2. Beginning in 1953 with the first commercial
NMR spectrometer, the early instruments
used permanent magnets or electromagnets
with field of 60, 80, 90 or 100 MHz,
respectively for proton resonance.
All instruments above 100 MHz are based on
helium- cooled superconducting magnets
(selenoids) and operate in pulsed FT mode.
The other basic requirements besides high
field are frequency-field stability, field
homogenity, and a computer interface.
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3. • Types of NMR high resolution spectra
Continuous wave Fourier transform
NMR NMR
• Imp parts of the NMR spectrometer:-
Permanent magnet/electromagnet
RF generator
RF detector
Sample holder
Magnetic coils
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4. • Principle:- based on
• frequency sweep field sweep
frequency of
RF source frequency is
is varied constant
Bo is constant Bo is varied
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5. Magnets:-
permanent:- constant Bo is generated that is 0.7;1.4;2.1
o adv:-
construction is simple
cheaper
electromagnet:-Bo can be varied which is done by winding
the electromagnetic coil around the magnet
most expensive components of the nuclear magnetic
resonance spectrometer system
Shim Coils
The purpose of shim coils on a spectrometer is to correct
minor spatial inhomogeneities in the Bo magnetic field.
These inhomogeneities could be caused by the magnet
design, materials in the probe, variations in the thickness of
the sample tube, sample permeability.
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6. A shim coil is designed to create a small magnetic field
which will oppose and cancel out an inhomogeneity in the Bo
magnetic field.
Superconducting solenoids:-
prepared from superconducting niobium-titanium wire and
niobium-tin wire
operated at lower temp.
kept in liquid He(mostly preferred) or liquid N2 at temp of 4
K
liquid N2 should be changed at 10 days while liquid He
shouldd be changed at 80-130 days
higher Bo can be produced that is upto 21 T.
o Advantage:-
High stability
Low operating cost
High sensitivity
Small size compared to electromagnets
simple
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7. RADIOFREQUNCY TRANSMITTER
It is a 60 MHz crystal controlled oscillator.
RF signal is fed into a pair of coils mounted at right angles to
the path of field.
The coil that transmit RF field is made into 2 halves in order
to allow insertion of sample holder .
2 halves are placed in magnetic gap
For high resolution the transmitted frequency must be highly
constant.
The basic oscillator is crystal controlled followed by a buffer
doubler, the frequency being doubled by tunning the variable
It is further connected to another buffer doubler tuned to 60
MHz
Then buffer amplifier is provided to avoid circuit loading.
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8. Signal amplifier and detector
Radiofrequency signal is produced by the
resonating nuclei is detected by means of a coil
that surrounds the sample holder
The signal results from the absorption of energy
from the receiver coil, when nuclear transitions are
induced and the voltage across receiver coil drops
This voltage change is very small and it must be
amplified before it can be displayed.
The display system
The detected signal is applied to vertical plates of
an oscilloscope to produce NMR spectrum
Spectrum can also be recorded on a chart recorder.
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9. Sample Probe
The sample probe is the name given to that part of
the spectrometer which accepts the sample, sends
RF energy into the sample, and detects the signal
emanating from the sample.
It contains the RF coil, sample spinner, temperature
controlling circuitry, and gradient coils.
It is also provided with an air driven turbine for
rotating the sample tube at several hundred rpm
This rotation averages out the effects of in
homogeneities in the field and provide better
resolution.
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10. The spectrum obtained either by CW scan or pulse FT at
constant magnetic field is shown in series of peaks whose
areas are proportional to the number of protons they
represent.
Peak areas are measured by an electronic integrator that
traces a series of steps with heights proportional to the peak
areas.
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11. A proton count from the integration is useful to
determination or confirm molecular formulas,
detect hidden peaks, determine sample purity, and
to quantitative analysis.
Peak positions are measured in frequency units
from a reference peak.
A routine sample for proton NMR on a 300MHz
instrument consist of about 2mg of the compound
in about 0.4 mL of solvent in a 5-mm o.d.glass
tube.
Under favorable conditions, it is possible to obtain
a spectrum of 1μg of a compound of a compound
of modest molecular weight in a microtube in a
300-MHz pulsed instrument.
Microprobe that accept a 2.5mm or 3 mm o.d. tube
are convenient and provide high sensitivity.
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12. Fig. illustrates basic elements of a classical 60-MHz NMR spectrometer.
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13. The sample is dissolved in a solvent containing no interfering
proton (usually CCl4 and CDCl3), and a small amount of TMS
is added to serve as an internal reference.
The sample is a small cylindrical glass tube that is suspended
in the gap between the faces of the pole pieces of the
magnet.
The sample is spun around the axis to ensure that all parts of
the solution experience a relatively uniform magnetic field.
Also in a magnetic gap is a coil attached to 60-MHz
radiofrequency generator. This coil supplies the
electromagnetic energy used to change the spin orientation
of the protons.
Perpendicular to the RF oscillator coil is a detector coil. When
no absorption of energy is taking place, the detector coil
picks up non of the energy given off by the RF oscillator coil.
