29. Analytical Chemistry

Question 1

Define ‘partition coefficient’.

Answer 1

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

Question 2

What are the components in paper chromatography?

Answer 2

Mobile phase - liquid solvent

Stationary phase - water trapped between cellulose fibres of filter paper

Question 3

Define ‘Rf value’.

Answer 3

distance travelled by solute / solvent front

Question 4

How can components be detected in paper chromatography / TLC?

Answer 4

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

Question 5

Describe two-way chromatography.

Answer 5

Used when solvents have similar Rf values.

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

Question 6

Describe partitioning in chromatography.

Answer 6

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

Question 7

What are the components in TLC?

Answer 7

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

Question 8

Describe how TLC works.

Answer 8

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

Question 9

What are the limitations of TLC?

Answer 9

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

Question 10

What are the strengths / uses of TLC?

Answer 10

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

Question 11

Define ‘retention time’.

Answer 11

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

Question 12

Which factors affect retention time?

Answer 12

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

Question 13

How can % composition be identified using GLC peaks?

Answer 13

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.

Question 14

What are the components in HPLC?

Answer 14

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

Question 15

How does HPLC work?

Answer 15

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

Question 16

What are the limitations of GLC?

Answer 16

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

Question 17

What are the strengths / uses of GLC?

Answer 17

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

Question 18

What does mass spec measure?

Answer 18

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

Question 19

How does mass spec work?

Answer 19

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.

Question 20

Define ‘molecular ion’.

Answer 20

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

Question 21

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

Answer 21

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)

Question 22

What are the ratios of the Cl and Br isotopes?

Answer 22

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.

Question 23

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

Answer 23

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

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

Question 24

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

Answer 24

3:4:1

Question 25

Describe the applications of mass spec.

Answer 25

- 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.

Question 26

Define 'soft ionisation'.

Answer 26

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

Question 27

Define 'soft ionisation'.

Answer 27

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

Question 28

How does NMR work?

Answer 28

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.

Question 29

Which molecule is used as a comparison in NMR?

Answer 29

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).

Question 30

How does low-res NMR work?

Answer 30

Single peak for each non-equivalent H atom. | - Area under peak = number of H atoms

Question 31

How does high-res NMR work?

Answer 31

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.

Question 32

What are the peak intensity ratios for the splitting patterns?

Answer 32

``` Singlet = 1 Doublet = 1:1 Triplet = 1:2:1 Quartet = 1:3:3:1 ```

Question 33

How are -OH and -NH signals identified?

Answer 33

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'.

Question 34

How does C13 NMR work?

Answer 34

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.

Question 35

What affects chemical shift values?

Answer 35

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).

Question 36

Which solvent is used to prepare NMR samples?

Answer 36

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

Question 37

Define 'resonance'.

Answer 37

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