We
know that a spin active nucleus behaves as a tiny magnet under the influence of
a strong magnetic field. Such nuclei have an intrinsic property of arranging
themselves in all possible spin states in the presence of a magnetic field. The
possible spin states can be calculated from the spin quantum number.
Spin
states = 2I + 1, where I is the spin of
the nucleus.
Another
important feature of spin active nuclei is that they start moving in a
particular fashion when the field is turned on. This peculiar movement is known
as precession, analogous to the movement of a gyroscopic top, just before it is
falling. Precession of the nucleus is associated with an angular frequency,
known as the larmor frequency, which normally lies in the range of radiofrequency.
According
to the Boltzmann’s distribution excess, there is always a small number of
nuclei in the lower energy state.
The
cumulative vector for the excess nuclei is shown as the bulk magnetization
vector (Mo). Mo is by default shown at z-axis which is
its equilibrium position.
Radiofrequency pulse
A
strong burst of radiofrequency (B1) is applied through the x-y plane
which flips the magnetization vector at an angle depending upon the duration of
the pulse. A 90o x pulse will flip Mo from z-axis through an angle
of 90o so that it comes down to lie on y-axis.
The
direction of the flip is determined by left hand rule, such that if the thumb
of your left hand points in the direction of Rf pulse, Mo will flip in the direction
of the curled fingers.
As
soon as the pulse goes off, the magnetization vector starts relaxing and moves
back towards the z-axis. The detector detects this movement and plots it in the
form of a decaying signal, known as the Free Induction Decay (FID). FID decays
with time therefore it is a time domain signal.
Fourier Transform
FID
is converted to frequency domain signal through a mathematical application
called the Fourier Transform. It is easier to study and extract information
from the frequency domain signal.
Related Topics: Principle of NMR Spectroscopy
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