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Nuclear magnetic resonance builds on the physics concepts of resonance and nuclear spin(angular momentum of elementary particles of an atom). Protons, electrons, atomic nuclei, and other elementary particles have an intrinsic spin as if they were spinning on their axes.When atomic nuclei, or other charged particles, spin they become like a magnetic dipole (like a bar magnet). A nucleus, or elementary particle, possessing spin will wobble or precess when they are placed in a magnetic field. A nucleus in a magnetic field precesses at a frequency, called the Larmor frequency, which is proportional to the magnetic field.
When an atom is in a constant magnetic field, it nucleus precesses at the Larmor frequency. If in addition to the constant magnetic field, there is also a weaker perpendicular magnetic field that varies at the same frequency as the Larmor frequency for the nucleus, then the nucleus resonates. The phenomenon is called Nuclear Magnetic Resonance (NMR).\[1\].Figure 1 shows an intuitive model.
*Analogy*: |
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This nuclear magnetic resonance (NMR) causes the spin of the nucleus to flip.The analogy for Earth spinning on its axis would be if Earth flipped so that the north and south poles interchanged and Earth were suddenly spinning in the opposite direction. Just as it would take considerable energy to flip Earth's spinning motion, the nucleus absorbs and emits energy as it flips its spin. With the correct electronic equipment physicists can measure the spectrum of absorbed and emitted energy.\[1\] |
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Note that the electron spin magnetic moment is opposite to the electron spin while the proton spin magnetic moment is in the direction of the proton spin. The electron spin or proton spin will tend to precess around the magnetic field with a Larmor frequency. For a 1 Tesla magnetic field this Larmor frequency would be: (shown in Figure 3. Larmor frequency)
Figure 3. Larmor frequency
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The Larmor frequency can be visualized classically in terms of the precession of the magnetic moment around the magnetic field, analogous to the precession of a spinning top around the gravity field. It can also be visualized quantum mechanically in terms of the quantum energy of transition between the two possible spin states for spin 1/2. \[2\] |
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Essential University Physics Volume 2 Chapter 38 Problem 38: An NMR spectrometer is described as a “300-MHz instrument,” meaning 300 MHz is the frequency supplied to its transmitter coil to flip the spin states of bare protons. What’s the strength of its unperturbed magnetic field?\[5\] |
Solution shown in Figure 5 below:
References :
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\[1\]\- Basics and Analogy of NMR: [http://suite101.com/article/understanding-nuclear-magnetic-resonance-a157340] \[2\]\- Physics of NMR : [http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/nmr.html] \[3\]\- Applications of NMR : [http://www2.chemistry.msu.edu/facilities/nmr/900Mhz/MCSB_NMR_applications.html] \[4\]\- MRI: [http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/mri.html] \[5\]\- Solution to problem - [http://www.chegg.com] |
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