GC-MS
In A2/A-Level Chemistry, GC-MS (gas chromatography and mass spectrometry) is a combined analysing tool; GC is used to separate components in a mixture and MS is used to identify components by comparing them to a database.
Applications:
- Forensics – identify substance at a crime scene, which can be used as evidence in a court of law
- Environmental analysis – monitoring and analysing organic pollutants, quality of waste and drinking water; and detecting pesticides in food
- Airport security – detect explosives in luggage and on people to prevent terrorist attacks and identify substance to prevent drug trafficking
- Space probes – analyse atmosphere of planets and materials on their surface
- Drugs – detect the presence of illegal drugs or performance enhancers in suspected drug users and athletes
Basics of NMR
In A2/A-Level Chemistry, NMR is an analytical technique used for examining molecular structure; the isotopes that are most commonly investigated are 1H, 13C and 31P.
A strong magnetic field, applied using an electromagnet, and low-energy radio frequency radiation are required for NMR spectroscopy.
Nuclear spin (non-examinable):
- Protons and neutrons are collectively known as nucleons.
- Nucleons can ‘spin’ in one of two directions
- Opposite spins pair up, but if there is an odd number of nucleons then a small residual nuclear spin is produced – which generates a magnetic field and a signal on an NMR spectrum.
- The nuclei, now tiny magnets, can either line up with the field or be opposed to the field
- Nuclei that oppose the field have a higher energy and if the external magnetic field is stronger there will be a larger energy gap.
Resonance (non-examinable):
- In A2/A-Level Chemistry, A nucleus can be promoted to its upper-energy spin state by providing energy equal to the gap – known as excitation.
- In NMR the excitation is provided by low-energy radio-frequency radiation.
- When the nucleus emits the energy it will drop back to its low-energy spin state – known as relaxation.
- This cycle of excitation and relaxation is known as resonance. It continues so long as there is a supply of energy (radio radiation) that exactly matches the energy gap – it is the basis for nuclear magnetic resonance (NMR).
Nuclear shielding (non-examinable):
- The strong external magnetic field and weaker magnetic field generated from the atoms electrons and surrounding atoms determine how much of the magnetic field is felt by the nucleus.
- An atom’s nucleus can be shield from the magnetic field by its electrons – nuclear shielding. The extent of the shielding depends on the density of nearby electrons.
- Therefore atoms in different environments have different resonance frequencies – from this we can calculate chemical shift.
Chemical shift:
In A2/A-Level Chemistry, Chemical shift (δ) – a scale that compares the frequency of an NMR absorption with the frequency of the reference peak of TMS at δ=0 ppm.
TMS (tetramethylsilane) – a standard compound used as the reference signal for chemical shift because it has 12 protons and gives rise to a single sharp peak. (Chemically unreactive and volatile so it can be easily removed from the system after calibration)
Chemical shift is the place on an NMR spectrum where the nucleus absorbs energy – it’s measured in ppm (parts per million) and calibrated using the delta scale.
Solvents used for NMR spectroscopy:
InA2/A-Level Chemistry, NMR analysis is usually carried out in solution and a solvent is needed but it can’t contain hydrogen or carbon (organic) as these both produce a signal.
An isotope of hydrogen, deuterium 2H, solves this problem; because it has an even number of nucleons it gives no signal.
CDCl3 is commonly used for both 1H-NMR and 13C-NMR – the peak is usually removed from the 13C-NMR spectrum.
The solvent can be evaporated off to recover the sample of running the NMR spectrum.
That's the end~
