Measurement of NO Using Electron Paramagnetic Resonance
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Absorption spectroscopy is based upon the principle that the absorption of radiation by a molecule involves the transition of the energy level from its ground state to an excited state. If we denote the energy difference as ΔE and the frequency of radiation as ν, then the relationship is expressed as
(1)
where ħ is the Planck’s constant. In optical-absorption spectroscopy, the absorption may be caused by π-electrons in proteins or conjugated double bonds. In infrared spectroscopy, the absorption may depend on bond angles and strength. In EPR, it is the magnetic interaction between the electron spin of a compound and the magnetic field applied by the instrument. In nuclear-magnetic resonance (NMR), the interaction between the nuclear spin and the applied field is detected. Table 1 shows approximate frequencies and wavelengths typical to these spectroscopic techniques. The exact wavelength (λ) can be calculated from the equation, c = νλ, where c is the light velocity (3 � 10 10 cm/s).
Table 1 Approximate Frequencies and Wavelengths of Absorption Spectroscopy for Biologic Materials
|
Transition frequency, n(Hz) |
Wavelength, λ (cm) |
|
|
UV absorption |
10 16 |
10 −6 – 10 −5 |
|
Visible absorption |
10 15 |
10 −5 – 10 −4 |
|
Infrared |
10 14 – 10 13 |
10 −4 – 10 −2 |
|
EPR (microwave) |
10 1 1 – 10 9 |
10 −1 – 10 1 |
|
NMR (radio frequency) |
10 9 – 10 8 |
10 1 – 10 2 |
