NMR Spectroscopy

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NMR Spectroscopy

• Tuning / Matching
• Shimming and Lock

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Tuning the probe

• Tuning
• change the resonant frequency
• Matching
• adjust the impedance

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Tuning Probe
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Tuning Probe
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Deuterium Lock
High resolution NMR measurements require a
special field-frequency stabilization to allow
accumulation of signals, which may be separated
by less than 1 Hz. Lock is to hold the resonance
condition by by separate NMR experiment running
parallel to the one on the observe
channel. Usually the deuterium resonance of the
deuterated solvent is used to provide the NMR
lock signal. Digital lock lock signal is fed
directly into ADC and displayed on the computer.
A narrower lock signals results in higher DC
voltage after rectification. By adjusting the
various shim currents one aims for an optimum
lock signals.
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Deuterium Lock
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Shimming
The process of optimizing the magnetic field
homogeneity for recording high resolution spectra
is called shimming a magnet. Small coils on pole
faces in carefully chosen geometry. Currents in
these coils are adjusted.
Two types of gradients spinning (Z0 to
Z5) non-spinning (X, Y, ) First order Z1,
X, Y Second Order Z2 Higher order Z3, Z4
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Shimming
Proton line width in a standard sample should be
less than 0.5 Hz with spinner on. Non-spinning
proton line width in a 5 mm probe can be easily
adjusted to about 2 Hz.
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Shimming
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Shimming
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Free Induction Decay (FID)

• FID represents the time-domain response of the
  spin system following application of an
  radio-frequency pulse.
• With one magnetization at w0, receiver coil would
  see exponentially decaying signal. This decay is
  due to relaxation.

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Free Induction Decay (FID)
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Sampling the Signal
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Sampling the Signal
ADC (Analog to Digital Converter)
The analogue signal generated in the coil by the
sample should be digitized before it can be
handled by the computer. The input of ADC is the
signal voltage, the output is a binary number
proportional to it.
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Sampling the Signal

• Factors determine the performance of ADC

• Number of bits the ADC uses to represent the
  signal voltage
• The maximum conversion speed
• The number of bits in single word of computer
  memory

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Sampling the Signal

• Spectral Width The maximum spectral width is
  determined by the ADC conversion time which
  limits the rate of signal sampling. 3-10 ms
  generates maximum spectral width of 50 kHz to 150
  kHz.
• Dynamic range The ability of ADC to digitize weak
  signals faithfully in the presence of string
  signals. 12 bits to 20 bits are often referred to
  ADC.

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NMR line shape
Lorentzian line A amplitude W half-line width