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Chemistry A-Level > Chromatography and NMR spectroscopy (Chapter 29) > Flashcards

Chromatography and NMR spectroscopy (Chapter 29) Flashcards

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1
Q

What is chromatography used for?

A

To separate individual components from a mixture of substances

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2
Q

What do all forms of chromatography have?

A

A stationary phase and a mobile phase

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3
Q

What is the stationary phase?

A

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

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4
Q

What is the mobile phase?

A

It does move and is normally a liquid or a gas

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5
Q

What can chromatography be used in?

A

The analysis of drugs, plastics, flavourings, air samples and has applications in forensic science

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6
Q

What does TLC stand for?

A

Thin Layer Chromatography

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7
Q

What is thin layer chromatography?

A

A quick and inexpensive analytical technique that indicates how many components are in a mixture

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8
Q

Describe what happens in TLC?

A
  • The technique used a TLC plate which is usually a plastic sheet or glass, coated with a thin layer of solid adsorbent substance (usually silica)
  • The adsorbent is the stationary phase
  • The different components in the mixture have different affinities for the adsorbent and bind with differing strengths to its surface
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9
Q

What is adsorption?

A

The process by which the solid silica holds the different substances in the mixture to its surface

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10
Q

How is separation achieved in TLC?

A

By the relative adsorptions of substances with the stationary phase

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11
Q

What is the mobile phase in TLC?

A

An organic solvent (liquid)

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12
Q

What determines separation in chromatography?

A

The strength of interaction with the stationary phase, NOT solubility

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13
Q

How are thin layer chromatograms analysed?

A
  • By calculating the value for the retention factor (Rf) for each component
  • Each component can be identified by comparing its Rf value with known values recorded using the same solvent system, and adsorbent
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14
Q

What formula do you use to calculate the Rf value in TLC?

A

Rf = distance moved by the component / distance moved by the solvent front (from the pencil line)

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15
Q

What range is the Rf value always in?

A

0-1

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16
Q

What is common to run alongside a TLC of a sample and why?

A
  • Pure samples of compounds that may be present

- It is then easy to identify the amino acids in the unknown sample visually, without needing to calculate any Rf values

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17
Q

What are some limitations of TLC?

A
  • Similar compounds will have similar Rf values
  • Unknown compounds have no Rf value to compare against
  • Polar compounds can be very “streaky”
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18
Q

What is gas chromatography (GC) useful for?

A

Separating and identifying volatile organic compounds present in a mixture

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19
Q

What is the stationary phase in GC?

A

A thin layer of a high boiling liquid (often a long-chain alkane) adsorbed onto an inert solid support or coated onto the inside of a capillary tube

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20
Q

What happens during GC?

A
  • A small amount of the volatile mixture is injected into the apparatus, called a gas chromatograph
  • The mobile carrier gas carries the components in the sample through the capillary column which contains the liquid stationary phase absorbed onto the solid support
  • The components slow down as they interact with the liquid stationary phase inside the column
  • The more soluble the component is in the liquid stationary phase, the slower it moves through the capillary column
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21
Q

What is the mobile phase in GC?

A

An inert carrier gas e.g. helium or neon

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22
Q

How are the components in a GC separated and detected?

A
  • The components of the mixture are separated depending on their solubility in the liquid stationary phase
  • The compounds in the mixture reach the detector at different times depending on their interactions with the stationary phase in the column
  • The compound retained in the column for the shortest time has the lowest retention time and is detected first
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23
Q

What is the retention time in GC?

A

The time taken for each component to travel through the column, from the point of injection to the detector (injection to detection)

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24
Q

What is each component detected as in GC?

A

As a peak on the gas chromatogram

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25
Q

What two pieces of information can be obtained from a gas chromatogram?

A
  • Retention times can be used to identify components present in the samples by comparing these to retention times for known components
  • Peak integrations (the area under each peak) can be used to determine the concentrations of components in the sample
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26
Q

How can the concentration of a component in a sample be determined from a GC?

