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Basic NMR Principles
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Figure 9.2: Decay of the NMR Signal Due to Transverse Relaxation
Secondly, the system naturally tends to relax back to the thermal equilibrium. This process is
equivalent to the polarization process described above and takes place with the time constant
T
1
. Various other processes are known from NMR-theory, depending on the pulse sequences
used to manipulate the magnetization. These processes are described by different relaxation
times, some examples are
T
1ρ
, T
1D
, and T
2e
.
Substances and materials can be characterized by relaxation measurements because the
relaxation times are highly sensitive to molecular properties. For example, they can be
sensors for various ranges of molecular motions. The large range of motions that can be
characterized by NMR is a main point which makes NMR to such a valuable tool in analysis.
For example, because of their different relaxation behavior solid and liquid parts in one
sample can be distinguished clearly and their amount determined as in Solid Fat Content
(SFC) measurements. As a consequence such materials as foodstuff, cosmetics, chemical
products and much more can be investigated. For a more detailed description refer to the
application descriptions available from Bruker.
Furthermore, the intensity of the NMR signal is a function of the number of atomic moments
contributing to the macroscopic magnetization. Therefore, the signal is a measure of the
number of nuclei in a sample. The minispec methods make use of this relationship to
determine the amount of, for example, oil or moisture in a wide range of samples.
In summary, NMR makes use of the nuclei in substances as sensors of their surroundings,
yielding important information about the matter. Since both excitation and detection are
performed via electromagnetic waves in the radio frequency region, this analytical method is
contactless, non-destructive, and rapid.