6.6 Spin Resonance Devices
199
with B), but external waves far from this frequency will not be effective in causing
a transition between the two energy states.
6.6.1 Electron Spin Resonance
For electron spin resonance (ESR) devices (also called electron paramagnetic
resonance devices (EPR)), the frequency of the stimulating wave is taken in the
microwave region (3–400 GHz) for convenience in the ease of their generation and
detection. Radio wave detection generally requires bigger antennae (comparable
to λ/4), while much higher frequencies need prohibitively larger magnetics. With
f = 9.3882 GHz, the B field is 0.3350 T (3500 gauss). For a 1 T field, the electronspin-flip resonance occur at f = 28.025 GHz.
In application to biological tissue, ESR works when there are significant numbers
of unpaired electrons in the tissue. A pair of electrons in one atomic or molecular
orbital typically have oppositely directed spins, causing their magnetic fields to
effectively cancel. Free radicals tend to have an unpaired electron. The concentration
of free radicals in normal cells is too low for ESR to work effectively, but that
concentration can be enhanced by UV irradiation. Alternatively, a ‘spin-probe’
can be used, i.e. a molecule built with an unpaired electron and with a uniquelyidentifiable ESR resonance behavior. A relatively stable organic radical called
nitroxide was synthesized in 1965. Nitroxide can be bound to proteins and lipids.
ESR with such molecules incorporated into cells then gives information about the
mobility of polypeptide chains and about lipids in biomembranes.
The atomic and molecular environment of the unpaired electron may affect its
ESR frequency, since this environment can generate its own magnetic field which
adds to the external one. As the other ’environmental electrons’ are paired, the
dominant environmental magnetic field comes from the nuclei of atoms that have
a magnetic moment. Hydrogen is plentiful in organic material, and its proton has
a relatively large magnetic moment. The nucleus in carbon-12 has no magnetic
moment, so it will not affect the ESR resonance frequency. In Table 6.1, magnetic
moments for the principal atoms in organic materials are given in units of the nuclear
magneton μ N = |e| ¯
h/(2m p c) = 0.15155 e fm. One can surmise that the effect
of hydrogen atoms bound within organic molecules will dominate the observed
electron spin resonances for these molecules.
The interaction of the electrons with nearby nuclei due to a magnetic dipolemagnetic-dipole interaction causes what is called ‘hyperfine splitting’ of the
resonance frequency observed for the unpaired electrons. The number of frequencies
into which the resonant frequency is split depends on the spin of the nucleus. The
strength of the resonances is determined by how far the interacting nucleus is from
the unpaired electron. A study of these splitting and their strength can reveal the
geometry of the molecule holding the unpaired electron(s) (Fig. 6.4).
199
with B), but external waves far from this frequency will not be effective in causing
a transition between the two energy states.
6.6.1 Electron Spin Resonance
For electron spin resonance (ESR) devices (also called electron paramagnetic
resonance devices (EPR)), the frequency of the stimulating wave is taken in the
microwave region (3–400 GHz) for convenience in the ease of their generation and
detection. Radio wave detection generally requires bigger antennae (comparable
to λ/4), while much higher frequencies need prohibitively larger magnetics. With
f = 9.3882 GHz, the B field is 0.3350 T (3500 gauss). For a 1 T field, the electronspin-flip resonance occur at f = 28.025 GHz.
In application to biological tissue, ESR works when there are significant numbers
of unpaired electrons in the tissue. A pair of electrons in one atomic or molecular
orbital typically have oppositely directed spins, causing their magnetic fields to
effectively cancel. Free radicals tend to have an unpaired electron. The concentration
of free radicals in normal cells is too low for ESR to work effectively, but that
concentration can be enhanced by UV irradiation. Alternatively, a ‘spin-probe’
can be used, i.e. a molecule built with an unpaired electron and with a uniquelyidentifiable ESR resonance behavior. A relatively stable organic radical called
nitroxide was synthesized in 1965. Nitroxide can be bound to proteins and lipids.
ESR with such molecules incorporated into cells then gives information about the
mobility of polypeptide chains and about lipids in biomembranes.
The atomic and molecular environment of the unpaired electron may affect its
ESR frequency, since this environment can generate its own magnetic field which
adds to the external one. As the other ’environmental electrons’ are paired, the
dominant environmental magnetic field comes from the nuclei of atoms that have
a magnetic moment. Hydrogen is plentiful in organic material, and its proton has
a relatively large magnetic moment. The nucleus in carbon-12 has no magnetic
moment, so it will not affect the ESR resonance frequency. In Table 6.1, magnetic
moments for the principal atoms in organic materials are given in units of the nuclear
magneton μ N = |e| ¯
h/(2m p c) = 0.15155 e fm. One can surmise that the effect
of hydrogen atoms bound within organic molecules will dominate the observed
electron spin resonances for these molecules.
The interaction of the electrons with nearby nuclei due to a magnetic dipolemagnetic-dipole interaction causes what is called ‘hyperfine splitting’ of the
resonance frequency observed for the unpaired electrons. The number of frequencies
into which the resonant frequency is split depends on the spin of the nucleus. The
strength of the resonances is determined by how far the interacting nucleus is from
the unpaired electron. A study of these splitting and their strength can reveal the
geometry of the molecule holding the unpaired electron(s) (Fig. 6.4).