2. NUCLEAR MAGNETIC RESONANCE
SPECTROSCOPY
NMR is a physical phenomenon in which nuclei in a magnetic field absorbs
and re-emit electromagnetic radiation.
This phenomenon occurs when the nuclei of certain atoms are immersed in
a static magnetic field and exposed to the second oscillating magnetic field.
It is an analytical chemistry technique used in quality control and research
for determining the content and purity of a sample as well as its molecular
structure.
It is used in various fields like scientific research, various industries, medical
field etc.,
It is now the most versatile spectroscopy technique that is used in regular
analysis of Bio macromolecules.
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3. FOURIER TRANSFORM NMR
SPECTROSCOPY
In FT-NMR instrument, small energy change takes place
in the magnitude, present in NMR and hence the
sensitivity of this instrument is very less.
The sensitivity in FT-NMR can be increased by adding the
square root of recorded spectra's together.
Simultaneous irradiation of frequency occurs in a
spectrum having a Radio frequency pulse and then the
nuclei returns back to thermal equilibrium on its normal
state.
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4. FOURIER TRANSFORM NMR
SPECTROSCOPY
The Fourier Transformation is the basic mathematical
calculation necessary to convert the data in time
domain(interferogram) to frequency domain(NMR
Spectrum).
i.e, time domain Intensity v/s Time.
Frequency domain Intensity v/s Frequency.
It was developed by JEAN BAPTISE JOSEPH FOURIER.
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5. THEORY OF FT-NMR
When magnetic nuclei are placed in a magnetic field and
irradiated with a pulse of radio frequency close to their
resonant frequency, the nuclei absorb some of the
energy and precess like little tops at their resonant
frequencies.
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6. THEORY OF FT-NMR (CONTINUED..)
This precession of many nuclei at slightly different
frequencies produces a complex signal that decays as the
nuclei loses the energy they had gained from the pulse.
This signal is called as free induction decay(FID) or
transient, it contains all the information needed to
calculate a spectrum.
The free induction decay can be recorded by a radio
receiver and a computer in 1-2 seconds and many FIDs
can be averaged in few minutes. A computer converts
the averaged transients into a spectrum.
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7. THEORY OF FT-NMR (CONTINUED..)
A Fourier transform is the mathematical technique used to compute
the spectrum from the free induction decay. This technique of using
pulses and collecting transients is called Fourier transform
spectroscopy.
A Fourier transform spectrometer is usually more expensive than a
continuous wave spectrometer, since it must have fairly
sophisticated electronics capable of generating precise pulses and
accurately receiving the complicated transients
A good 13C NMR instrument usually has the capability to do 1H NMR
spectra as well. When used with proton spectroscopy, the Fourier
transform technique produces good spectra with very small
amounts( less than milligram) of sample.
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8. CHEMICAL SHIFT:
• Resonance frequencies of the same isotopes in different
molecular surroundings differ by several ppm (parts per
million). For resonance frequencies in the 100 MHz range
these differences can be up to a few 1000 Hz. After
creating a Mx,y coherence, each spin rotates with its own
specific resonance frequency w, slightly different from
the B1 transmitter (and receiver) frequency w0. In the
rotating coordinate system, this corresponds to a rotation
with an offset frequency W = w - w0.
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10. SENSITIVITY:
The signal induced in the reciever coil depends
• On the size of polarization Mz to be converted into Mxy
coherence by a 900 pulse.
• On the signal induced in the receiver coil at detector,
depending on the magnetic moment of the nucleus
detected and its precession frequency
• unfortunately the noise also grows with the frequency.
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11. MICHELSON INTERFEROMETER
A Michelson interferometer is used to observe interference.
It does this through a setup involving a light source, a light
detector, a beam splitter, and mirrors.
By splitting the beam of light and introducing differences in path
length for the resulting beams, interference can be induced.
12. MICHELSON INTERFEROMETER
The following slides will explain this concept in greater detail.
Legend
Light wave (original)
Light wave (split)
Light wave (recombined)
Mirror
Light source
Beam splitter
Light detector
13. MICHELSON INTERFEROMETER
First, the Michelson
interferometer emits a
beam of light of a fixed
wavelength from the
source.
This beam travels
through the beam
splitter, resulting in 2
waves (still same
wavelength) being sent
to different mirrors.
14. MICHELSON INTERFEROMETER
The mirrors each reflect
their respective beam
back toward the
splitter. In this case, the
distance between each
mirror from the splitter
is the same.
15. MICHELSON INTERFEROMETER
When the beams reach
the splitter, they are
both in the same spot
and aimed in the same
direction. Because they
occupy the same space,
interference must occur.
In this case, it is
constructive because the
mirrors are the same
distance away, thus the
number of wavelengths
is the same. Note the
resulting amplitude is
now 2A. This should
result in a bright light
being observed on the
detector.
16. MICHELSON INTERFEROMETER
Now let’s modify the experimental settings by moving the right
mirror to the right by λ/4
(one quarter of the beam’s wavelength)
17. MICHELSON INTERFEROMETER
Just as before, a light of
a fixed wavelength is
emitted, is split into
two, and each beam
travels to its respective
mirror.
This time, however, the
right beam’s mirror is
slightly further away, a
length of λ/4.
18. MICHELSON INTERFEROMETER
Because the right
mirror has been
shifted, a phase
difference has been
introduced between
the waves
corresponding to the
two mirrors. Because
the distance moved is
λ/4, and that distance
is travelled twice
(oncoming and
reflected beam) the
phase is now λ/2 or π.
19. MICHELSON INTERFEROMETER
Once these 2 waves
combine at the same
spot as before, their
phase difference results
in complete destructive
interference.
As a result, it is
expected that no light
will be observed at the
detector.
20. CONCLUSION:
• importance of detecting the nucleus with the highest γ
(i.e., 1H), important in heteronuclear H,X correlation
experiments: "inverse detection"
• double sample concentration gives double sensitivity, but
to get the same result from longer measuring time, one
needs four times the number of scans
• sensitivity should increase at lower temperatures (larger
polarisation), but lowering the temperature usually also
reduces T2 , leading to a loss of sensitivity due to larger
line widths.
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