Basic NMR Principles
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9 Basic NMR Principles
NMR (Nuclear Magnetic Resonance) is made possible by the properties possessed by some
atoms that cause their nuclei (part of every atom) to have a magnetic moment. The atom
most commonly investigated by NMR is hydrogen (
1
H), the nucleus of which is composed of
a single proton. Hydrogen is present in many natural and synthetic substances like oil, water,
polymers, pharmaceutics and foodstuffs. Another nuclei often investigated is fluorine, typical
applications include fluorine content in toothpaste or fluorine in chemical compounds.
In a strong external magnetic field (called polarizing field) produced by the permanent
magnets in the Magnet Unit, the magnetic moments can be oriented, and the vector sum of
all moments results in a macroscopic magnetization. The amount of magnetization depends
on the strength of the external magnetic field and the temperature.
Pulses of radio frequency (RF) at a suitable frequency can influence the magnetization in the
sample. In NMR this resonance frequency, called Larmor frequency, depends on the field
strength of the external magnetic field and the nucleus investigated. For example, at a field
strength of 0.47 Tesla, protons (
1
H) have a resonance frequency of 20 MHz. The orientation
of the magnetic field generated by the RF pulse is perpendicular to the static external
polarizing field.
When, for example, a sample containing NMR-active nuclei is exposed to a strong external
magnetic field by inserting a sample into the sample orifice of the magnet, the process of
buildup of macroscopic magnetization will start immediately, but it takes some time until the
equilibrium is reached. The time constant of this built-up process is called longitudinal
relaxation time T
1
(see figure).
Figure9.1: Increase of the NMR Signal Due to Longitudinal Relaxation
With skillful sequences of RF-pulses (generated by the minispec electronics and transmitted
to the sample via the probe) the macroscopic magnetization can be influenced. One
possibility is the generation of so-called transverse magnetization. This is not an equilibrium
state, and therefore different relaxation processes arise. The transverse magnetization is still
the vector sum of discrete atomic magnetic moments. Each one of these moments underlies
a specific time evolution, and a loss of phase coherence results. The time scale of this
magnetization loss is described by the transverse relaxation time T
2
(see figure) in the easiest
case.
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