✅19 - Modern Analytical Techniques II Flashcards
What is formed in a mass spectrometer?
A molecular ion
How is a molecular ion formed?
When a molecule looses an electron
What is the m/z value?
The mass/charge ratio
What is the m/z value the same as?
The molecular mass
What does high resolution mass spectrometry measure?
Precise masses
What can high resolution mass spectrometry be useful for identifying?
Compounds that appear to have the same Mr when rounded to the nearest whole number
What information does NMR give you?
About the structure of molecules
What does NMR stand for?
Nuclear Magnetic Resonance
How does NMR work?
A sample of a compound is put into a strong magnetic field and exposed to a range of different frequencies of radio waves
Nuclei of certain atoms absorb energy from the radio waves
What information does Carbon-13 NMR give you?
About the number of carbon atoms that are in a molecule, and the environments that they are in
What information does High resolution Proton NMR give you?
About the number of hydrogen atoms that are in a molecule and the environments that they are in
What does the amount of energy that a nucleus absorbs depend on?
The environment that it is in
What is a nucleus shielded by?
Its surrounding electrons
What can electron shielding be affected by?
The other atoms and groups of atoms around the nucleus
How would an electronegative element affect the electron shielding?
It would reduce it, as the electrons would be drawn away
What does an atom’s environment depend on?
All the groups that it is connected to
What is used as a standard in NMR?
Tetramethylsilane - TMS
What are absorbances measured relative to?
TMS
Why is TMS used as a standard?
Because its absorption peak is at a lower frequency than all other molecules
What is chemical shift?
The difference in the radio frequency absorbed by the nuclei in the molecule being analysed and by the same nuclei as in TMS
What does the number of peaks on a C13 NMR spectrum show you?
The number of carbon environments
What do the chemical shifts show you?
The different carbon nuclei
What does proton NMR tell you?
The number of hydrogen environments
What splits the peaks in C13 NMR?
Spin-spin coupling
Which hydrogens can affect splitting?
Only those on the adjacent carbon
What would a triplet peak indicate?
That there are two hydrogens in adjacent carbons
What do integration traces show?
The ratio of protons in each environment - the relative areas under each peak is the relative number of H atoms in each environment
What can proton NMR be used to work out?
Structures
What is chromatography useful for?
Separating and identifying things
What is a mobile phase?
Where the molecules can move, always a liquid or a gas
What is a stationary phase?
Where the molecules can’t move, must be a solid, or a liquid on a solid support
What is the basic principle of chromatography?
Mobile phase moves through or over the stationary phase
The distance each substance moves up the plate depends on its solubility in the mobile phase and its retention by or adsorption to the stationary phase
Components more soluble will travel further
What does the distance each substance moves depend on?
The solubility in the mobile phase and the retention or adsorption to the stationary phase
What can Rf values be useful for?
Identifying compounds in a mixture
How would you calculate an Rf value?
Distance travelled by spot/distance travelled by solvent
What can affect Rf values?
Composition of the paper, the solvent or the temperature
Why would spots of two different substances be put on the same paper?
Because conditions are difficult to keep identical
What is HPLC?
High Performance Liquid Chromatography
What conditions is HPLC done under?
High pressure
What is usually the stationary phase in HPLC?
Silica bonded to various hydrocarbons
What is often the mobile phase in HPLC?
A polar mixture of methanol and water
Why does HPLC separate the mixture into different parts?
Because they are attracted by different amounts to the solid, so take different lengths of time to travel through the column
What happens as the liquid leaves the HPLC column?
UV light is passed through it and absorbed by parts of the mixture
What does a HPLC chromatogram show?
The retention times of the components of the mixture
What is the mobile phase in gas chromatography?
A gas
What happens to gases as they move through gas chromatography?
They constantly dissolve in the oil or adsorb onto the solid, evaporate back into the gas then redissolve as they travel through
