EPR Flashcards
How are EPR and NMR similar?
Both use less energetic photons and must be observed in a magnetic field. Both observe spin transitions.
What are the major differences between EPR and NMR?
Microwaves are used in EPR and radiowaves are used in NMR. EPR observes electron spin transitions and NMR observes nuclear spin transitions.
What does EPR measure?
The energy difference between the two states of a paramagnet when in a magnetic field.
Is an energy difference between states when the molecule has not been placed in a magnetic field?
No
Which has a higher magnetic moment, the unpaired electron spin in EPR or the unpaired nuclear spin in NMR?
Electron spin in EPR
What is a paramagnet?
An atom or molecule with an unpaired electron spin.
What is the zeeman effect?
When in a magnetic field, the magnetic moment of the paramagnet will either align with or against the magnetic field.
What is the highest energy state of a paramagnet?
When the paramagnet aligns against the magnetic field.
Describe most paramagnets in the ground state at low temperatures.
Aligned with the magnetic field (low energy state)
What is the result of using a bigger magnetic field?
More energy is required to make the transition between the two states.
Why are microwaves used in EPR?
The energy gap between the two electron spin states corresponds to microwaves in the GHz range.
In EPR, which is kept constant the microwave frequency or the magnetic field?
Microwave frequency
When does absorption occur?
At the resonance position - where ΔE matches the energy of the microwaves
What is the advantage of scanning the magnetic field rather than scanning the microwave frequency?
It is easier to scan the magnetic field. This gives a better signal:noise ratio and gives more control of the system.
At what frequency is the field modulation in EPR?
100 kHz for measurements at X band microwave frequency
What is the advantage of field modulation?
Gives better signal:noise, overcomes wide EPR signals by giving the first derivative graph.
What does the zero point on the first derivative graph?
The absorption peak when there is no field modulation.
How do most EPR instruments generate microwaves?
Using a gun diode
Where is the sample loaded?
In the resonant cavity between the two magnets
How are microwaves guided through the EPR instrument?
Using waveguides - open air, brass rectangular channels with dimensions corresponding to the X band wavelength (3cm)
Why is liquid helium used?
To keep samples cool and to allow slower relaxation of the sample.
What is the purpose of the circulator?
Directs generated microwaves from the microwave source to the resonant cavity and directs reflected radiation to the detector.
When is it particularly important to use liquid helium and why?
When studying metalloproteins as the excited state of a metalloprotein relaxes very quickly, giving broad peaks.
Describe X band microwave radiation.
Microwaves resonating at a wavelength of 3cm and a frequency of 9.4 GHz.
What happens if the frequency of microwave radiation is increased?
Resonance will be seen at higher resolution but this requires a stronger magnetic field.
What are the sources of unpaired electrons in biology?
Transition metals, free radicals and triplet states.
What is a triplet state?
An atom or molecule with two unpaired electrons.
How can integer spin be detected in EPR?
Using parallel mode EPR - where the modulating field is parallel to the applied magnetic field
Describe the spectra given by radicals.
Sharp, defined features with narrow bands. Tend to have spin 1/2 and are found around g=2.002
Can radicals be detected at room temperature?
Yes, as they are slow relaxing.
