What are the five stages of mass spectrometry?
What does a mass spectrometer measure?
Where is the molecular ion peak on a substance's mass spectrum?
The molecular ion peak is the furthest sizeable peak to the right of a mass spectrum.
Why are fragmentation patterns helpful?
They help determine formula and structure.
What ways is it possible for the molecular ion CH3CH2CH3+∙ to fragment?
When a molecule fragments, what determines the likelihood of a particular ion being formed?
That ion's stability - inductive effects lend stability to ions, so carbons with more groups attached are more likely to form.
What ion is usually formed when a carbonyl compound fragments in mass spectrometry? Why is this ion usually formed?
An acylium ion
RCOR+∙ ----> RCO+ + R∙
Acylium ions exist in a resonant structure in which stabilizes it.
When do M+2 peaks occur in a mass spectrum? What is an M+2 peak?
When the compound contains chlorine or bromine (Cl = 35/37, Br = 79/81). An M+2 peak occurs at the m/z value for the molecular peak, +2.
In what ratio do chlorine isotopes exist?
3:1 for Ar 35 to 37
In what ratio do bromine isotopes exist?
1:1 for Ar 79 to 81.
What would an M+4 peak in a mass spectrum suggest about a compound?
That the compound contains two halogen atoms.
What are the two types of nuclear magnetic resonance (NMR) spectroscopy?
13C NMR and 1H NMR.
How does NMR spectroscopy work?
- An atomic nucleus with an odd number of nucleons has a spin, which creates a magnetic field.
- When an external field is applied across the nuclei, the nuclei's field either aligns parallel to (low energy) or against (high energy) this field.
- The nuclei in the high-energy state are able to absorb EM radiation and flip to the higher state.
- More nuclei occupy the lower state therefore there is an overall absorption of radiation, producing an NMR absorption spectrum.
What factors affect the frequency of radiation that a nucleus will absorb?
Electron shielding, which is further affected by neighbouring atoms and groups.
What is the frequency radiation absorbed (chemical shift) in NMR spectroscopy measured relative to?
Tetramethylsilane - TMS.
Why is tetramethylsilane used as a base against which all chemical shifts are measured?
Because it produces a single absorption peak - all of its carbon and hydrogen atoms are in the same environment.
It is also non-toxic, won't react with other substances and volatile (making it easy to remove).
What does each seperate peak in 1H NMR spectroscopy represent? What does the area under each of these peaks represent?
Each peak represents a different hydrogen environment, and the area under each peak represents the relative number of hydrogen atoms in that environment.
What does a singlet pattern in 1H NMR suggest?
That there are no hydrogen atoms on the adjacent carbon.
What does a triplet splitting pattern on a 1H NMR spectrum suggest?
That there are two hydrogen atoms on the adjacent carbom atom.
What type of solvent must be used when conducting 1H NMR spectroscopy? Why is this?
A hydrogen-free solvent, such as one containing deuterium rather than 1H, or CCl4, which does not contain any hydrogen. This is because a 1H-containing solvent would disrupt the spectrum.
What is infrared spectroscopy? How does it work?
Inter-atomic bonds absorb different wavelengths of radiation depending on their type and environment. This fact can be used when analysing substances to help establish their constituents.
What is chromatography used for?
Seperating the elements of a mixture.
Name and explain the two phases of any chromatography process.
The mobile phase: This is either a liquid or gas which is a able to move inside the chromatography apparatus.
The stationary phase: This is either a solid or a liquid held in a solid and it is not able to move inside the chromatography apparatus.
Describe how a mixture could be seperated using column chromatography.
The column is packed with an absorbent material such as Al2O3, then the mixture is poured into the column along with the mobile phase (solvent), which should be constantly poured through the column. The different elements of the mixture will move with the solvent at different rates according to their solubility and retention and it will be possible to siphon off each element individually at the base of the column.
What is retention?
How strongly a substance adsorbs onto the surface of a solid substance.
Which elements of a mixture will pass through the column in column chromatography the most quickly?
Those which are most soluble in the selected solvent.
/What is column chromatography usually used for?
Purifying organic products and seperating mixtures.
What is Gas-Liquid Chromatography used for?
Seperating mixtures of volatile liquids.
Describe the apparatus required for Gas-Liquid Chromatography.
A coiled glass tube packed with a viscous liquid (oil) inside an oven, into which there a seperate inlets for the mobile phase (an inert gas) and mixture, as well as an outlet to a detector, which measures the amount of substance received at a given time.
How are components differentiated between by Gas-Liquid Chromatography?
By measuring their retention times - the time between the mixture being injected and that particular component arriving at the detector - and comparing them to the retention times of pure substance's known times.
How is the relative amount of substance in a mixture detailed by a GLC chromatogram?
By the area under the peak for that substance.
What are the uses of Gas-Liquid Chromatography?