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MRS Science

Nuclear Magnetic Resonance Spectroscopy is fundamentally different than other forms of spectroscopy in that it is non-ionizing and provides universal detection of the target nucleus, typically 1H protons for metabolite quantitation.

The basic experiment immerses a sample in a strong static magnetic field, termed the B0 field to induce a net magnetization M0. Because the proton has spin, charge, and mass it has an intrinsic magnetic moment and angular momentum, and the ratio of these, known as the gyromagnetic ratio, is known to about 9 significant digits. Applying an external B0 field exerts a torque on this net magnetic moment. Consequently, like spinning top precession in a gravitational field, the induced magnetization will precess about the axis along which B0 is directed at a precise angular frequency, known as the Larmor frequency, given by the product of the gyromagnetic ratio and the B0 field magnitude.
Interrogation of the sample is performed with an oscillatory excitation field, called B1, nominally tuned to the Larmor precession frequency of the protons. The induced magnetic resonance, known as the free-induction decay (FID) signal, has an exponential decay as it relaxes back to equilibrium due to spin-spin coupling with time constant T2. Full longitudinal recovery of M0 occurs through spin-lattice relaxation with exponential time constant T1.

As may be expected the orbiting electron(s) also produces a net magnetic moment, generally opposing the nuclear magnetic moment given its opposite charge, and the magnitude of this effect depends on the chemical bonding environment of the proton. Thus, each species of proton in a molecule has a resonance frequency offset from the nominal Larmor frequency. This offset frequency is termed chemical shift (expressed in ppm) relative to the Larmor frequency of a reference compound in the same B0 field and is precisely known for the protons and natural abundance 13C in common metabolites. Furthermore spectral line groupings and relative amplitudes indicate coupling between neighboring spins. Thus high field NMR plays a vital role performing structural elucidation of small molecules in the life sciences. NMRS has incredible selectivity, although the resonance amplitudes and frequency spread of chemical shifts are smaller at lower B0 fields making measurements more technically challenging than at high field.

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