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30
1.2.8. NMR of alginates – a brief
course
for polysaccharide
chemists
MR imaging and NMR spectroscopy are both based on the same principles,
namely the quantum mechanical properties of atomic nuclei. When placed in a
strong magnetic field different nuclei have characteristic behaviour, which can
be detected and processed to obtain useful (quantitative and qualitative) data.
Atomic nuclei have mass, charge (positive) and a nuclear spin. The nuclear
spin (I) has an associated spin quantum number (I):
I = 0,1/2, 3/2.. (units h/2
π
)
The nuclear spin (I) depends on both nuclear mass and atomic number (or
number of charges) according to the table:
Masse
number
Atomic
number
I
Odd
All
1/2, 3/2, 5/2..
Even
Even
Odd
0
1, 2, 3..
12
C,
16
O,
32
S have I = 0 and cannot be observed by NMR. However,
1
H,
13
C,
19
F,
31
P have I = ½. They are ideal for NMR, as is for instance
2
H (D), with I =
3/2.
Nuclei with I
≠
0 possess, due to their spin, a nuclear magnetic moment (
µ
),
which is proportional to the spin:
µ
=
γ
Ih/2
π
, where
γ
is a constant
(magnetogyric ratio) depending on type of nucleus.
When a magnetic field (B) is applied, the nuclear moments orient themselves
with only certain allowed orientations (quantum mechanical system): A
nucleus of spin I has 2I+1 possible orientations. For I = ½ we thus have only
two levels. Each orientation is characterized by a corresponding magnetic
quantum number (m
I
). For I = ½ m
I
has values –½ and ½. The energy
difference between the two levels is given by:
ž
Δ
E
=
γ
hB
2
π