What must a nucleus be able to do in order to produce and NMR spec and how does it do this?
Must be able to absorb energy in the radio frequency part of the electromagnetic spec. To do this, it must have an odd number of protons or neutrons or both
Definition of chemical shift
A scale that compares the frequency of an NMR absorption with the frequency of the reference peak of TMS at delta = 0ppm (It is the place in an NMR spec. at which the nucleus absorbs energy)
How many signals/peaks does TMS produce
1, all four carbon atoms are equivalent and so are the12 protons, thus it is easily identifiable
Why is TMS usually added when NMR spectra are recorded and are there any problems with this?
To calibrate the spectrometer against the TMS reference peak TMS is chemically unreactive and volatile so it can easily be removed from the sample after
Why can't water be used as an NMR solvent?
Many organic compounds do not dissolve in polar solvents
Why can't organic solvents be used in NMR?
They contain hydrogen and carbon atoms which would produce peaks
What solvents are used in NMR? Why? / what is D?
Deuterated solvents. D, deuterium, is an isotope of hydrogen with one neutron and one proton, it's even number of nucleons mean that there is no peak produced on an NMR spec
What is a common solvent used, how are problems with it overcome?
CDCl3 is commonly used for both types of NMR. For C-13 spec . the carbon peak from the solvent is usually removed from the spectrum. After running the NMR spec. the sample can be recovered by evaporating off the solvent
What are the requirements for NMR?
- strong magnetic field applied using an electromagnet - low energy radio frequency radiation
Limitations of GC
GC can separate components in a mixture but cannot identify them conclusively. It is difficult to identify unknown components by gas chromatography alone unless a pure sample has been run under identical conditions for analysis - potentially thousands of chemicals may have the same retention time, peak shape and detector response
Use of GC-MS in forensics
Used to analyse minute particles found at the scene of a crime. Mass spectrometer positively identifies presence of particular substances , provides critical evidence during a trial.
GC-MS in environmental analysis
Used for monitoring and analyzing organic pollutants in the environment, including the quality of waste water and detecting pesticides in food.
GC-MS in airport security
Used to detect explosives in luggage and on passengers
GC-MS in spaceprobes
Sent to collect and analyze material on Mars, and to analyze the atmosphere of venus.
What is the stationary phase in GC?
A thin layer of a liquid or solid, coated on the inside of some capillary tubing.
What is the mobile phase in GC?
What is retention time?
The time for a component to pass from the column inlet to the detector
Producing a chromatogram (GC)
- mixture injected into gas chromatograph where it is vaporised - mobile carrier gas flushed the mixture through the column - as mixture moves through the column, components slow down as the interact with the stationary phase lining of the column. If the lining is a liquid, components may dissolve and if it is a solid then they may become adsorbed to the surface. The carrier gas flushes the components further along the tube. - each component leaves the column at a different time and is detected as it leaves the column
What affects retention time?
The greater the solubility or adsorption, the more the components are slowed down
In GC what does are under the peak show?
It is a direct measure of the proportion of that substance present in the original sample
What is a chromatogram?
A visible record showing the result of separation of the components of a mixture by chromatography
What is a mobile phase?
The phase that moves in chromatography
What is a stationary phase?
The phase that does not move in chromatography
What is adsorption?
The process by which a solid holds molecules of a gas or liquid or solute as a thin film on the surface.
What is chromatography?
An analytical technique used to separate the components in a mixture
How does chromatography work?
On the basis that different components have different affinities for a stationary phase and for a mobile phase. The stationary phase interacts with the components in the mixture slowing them down.
What are the states of the stationary and mobile phases in TLC and GC?
TLC: stationary phase is solid, mobile is a liquid GC: stationary phase is liquid or solid on a solid support, mobile is a gas
How does separation in chromatography work?
Solid stationary phases separate by adsorption, as the mobile phase passes over the stationary phase, component molecules are able to bind to the solid making up the stationary phase, happening at the surface Liquid stationary phases separate by relative solubility, some components dissolve more readily in the liquid
What is a retention factor?
Distance moved by a particular substance/Distance moved by the solvent front
Limitations of TLC
Similar compounds often have similar Rf values Unknown compounds have no reference Rf for comparison It may be difficult to find a solvent that separates all the components in a mixture. If the components are very soluble in the solvent, they will just be washed up the TLC plate with the solvent front. If the components have little solubility they will hardly move.
Why is the delta scale employed when plotting NMR spec as opposed to the actual resonance frequencies?
To compare spectra from different instruments
How are the relative numbers of hydrogen atoms determined from an H- NMR spec?
Area under peak - integration
What is the significance of the delta value in H- NMR?
Indicates the type of proton chemical environment
What does peak splitting show?
Number of protons on adjacent carbon atoms + 1
How does MRI scan work?
Patient placed inside cyclindrical electromagnet and radio waves are sent through the body. Protons in their bodies resonate in response to the magnetic field and radio waves
Hazards of MRIs
Hamrless as low energy radio frequency radiation. Patients with ferromagentic metal implants (which would be attracted to magnet in scanner) and pacemakers should not have these scans. The electromagnetic magnet is very strong!
MRIs in diagnosis of medical problems
Many diseases channge the water content within tissues and organs and the scanner detects these differences. A 3D image that reflects the chemical struture of the tissues. Diagnosis and treatment of tumours.
MRIs in sport
Used to identify the extent of injuries such as tears in muscles, tendons and ligaments