Chemistry - Rings, Polymers and Analysis - F324 - Separation by Chromatography (3) Flashcards
What is the advantage of chromatography?
Chromatography can separate out components with similar physical properties in very complex mixtures.
Define a phase in chromatography?
A phase is a physically distinctive form of a substance, such as the solid, liquid and gaseous states of ordinary matter.
Define the mobile phase.
The mobile phase is the phase that moves in chromatography.
Define the stationary phase.
The stationary phase is the phase that does not move in chromatography.
On which basis does chromatography work?
Chromatography works on the basis that different components have different affinities for a stationary phase and for a mobile phase.
What is the stationary phase in thin layer chromatography?
In thin layer chromatography, the stationary phase is a solid.
What is the mobile phase in thin layer chromatography?
In thin layer chromatography, the mobile phase is a liquid.
What is the stationary phase in gas chromatography?
In gas chromatography, the stationary phase is a solid or a liquid mounted on a solid support.
What is the mobile phase in gas chromatography?
The mobile phase in gas chromatography is a gas.
What does a solid stationary phase separate via?
A solid stationary phase separates via adsorption.
What does a liquid stationary phase separate via?
A liquid stationary phase separates via relative solubility.
Define adsorption.
Adsorption is the process by which a solid holds more molecules of a gas or liquid or solute as a thin film on the surface of a solid or, more rarely, a liquid.
What is usually used as the stationary phase of thin layer chromatography?
The stationary phase of TLC is usually a thin layer of an adorbent such as silica gel (SiO2) or alumina (AlO3) coated on a flat, inert support, usually a sheet of glass.
How is the Rf value calculated?
Rf = distance moved by component / distance moved by the solvent front
What are some of the negatives of chromatography?
- similar compounds will have similar Rf values
- unknown compounds have no reference Rf value
- it may be difficult to find a solvent that separates all the components in a mixture
What needs to be considered when choosing a suitable solvent for chromatography?
- if the components are very soluble then they will be washed all the way up the plate
- if the components are not very soluble then there will be very little movement
- trial and error may be necessary to find a suitable solvent.
What type of compounds is gas chromatography most useful for separating?
Gas chromatography is particularly useful for organic compounds that have a low boiling point and evaporate easily
What is the stationary phase in gas chromatography?
The liquid for the stationary phase id often a long chain alkane with a high boiling point. e.g silicone polymers.
What is the tubing in gas chromatography referred to as?
The tubing in gas chromatography is referred to as the chromatography column.
What is the mobile phase in gas chromatography?
The mobile phase is an inert or unreactive carrier gas such as helium or nitrogen.
How can the separation in gas chromatography be improved?
The separation can be improved by using different oven temperatures and different flow rates for the carrier gas.
What are some of the negatives of gas chromatography?
- potentially thousands of chemicals have the same retention time, peak shape and detector response
- not all parts of the mixture will necessarily be separated or detected.
- unknown compounds have no reference retention time.
What are the isotopes most commonly used in NMR spectroscopy?
1H, 13C, 19F and 31P
What creates the magnetic field for NMR spectroscopy?
Unpaired nucleons and their residual spin creates the magnetic field.
What is resonance?
The excitation and relaxation of the nucleus is called resonance.
What is used as the reference peak in NMR?
Tetramethylsilane (TMS) (CH3)4Si is used as the reference peak.
Why is TMS used as the reference?
TMS is used as the reference because it is chemically unreactive and volatile so it can easily be removed from the sample after running an NMR spectrum.
Why are NMR spectra run with deuterated solvents?
Because you cannot use a regular organic solvent because the carbon and hydrogen within them also produce a signal. Deuterium has an even number of nucleons so produces no signal.
What does the number of peaks in carbon-13 NMR tell you?
The number of peaks in carbon-13 NMR tells you the number of different carbon environments.
What do the chemical shifts in carbon-13 NMR spectra tell you?
The chemical shifts in carbon-13 NMR tell you the types of carbon environment.
What does the size of the peaks in carbon-13 NMR tell you?
The size of the peaks in carbon-13 NMR tells us nothing.
Why does proton NMR require far less material than carbon NMR?
1H is the commonest isotope of hydrogen with a relative abundance of 99.9% compared with 13C which has a relative abundance of 1.1%.
What does the number of peaks in proton NMR tell us?
The number of peaks in proton NMR tells us the number of proton environments.
What does the chemical shift in proton NMR tell us?
The chemical shift in proton NMR gives the type of proton environment of each peak.
What do the relative peak areas in proton NMR tell us?
The relative peak areas in proton NMR give the proportions of protons in each environment.
What does spin-spin coupling tell us?
Spin-spin coupling gives us information about adjacent protons.
What is the rule for predicting the splitting pattern due to spin-spin coupling?
We predict the splitting pattern using the n+1 rule.
Why can it be difficult to identify the protons of OH and NH peaks?
It can be difficult to identify the OH and NH peaks as they can appear over a wide range of different chemical shift values depending on the solvent and concentration of the sample.
What is the common name for deuterium oxide?
The common name for deuterium oxide is heavy water.
How can OH and NH peaks be specifically identified?
A proton NMR is run, D2O is then added and shaken, then a second NMR is run which makes any peaks due to OH or NH disappear.
How does an OH or NH peak usually show on a spectrum?
An OH or NH peak usually shows as a singlet which may be broad.
What are the advantages of MRI scanners?
MRI scanners are harmless and non-invasive.
Patients with what should not have MRI scans?
Patients with ferromagnetic implants should not undergo MRI scanning.
What key information does mass spectrometry tell us?
Mass spectrometry tells us the percentage by mass of each element.
What key information does infrared spectroscopy tell us?
Infrared spectroscopy gives information about bonds and functional groups that are likely to be present.
What key information does C13 NMR tell us?
C13 NMR gives information about the number and types of carbon environments in each molecule.
What key information does proton NMR tell us?
Proton NMR gives information about numbers and types of proton environment and the number of protons on adjacent carbon atoms.