A

By comparing its peak integration with values obtained from standard solutions of the component

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27
Q

Describe how you would determine the concentration of a component in a sample from a gas chromatogram

A

1) Prepare standard solutions of known concentrations of the compound (component) being investigated
2) Obtain GCs for each standard solution
3) Plot a calibration curve of peak area against concentration (external calibration)
4) Obtain a GC of the compound being investigated under the same conditions
5) Use the calibration curve to measure the concentration of the compound

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28
Q

What happens during GC-MS (Gas chromatography-mass spectrometry)

A
  • When a peak is detected on a GC, a selected component can be sent to a mass spectrometer for further analysis (it separates out the compounds)
  • The mass spectrum can be analysed and compared with a spectral database using a computer for positive identification of a compound
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29
Q

What is GC-MS used in?

A

Forensics, environmental analysis, airport security and space probes (it’s very powerful)

30
Q

What are limitations of GC?

A
  • Similar compounds have similar retention times
  • Peaks can get lost under nearby peaks with a higher concentration
  • Unknown compounds have no reference retention times
  • Only volatile compounds can be used (<350Mr)
31
Q

What does NMR stand for?

A

Nuclear Magnetic Resonance

32
Q

How does NMR spectroscopy work?

A
  • It uses a combination of a very strong magnetic field and radio frequency radiation
  • With the right combination of magnetic field strength and frequency, the nuclei of some atoms absorb this radiation
  • The energy for absorption can be measured and recorded as an NMR spectrum
33
Q

What is nuclear spin?

A

It is similar to electron spin but just for the nucleus and is significant if there is an odd number of nucleons (protons and electrons)

34
Q

What is resonance?

A
  • The nucleus has two different spin states each with different energies
  • With the right combination of a strong magnetic field and radio frequency radiation, the nucleus can absorb energy and rapidly flips between the two spin states
35
Q

Describe how the frequency shift is derived

A
  • In an organic molecule, every carbon and hydrogen atom is bonded to other atoms
  • All atoms have electrons surrounding the nucleus, which shifts the energy and radio frequency needed for NMR to take place
36
Q

What is the scale that frequency shift measured on called?

A

Chemical shift (𝛿)

37
Q

What are the units of chemical shift?

A

Parts per million (ppm)

38
Q

What is used as the standard reference chemical against which all chemical shifts are measured?

A

Tetramethylsilane (TMS) - (CH3)4Si

39
Q

What is the chemical shift value of TMS?

A

0 ppm

40
Q

What is the amount of chemical shift determined by?

A

Chemical environment, especially the presence of nearby electronegative atoms

41
Q

What is a deuterated solvent?

A

A solvent in which the 1H atoms have been replaced by 2H atoms (deuterium, D)

42
Q

Why is a deuterated solvent (Deuterated trichloromethane, CDCL3) often used in carbon and proton NMR?

A
  • Deuterium produces no NMR signal in the frequency ranges used in proton NMR spectroscopy because it has no protons
  • CDCl3 will still produce a peak in a carbon NMR spectrum but the computer usually filters out this peak before displaying the spectrum
43
Q

What two important pieces of information does a carbon-13 NMR provide about a molecule?

A
  • The number of different carbon environments - from the number of peaks
  • The types of carbon environments present- from the chemical shift
44
Q

What will happen to the carbon NMR if two carbon atoms are positioned symmetrically within a molecule?

A
  • They are equivalent and have the same carbon environment

- They will then absorb radiation at the same chemical shift and contribute to the same peak

45
Q

What is the chemical shift range for carbon-13 NMR?

A

0-220 ppm

46
Q

If carbon atoms are in different environments what does this mean for the carbon-13 NMR?

A

That they will absorb at different chemical shifts and contribute to different peaks

47
Q

What two pieces of information does a proton NMR provide which is similar to a carbon-13 NMR? (a low resolution proton NMR)

A
  • The number of different proton environments - from the number of peaks
  • The types of proton environments present - from the chemical shift
48
Q

What two new pieces of information does a proton NMR provide? (a high resolution proton NMR)

A
  • The relative numbers of each type of proton (the number of protons in each different proton environment) - from integration traces or ratio numbers of the relative peak areas
  • The number of non-equivalent protons adjacent to a given proton - from the spin-spin coupling pattern
49
Q

What is the chemical shift referenced against in both carbon and proton NMR?

