The document discusses nuclear magnetic resonance (NMR) spectroscopy. It describes the key components of an NMR instrument, including the superconducting magnet, RF generator, sample probe, and electronics for data processing. Modern NMR spectrometers use powerful superconducting magnets that require liquid helium to maintain the coils in a superconducting state. Shim coils are used to correct for minor field inhomogeneities. The sample is dissolved and placed in a glass tube within the probe. The probe contains RF coils and systems to spin and temperature control the sample. Fourier transform NMR provides sensitivity improvements over continuous wave instruments.

1. Guided by,
Dr. Preeti Gupta
Presented by,
Anshu singh

2.  Nuclear magnetic resonance (NMR) spectroscopy is
based on the measurement of absorption of
electromagnetic radiation in the radio-frequency
region of roughly 4 to 900 MHz.
 Nuclei of atoms rather than electrons are involved
in the absorption process.s
 In order to cause nuclei to develop the energy
states required for absorption to occur, it is
necessary to place the analyte in an intense
magnetic field.
 Nuclear magnetic resonance spectroscopy is one of
the most powerful tools for elucidating the
structure of chemical species.

3. The most important parts of an NMR instrument
are:
 The magnet,
 The RF generator,
 The sample chamber or probe,
(which not only houses the sample but also the
RF transmission and detection coils).
In addition, the instrument requires:
 A pulse generator,
 An RF receiver,
 Lots of electronics, and a computer for data
processing.

5. Modern NMR spectrometers use superconducting
solenoids magnets,
The magnet in an NMR spectrometer must be strong,
stable, and produce a homogeneous field.
The magnet consist a main field coil made of
superconducting Nb/Sn or Nb/Ti wire.
To maintain the wire in its superconducting state the
coil is immersed in a bath of liquid helium.
These coils are adjusted with every new sample placed
in the probe to compensate for sample composition,
volume, and temperature.

6. Advantages of SCM ?
1.Strongest Magnet;
2. Stable & homogeneous magnet field Bo;
3. Low running cost.

7. 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.
 Surround the sample with a set of shim coils, each of
which produces a tiny magnetic field with a
particular spatial profile to canceling out the small
residual inhomogeneities in the main magnetic field.

8.  The sample holder in NMR is normally tube-
shaped and is therefore called the sample
tube.
 The tube must be transparent to RF
radiation, durable, and chemically inert.
 Glass or Pyrex tubes are commonly used.
 These are sturdy, practical, and cheap.
 They are usually about 6–7 in. long and
~1/8 in. in diameter, with a plastic cap to
contain these sample.
 This type of tube is used for obtaining
spectra of bulk samples and solutions.
 Sample tubes range in size from this
“standard” size down to tubes designed to
hold 40 L of sample, such as the Nano
Probe version from Varian Associates

10. The sample chamber into which the sample holder is placed is
called the probe in an NMR spectrometer.
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 contains an air turbine to spin the sample holder while the
spectrum is collected and houses the coil(s) for transmitting
and detecting NMR signals.

12. 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..

13. 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.

14. 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.

16.  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.

17.  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.

18. In fourier transform NMR Instrument small
energy change takes place in the magnitude.
Present in NMR and hence the senstivity of
this instrument is very less.The sensitivity in
fourier transform is increased by adding the
square root of spectras together.Thisprocess
takes place by a continuous wave instrument
. Simultaneous irradiation of frequency occurs
in a spectrum having a radio frequency pulse
and the nuclie returns back to thermal
equilliberium or its normal state.

20.  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.

21. THANK
YOU
FOR
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PATIENCE