Report by the analytical methods committee: evaluation of analytical instrumentation Part XVI Evaluation of general user NMR spectrometers

• Published in: Accreditation and quality assurance Vol. 11; no. 3; pp. 130 - 137
• Format: Journal Article
• Language: English
• Published: Heidelberg Springer Nature B.V 01.05.2006
• Subjects: Angular momentum; Boltzmann distribution; Charged particles; Chemical equilibrium; Committees; Energy; Energy levels; Ethanol; Gyromagnetic ratio; Line broadening; Magnetic fields; Magnetic moments; Molecular motion; Molecular structure; NMR; Nuclear magnetic resonance; Nuclei (nuclear physics); Particle spin; Protons; Radiation; Spectrometers; Spectrum analysis
• ISSN: 0949-1775; 1432-0517
• DOI: 10.1007/s00769-005-0006-7

[...]the Committee considers that it would be invidious to do so: rather it has tried to encourage the purchasers to make up their own minds as to the importance of the various features of the equipment that is on offer by the manufacturers. In many cases this causes severe line broadening through its effect on relaxation processes (see below) and signals are consequently difficult to observe. Since they are charged particles, nuclei with a spin angular momentum also possess a nuclear magnetic moment, colinear with the spin axis, which has the valueγ, the magnetogyric ratio (and thus the magnetic moment) is a constant unique to the nuclear species. Where this axis is provided by an external magnetic field the different orientations differ in energy. [...]for 1H (and other I=1/2 nuclei) two orientations are allowed: that with the nuclear magnetic moment oriented (roughly) in the field direction being of lower energy than that oriented the other way. If a large assembly of identical spin 1/2 nuclei (e.g. protons) is exposed to a magnetic field a Boltzmann distribution of population is established between the energy levels.This occurs through relaxation processes involving interaction of the nuclear magnetic moments with fluctuating magnetic fields generated in the environment by molecular motion. The chemical shift Within a molecule a nucleus does not experience the full applied magnetic field since it is slightly shielded by the surrounding electrons to an extent which is characteristic of the local chemical environment. [...]in a molecule which has the same nuclear species in several chemically distinct locations, for example ethanol, the protons of the CH3, CH2 and OH groups experience slightly different magnetic fields and consequently resonate at different frequencies.