- Wang NMR Inc.
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Nuclear magnetic resonance (“NMR”) was discovered in 1946 by Bloch
and Purcell and has gradually become a standard technique for chemical,
biological, medical and physical research. Additionally, due to the last
eight years of effort, NMR imaging systems have also become a powerful
medical diagnostic tool.
The NMR phenomenon occurs because atomic
nuclei behave like tiny magnets under an external magnetic field. When the
small magnetic fields generated by the spinning nuclei are stimulated with a
specific frequency of electro-magnetic energy, the nuclide reacts by
emitting radio waves. By measuring the radio waves, one can create an image
of the molecular structure and environment of the sample under study.
In NMR spectroscopy, a solid, liquid, or
gas sample is subjected to a strong uniform magnetic field. Simultaneously,
the NMR spectrometer applies RT (radio frequency) pulses at right angles to
the magnetic field. The nuclei in the sample absorb the energy, causing the
nuclei to go from parallel to nonparallel orientation (spin) in the magnetic
field.
Maximum absorption occurs when the
characteristic frequency of the nucleus is the same as the frequency of the
RT pulse. The intensity of the absorption is directly proportional to the
number of nuclei changing orientation at the specific frequency. So peak
areas correspond to the numbers of nuclei in a sample and also the absolute
concentrations.
A second, more subtle effect is created
because each nuclei is also partially shielded from the field by its
electrons. The shield slightly alters the frequency at which the nucleus
absorbs RT energy. This phenomenon, called a chemical shift, explains the
various region in the typical NMR spectrum and helps identify the chemical
structure of the sample.
Rather than measure the amount of the
energy absorbed by a sample’s nuclei, detectors actually measure their
energy (or spin) decay. Between RT pulses, with the atoms no longer excited
the energy absorbed by each nucleus decays (free induction decay) is
emitted. A highly sensitive detector measures and records these emissions as
a decay pattern, which is then transformed into a spectrum.
Thus, NMR spectroscopy non-destructively determines a substance’s composition. It is not limited to samples that reflect or transmit radiation.