

what acts as the stationary phase in TLC?

Answer 10

a glass, metal or plastic sheet coated with a thin layer of silica gel or alumina (plates)

Question 11

what are the advantages of TLC over paper chromatography?

Answer 11

it runs faster
the plates are more robust than paper
smaller amounts of mixtures can be separated

Question 12

what acts as the stationary phase in column chromatography?

Answer 12

a powder (silica or aluminium oxide) or a resin

Question 13

how does column chromatography work?

Answer 13

the eluent runs down the column and the components of the mixture move at different rates and so can be collected separately in flasks at the bottom

Question 14

how can you get a better separation in column chromatography?

Answer 14

use more than one eluent

Question 15

what acts as the stationary phase in gas-liquid chromatography?

Answer 15

a powder coated with oil, packed into or coated onto the inside of a long capillary tube

Question 16

what is the mobile phase in gas-liquid chromatography?

Answer 16

an unreactive gas e.g. nitrogen, helium

Question 17

how does gas-liquid chromatography work?

Answer 17

the sample is injected into the apparatus (under pressure at high temp) and carried along by the gas
the mixture separates because the components have different retention times so they reach the detector at different times

Question 18

how can colourless spots be seen on a TLC chromatogram?

Answer 18

by shining UV light on the plate or by spraying the plate with a locating agent e.g. iodine (which reacts to give coloured compounds)

Question 19

what does the number of spots on a TLC plate tell you?

Answer 19

the number of substances in the mixture

Question 20

what would change the Rf value for a given substance?

Answer 20

if the temperature, solvent or make-up of the TLC plate changes

Question 21

what is column chromatography often used for?

Answer 21

separating and purifying larger quantities of a mixture (TLC is useful for smaller quantities)

Question 22

what is gas chromatography useful for?

Answer 22

to separate a mixture of liquids that are volatile

Question 23

what is GCMS?

Answer 23

gas chromatography mass spectrometry (two techniques in one!)

Question 24

how does GCMS work?

Answer 24

a mass spectrometer is used as the detector for a gas chromatography system

Question 25

what does separation depend on?

Answer 25

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

Question 26

what is used to identify substances in chromatography?

Answer 26

Rf values and retention times