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14. when sample absorbs energy, however, the
reorientation of the nuclear spins induces a
radiofrequency signal in the plane of the detector
coil, and the instrument responds by recording this
as a resonance signal, or peak.
Rather than changing the frequency of the RF
oscillator to allow each of the protons in a
molecule to come into a resonance, the typical
NMR spectrometer uses a constant frequency RF-
signal and varies the magnetic field strength.
As the magnetic field strength is increased, the
precessional frequency of all the protons increase.
When the precesional frequency of a given type
proton reaches 60MHz, it has resonance.
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15. As the field strength is increased linearly, a pen travels across
a recording chart.
A typical spectrum is recorded as shown in fig below.
The 60MHz 1H NMR spectrum of phenylacetone.
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16. As each chemically distinct type of proton comes into
resonance, it is recorded as a peak on the chart.
The peak at δ = 0 ppm is due to the internal reference
compound TMS.
IN THE CLASSICAL NMR EXPERIMENT THE INSTRUMENT SCANS
FROM “LOW FIELD” TO “HIGH FIELD”
scan
HIGH
FIELD
LOW
FIELD
UPFIELD
DOWNFIELD
NMR CHART
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17. Since highly shield proton precess more slowly than unshield
proton, it is necessary to increase field to induce them to
precess at 60MHz and hence highly shield proton appear to
the right of this chart and deshield proton appear to the left.
Instruments which vary the magnetic field in continuous
fashion, scanning from downfield to upfield end of the
spectrum, are called continuous-wave(CW) instrument.
Because the chemical shift of the peak in this spectrum are
calculated from frequency difference from TMS, this type of
spectrum is said to be a frequency-domain spectrum.
Peaks generated by a CW instrument have ringing. Ringing
occurs because the excited nuclei do not have time to relax
back to their equilibrium state before the field. And pen, of
the instrument have advanced to a new position. Ringing is
most noticeable when a peak is a sharp Singlet.
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19. FTNMR uses a pulse of RF radiation which causes a nuclei in a
magnetic field to flip into higher energy alignment
Applying such a pulse to a set of nuclear spins simultaneously
excites all the nuclei in all local environment.
All the nuclei will re emit RF radiation at their resonance
frequencies which induces a current in a nearby pickup coil,
creating an electrical signal oscillating at the NMR frequency.
This signal is known as the free induction decay (FID) and
contains the sum of the NMR responses from all the excited
spins.
In order to obtain the frequency-domain NMR spectrum
(intensity vs. frequency) this time-domain signal (intensity vs.
time) must be Fourier transformed. Fortunately the development
of FT-NMR coincided with the development of digital computers
and Fast Fourier Transform algorithms.
. This is the principle on which a pulse Fourier transform
spectrometer operates. By exposing the sample to a very
short (10 to 100 μsec), relatively strong (about 10,000 times
that used for a CW spectrometer) burst of RF energy, all of
the protons in the sample are excited simultaneously.
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20. PULSED EXCITATION
CH2 C
O
CH3
BROADBAND
RF PULSE
All types of hydrogen are excited
simultaneously with the single RF pulse.
contains a range
of frequencies
N
S
n1
n2
n3
(n1 ..... nn)
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21. CH2 C
O
CH3
FREE INDUCTION DECAY
( relaxation )
n1
n3
n1, n2, n3 have different half lifes
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23. FOURIER TRANSFORM
A mathematical technique that resolves a complex
FID signal into the individual frequencies that add
together to make it.
COMPLEX
SIGNAL n1 + n2 + n3 + ......
computer
Fourier
Transform
FT-NMR
individual
frequencies
TIME DOMAIN FREQUENCY DOMAIN
a mixture of frequencies
decaying (with time)
converted to
converted to a spectrum
( Details not given here. )
FID NMR SPECTRUM
DOMAINS ARE
MATHEMATICAL
TERMS
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24. The Composite FID is Transformed into a
classical NMR Spectrum :
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CH2 C
O
CH3
“frequency domain” spectrum
25. Advantage over continuous wave NMR
Sample of low conc. can be determined
Magnetic nuclei with low natural isotopic
abundance can be determined eg 13C
Very rapid pulse repetition can be possible
Entire spectrum can be recorded, computerized
and transformed in a few seconds that is every 2
sec For e.g. in 13 min 400 spectra can be recorded.
So thus 20 times signal enhancement is seen
Analysis can be possible where magnetogyric ratio
is low
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26. Properties:-
Nonviscous.
Should dissolve analyte.
Should not absorb within spectral range of analysis.
All solvents used in NMR must be aprotic that is they should not
possess proton.
Chloroform-d (CDCl3) is the most common solvent for
NMR measurements
other deuterium labeled compounds, such as deuterium
oxide (D2O), benzene-d6 (C6D6), acetone-d6
(CD3COCD3) and DMSO-d6 (CD3SOCD3) are also
available for use as NMRsolvents.
DMF, DMSO, cyclopropane, dimethyl ether can also be
used
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