Nuclear Magnetic Resonance in Metals Analysis
Steel and Related Materials
Published Online: 15 SEP 2006
Copyright © 2000 John Wiley & Sons, Ltd. All rights reserved.
Encyclopedia of Analytical Chemistry
How to Cite
Neue, G. 2006. Nuclear Magnetic Resonance in Metals Analysis. Encyclopedia of Analytical Chemistry. .
- Published Online: 15 SEP 2006
Nuclear magnetic resonance (NMR) is a spectroscopic technique which uses the magnetic component (B1) of radiofrequency (RF) electromagnetic fields to excite nuclear spins which are subject to a static magnetic field (B0). The resonance frequency is proportional to this field. Most stable elements possess at least one isotope with spin I > 0 and, hence, are observable by NMR. Magnetic field gradients make it possible to label spatial coordinates within a sample. Magnetic resonance imaging (MRI) uses this principle to reconstruct images from NMR signals. Skin effect enhanced imaging nuclear magnetic resonance (SEEING-NMR) derives its submicrometer resolution from the strong shielding properties of metals with respect to RF fields. Resonance frequencies of most isotopes are well separated from each other, making NMR an element-specific technique. Interactions of spins with their local environment lead to characteristic spectral changes that reflect the chemical state, the physical state or local geometry. The possibility of coherent manipulation and detection of spin states allows for an extreme flexibility in designing new and highly selective NMR experiments. Integrated signal intensities are strictly proportional to concentrations. The disadvantage of NMR is the small signal-to-noise ratio, especially for nuclei that have low resonance frequencies. Except for sensitive isotopes like 1H or 19F, concentrations should therefore be higher than 1% and samples weighing 10 to 100 mg are recommended. The sensitivity to local interactions often makes it impossible to predict the visibility of NMR signals in unknown substances.