4.9 Spectroscopy and Chromatography
| EM Radiation: | Microwaves | Ultraviolet |
| Wavelength: | 1mm-1m | 400nm-10nm |
| Why: | Heating | Initiating reactions |
| How: | Radiation causes electric field; food (also polar e.g. fats, sugars) rotate to line up with the field. Dryer food with less water content will take longer to cook as water has polar Oδ-—Hδ+ bonds. | Has enough energy to split molecules and produce free radicals |
| Example: | Cooking – Microwave oven
Surgery – to kill cancer cells Chemical industry – heating |
Initiating reactions such as substitution between halogen and alkane
– Cl2 —UV—> 2Cl•
– CF3Cl —UV—> CF3 + Cl• |
| Danger: | n/a | This initiation can cause one Cl• can cause the destruction of two O3 molecules and another Cl•
Massive chain reaction. |
Mass Spectroscopy
The base peak is the 100% relative abundance which is used to find the RFM
M peak is caused by the whole molecular ion which breaks up into fragments of free radicals and positive ions, but only the positive ion shows up on a mass spectrometer.
The other peaks are fragment ions of a broken ethanol molecule. See below.
Some common RFM of fragment ions:
CH3+ 15
C2H6+ 29
C3H7+ 43
OH+ 17
CHO+ 29
COOH+ 45
NMR Spectroscopy
This gives you information about the structure using the idea that every atomic nucleus (with an odd number of protons/neutrons) has a weak magnetic field due to its nuclear spin, and applying a strong magnetic field will display accordingly.
Hydrogen is a single proton and so we can use proton NMR to find how many hydrogens there are and how they’re arranged…
Normally protons are spinning randomly, however when you apply a STRONG EXTERNAL MAGNETIC FIELD all the protons line up. Some protons are aligned in the direction of the magnetic field and others are opposing it. Those opposing it are at a higher energy level and can emit a radiowave to move to the lower energy level. Those in the direction of the magnetic field are at a lower energy level and can absorb a radiowave and move to a higher radiowave.
NMR measures the absorption of energy.
Protons in different environments absorb different amounts of energy; due to them being shielded by electrons experiencing the effects of the strong magnetic force instead.
Examples of different environments:
2 environments: 
4 environments:
Chemical shift – is the difference in absorption of a proton
relative to TetraMethylSilane (Si(CH3)4).
Where δ = 0 is the value of TMS.
Each peak = one environment. In the graph opposite,
there are two environments (2 peaks)
The less shielded a proton is, the further left the shift will be.
Spin-spin coupling – in high res, the peaks of an NMR usually split into smaller peaks, this is because the magnetic field of neighbouring protons interact. The peaks follow an n+1 rule whereby;
2 splits [doublet] = 1 neighbouring proton (or hydrogen)
3 splits [triplet] = 2 neighbouring protons (or hydrogens)
4 splits [quartet] = 3 neighbouring protons (or hydrogens)
Magnetic Resonance
– Patient is placed in a very large magnet and irradiated with radio waves
– Hydrogen nuclei in the water in patients body interacts with the radiowaves
– Different frequencies of wave are absorbed by different densities of tissue
– A series of images can be produced by moving the beam to build a 3D image
USES: cancer/bone and joint treatment, brains studies, checking purity in pharmaceutical industry
ADV: non invasive, X-ray would be harmful
Infrared Spectroscopy
- IR beam goes through sample
- IR energy is absorbed by the bonds, increasing their energy (vibrational)
- Different bonds in different environments absorb different wavelengths
- Any wavelengths that you need to know will be in the data book
USES: in the chemical industry to determine the extent of a reaction by seeing what bonds are present
Chromatography – good at separating and identifying things
Mobile phase – where molecules can move i.e. liquid/gas
Stationary phase – where molecules can’t move i.e. solid
| Gas chromatography GC | High performance liquid chromatography HPLC |
| Stationary phase is a viscous liquid in a long coiled tube e.g. oil | Stationary phase is small particles of a solid packed into a tube e.g. silica |
| Tube is built into an oven | Tube is not heated |
| Sample injected and vaporised | Sample forced through tube by high pressure |
| Both rely on different amounts of the sample being moved from the top of the tube to the bottom known as the “retention time” | |
| ADV of HPLC over GC: HPLC can be used if sample is heat sensitive or has a high boiling point | |