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Chemistry > 29. Analytical Chemistry > Flashcards

29. Analytical Chemistry Flashcards

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1
Q

Define ‘partition coefficient’.

A

EQM constant (ratio) relating conc. of a solute distributed between two immiscible solvents at a particular temperature.

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2
Q

What are the components in paper chromatography?

A

Mobile phase - liquid solvent

Stationary phase - water trapped between cellulose fibres of filter paper

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3
Q

Define ‘Rf value’.

A

distance travelled by solute / solvent front

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4
Q

How can components be detected in paper chromatography / TLC?

A
  • Ninhydrin / other chemical spray
  • UV light and fluorescence
  • Iodine tank
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5
Q

Describe two-way chromatography.

A

Used when solvents have similar Rf values.

- normal chromatography carried out, then rotated 90 degrees to proceed again using a different solvent.

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6
Q

Describe partitioning in chromatography.

A

The components have different Kpc values in the two phases - the more soluble in the mobile phase, the further the component travels.

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7
Q

What are the components in TLC?

A

Mobile phase - liquid solvent
Stationary phase - solid which adsorbs the solute molecules onto its surface.
- alumina or silica gel, made into slurry with water, spread on microscope slide, dried in oven
- polar surface due to OH groups

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8
Q

Describe how TLC works.

A

Polar components are adsorbed more strongly, so they travel less. Components can be identified using known substances or by calculating Rf values.

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9
Q

What are the limitations of TLC?

A

Apply to non-volatile compounds only, can only be used for small samples, less accurate.

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10
Q

What are the strengths / uses of TLC?

A

Quicker than paper, can be used on smaller samples, forensic science, testing drugs.

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11
Q

Define ‘retention time’.

A

Time taken for component to pass through column from injector, and reach detector, showing maximum peak height.

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12
Q

Which factors affect retention time?

A
  • BP of component (higher or lower than column temp)
  • Column temperature
  • Solubility in stationary phase
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13
Q

How can % composition be identified using GLC peaks?

A

Relative peak area (triangle formula), expressed as a percentage of the total peak areas. Where there are narrow peaks with similar base widths, the height is used.

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14
Q

What are the components in HPLC?

A

Like GLC except the mobile phase is a polar liquid solvent.

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15
Q

How does HPLC work?

A

Components that are more polar are more soluble in the mobile phase, so they move faster and have a shorter retention time.

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16
Q

What are the limitations of GLC?

A

Can only use volatile compounds. Some compounds will have similar retention times, and new compounds don’t have a match in databases.

17
Q

What are the strengths / uses of GLC?

A

High resolution and sensitivity. Conditions like temperature can be maintained and kept constant.

18
Q

What does mass spec measure?

A

Relative isotopic masses and their abundances, to calculate the Mr of and identify a given molecule.

19
Q

How does mass spec work?

A

Vaporisation -> ionisation -> acceleration -> deflection -> analysis and detection.
- Molecules are bombarded with high energy electrons, causing the covalent bonds to break and fragment the molecule into daughter ions.

20
Q

Define ‘molecular ion’.

A

M+. - the peak with the highest m/z value, formed by the sample molecule with one electron removed. Fragments into X+ and Y^. (radical)

21
Q

How do you deduce the number of carbons in a molecule?

A

Using the M+1 peak. The ratio is such because there is always approx. 1.1% of the 13C isotope in an organic compound.
n = (100/1.1) x (M+1 abundance / M+ abundance)

22
Q

What are the ratios of the Cl and Br isotopes?

A

35Cl : 37Cl = 3:1
79Br : 81Br = 1:1
These ratios correspond to the peak heights.
The larger isotopes form the M+2 peak in a spectrum where there is one Cl / Br present.

23
Q

What are the ratios when two Cl / Br atoms are present in a molecule?

A

For Cl - M+ : M+2 : M+4 = 9:6:1

For Br - M+ : M+2 : M+4 = 1:2:1

24
Q

What is the ratio when one Cl and one Br are present in a molecule?

A

3:4:1

25
Q

Describe the applications of mass spec.

A
  • Linked with GLC/HPLC to rapidly separate and identify molecules in a sample. 3D graph generated (time/abundance/mz)
  • Analysing complex mixtures eg. crude oil
  • Medical research to identify amino acid sequences in whole proteins or partial peptides.
  • Forensics, carbon dating, monitoring pollutants, airport security, drug testing.
26
Q

Define ‘soft ionisation’.

A

Adding a proton instead of using electrons, gives MH+ peak.

27
Q

Define ‘soft ionisation’.

A

Adding a proton instead of using electrons, gives MH+ peak.

28
Q

How does NMR work?

A

The nucleus of each H atom / C atom acts as a small magnet, with spin. The spin can line up with or against the field applied, forming two different states of energy. Strong fields allow the energy state to flip to the opposite spin. The field strength’s effect on the E gap depends on the environment of the nucleus.

29
Q

Which molecule is used as a comparison in NMR?

A

TMS - tetramethylsilane.

  • Inert, volatile, mixes well with organic compounds
  • Only gives one peak because all H and C are in the same environments.
  • Absorptions are measures by their shift away from the TMS peak (chemical shift).
30
Q

How does low-res NMR work?

A

Single peak for each non-equivalent H atom.

- Area under peak = number of H atoms

31
Q

How does high-res NMR work?

A

Spin-spin coupling results in splitting patterns. Follows the n+1 rule (n = number of H on adjacent carbons).
-OH doesn’t usually split as its H is being rapidly exchanged with other molecules.

32
Q

What are the peak intensity ratios for the splitting patterns?

A 
Singlet = 1
Doublet = 1:1
Triplet = 1:2:1
Quartet = 1:3:3:1
33
Q

How are -OH and -NH signals identified?

A

Using deuterium oxide. The proton will swap with a deuterium and the peak will disappear due to deuterium’s not showing up on an NMR spectrum. The H being exchanged is called a ‘labile proton’.

34
Q

How does C13 NMR work?

A

Same way as H NMR except identifying different C environments. There is no splitting, and the vertical peaks do not tell us about the number of C atoms present.

35
Q

What affects chemical shift values?

A

The shielding on the C / H - more electrons means the field affects them less, so the shift is lower. Having benzene rings / EN groups attached shifts them further, because of less shielding (electron-withdrawing).

36
Q

Which solvent is used to prepare NMR samples?

A

CDCl3 - only one C atom, no 1H present so will not interfere with other peaks.

37
Q

Define ‘resonance’.

A

Ability to absorb energy at certain frequencies, depending on the number of C / H present.

Chemistry (12 decks)

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