Nuclear Magnetic Resonance
Proton NMR
Nuclear Magnetic Resonance was first applied to protons. It is used mainly for the identification of unknowns and for the determination of molecular structures. Single scans can deliver resonable spectra, because H-1 comprises 99.98% of naturally occurring hydrogen.
In most Magnetic Resonance Imaging studies, the protons in water molecules provide the physican with his or her diagnostic information.
An important feature of proton NMR is the spin-spin coupling of protons on neighboring carbon atoms. This results in the signal being split into multiple peaks. At low instrument frequencies, these split signals can overlap other signals. This problem is addressed by raising the instrument frequency. Early studies were made with permanent magnets at a frequency of 60 MHz. As more powerful electromagnets became available, the instrument frequency was raised to 90 and 100 MHz. Still higher frequencies have been achieved using liquid nitrogen and liquid helium cooled superconducting solenoids. Frequencies of 750 MHz and above are now being used.
The signal can also be enhanced by operating at lower temperatures. The enhancement can be calculated using the equation
Nj/No = exp (-DE/kT) = exp (-hn/kT)
where Nj is the number of atoms in the excited state, No is the number of atoms in the gound state, DE is the difference in energy between ground and excited state, k is the Boltzmann constant, h is Planck's constant, n is the resonant frequency, and T is the absolute temperature.
The signal is enhanced about 70-fold by reducing the sample temperature from 293 K to 4 K, while operating at 650 MHz. There is an approximate 10-fold enhancement in signal brought about by increasing the instrument frequency from 60 MHz to 750 MHz.
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