A

TMS at 𝛿=0ppm

50
Q

What is the chemical shift range for proton NMR?

A

0-12ppm

51
Q

What factors may move a peak outside of its ranges in carbon or proton NMR?

A

Solvent, concentration and substituents (what it is next to)

52
Q

What does it mean for a proton NMR spectrum if two or more protons are equivalent?

A

They will absorb at the same chemical shift, increasing the size of the peak

53
Q

What does it mean for a proton NMR spectrum if two or more protons are non-equivalent?

A

They will absorb at different chemical shifts as they have different chemical environments

54
Q

In proton NMR, what shows the ratio of the number of protons responsible for each peak?

A

The ratio of the relative areas under each peak

55
Q

How does the NMR spectrometer measure the area under each peak?

A

As an integration trace

56
Q

How is the integration trace shown?

A

Either as an extra line of the spectrum or as a printed number of the relative peak areas

57
Q

What causes the splitting patterns in proton NMR?

A

The proton’s spin interacting with the spin states of nearby protons that are in different environments

58
Q

What information do splitting patterns provide?

A

The number of protons bonded to adjacent carbon atoms

59
Q

What is the splitting of a main peak into sub-peaks called?

A

Spin-spin coupling or spin-spin splitting

60
Q

How do you work out the splitting pattern in proton NMR, and what is the n+1 rule?

A
  • The number of sub-peaks is one greater than the number of adjacent protons causing the splitting
  • For a proton with n protons attached to an adjacent carbon atom, the number of sub-peaks in a splitting pattern = n+1
61
Q

What would happen to the splitting in molecules such as CH3CH2CH2COOH?

A
  • The central CH2 would be split differently by the CH2 and CH3 protons
  • The resulting splitting would then be shown as a multiplet
62
Q

What protons are difficult to identify on a proton NMR spectrum?

A

Protons not bonded to carbon atoms e.g. -OH or -NH

63
Q

What functional groups contain -OH protons?

A

Alcohols (ROH), Phenols (ArOH) and carboxylic acids (COOH)

64
Q

What functional groups contain -NH protons?

A

Amides (RNH2), amides (RCONH2), and amino acids (RCH(NH2)COOH)

65
Q

What makes assigning -OH and -NH protons to a peak difficult?

A
  • In solutions, NH and OH protons may be involved in hydrogen bonding and the NMR peaks are often broad and can occur at almost any chemical shifts
  • The broadening of the peaks also means that OH and NH protons are not usually involved in spin-spin coupling
66
Q

What technique have chemists devised for identifying OH and NH protons?

A

Proton exchange

67
Q

How would you carry out a proton (deuterium) exchange?

A

1) Run a proton NMR spectrum as normal
2) Add a small volume of deuterium oxide (D2O), shake the mixture
3) Run a second proton NMR
4) Analyse the spectrum to see which peaks have changed

68
Q

What is happening during proton exchange?

A
  • Deuterium exchanges and replaces the OH and NH protons (because the OH bond is labile)
  • e.g with methanol: CH3OH + D2O <=> CH3OD + HOD
  • So the second spectrum is being run on CH3OD
  • As deuterium does not absorb in this chemical shift range, the OH peak disappears
69
Q

What peak might appear in carbon-13 NMR that you can ignore?

A

A triplet at 77 which is due to the solvent (CDCl3)

70
Q

What name is given to the process by which components in a mixture are separated during gas/liquid chromatography?

A

Relative solubility

71
Q

How can a gas/liquid chromatogram be used to determine the percentage composition of each component in a mixture?

A

Using the relative area underneath each peak

  • The area under each peak is proportional to the amount of substance in the sample
  • % of component = area/total area x 100
72
Q

Why is TMS added to solvents used in proton NMR?

A

As a reference compound

Chemistry A-Level (27 decks)

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