[29.1] chromatography & functional group analysis

Question 1

what is the purpose of chromatography?

Answer 1

to separate individual components from a mixture

Question 2

what are some practical applications of chromatography?

Answer 2

• forensic science
• analysis of: drugs, plastics, air samples, flavourings

Question 3

describe the 2 phases of chromatography

Answer 3

• stationary: does not move, normally solid or liquid supported on a solid
• mobile: does move, normally a liquid or gas

Question 4

what is thin layer chromatography (TLC)?

Answer 4

• analytical technique that indicates how many components are in a mixture
• uses TLC plate which is usually a plastic sheet or glass, coated with a thin layer of a solid adsorbent substance (usually silica)

Question 5

what are the stationary and mobile phases in TLC?

Answer 5

• stationary = TLC plate covered with a thin layer of a solid absorbent (usually silica, SiO₂)
• mobile = the solvent

Question 6

how does TLC work?

Answer 6

• the different components have different affinities for the absorbent and bind with different strengths to its surface
• the more the components interact with the stationary phase, the more difficult it is for the mobile phase to move them
• this results in a smaller Rf value

Question 7

how do you calculate the retention factor (Rf) for a TLC plate?

Answer 7

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

Question 8

what is gas chromatography used for?

Answer 8

to separate volatile substances in a mixture

Question 9

how is gas chromatography different from TLC?

Answer 9

• capillary column (long hollow glass or metal tube) filled with stationary phase inside oven
• the much greater length of the column compared to the length of the TLC plate dives a much greater separation of components

Question 10

what is the stationary and mobile phase in gas chromatography?

Answer 10

• stationary = powdered solid (GC) or powdered solid with particles coated with a non-volative liquid (GLC). high boiling point liquid is adsorbed on to the solid support
• mobile = inert carrier gas eg. helium / neon

Question 11

describe the process of gas chromatography

Answer 11

1. mixture is injected into column at injection port
2. carrier gas (eg. He) moves particles along column at slightly different rates, depending on their different affinities for the stationary phase if a solid, or their solubilities in the liquid stationary phase
3. first compound detected is the one which has the fewest interactions with the stationary phase, and will have the lowest retention time

Question 12

describe the findings from gas chromatography

Answer 12

• greater affinity / solubility for the stationary phase = longer retention time (more time from injection to detection)
• less affinity / solubility for the stationary phase = shorter retention time (less time from injection to detection)

Question 13

what can retention times be used to identify in gas chromatography?

Answer 13

components present by comparison with retention times for known components using the same stationary and mobile phases

Question 14

what can peak integrations be used to determine in gas chromatography?

Answer 14

• peak integration = area under the peaks
• determine concentrations of components in the sample

Question 15

gas chromatography: calibration to determine concentrations of components

Answer 15

• solutions of known concentrations under investigation are prepared
• gas chromatogram for each concentration is obtained
• calibration curve of peak area against concentration is plotted
• gas chromatograph of the compound under investigation is obtained under the same conditions
• concentration is determined by comparison with the calibration curve

Question 16

how can gas chromatography be linked with a mass spectrometer for further analysis?

Answer 16

• samples are analysed by mass spectrometer as they leave GC column
• each component can then be identified separately