Transcript NMR Spectroscopy

NMR Spectroscopy
Tuning / Matching
Shimming and Lock
Tuning the probe
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Tuning
change the resonant frequency
Matching
adjust the impedance
Tuning Probe
Tuning Probe
Deuterium Lock
High resolution NMR measurements require a special fieldfrequency 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.
Deuterium Lock
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…
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.
Shimming
Shimming
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.
Free Induction Decay (FID)
Sampling the Signal
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.
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
Sampling the Signal
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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.
NMR line shape
Lorentzian line
y
AW
2
W  4x0  x 
2
A
amplitude
W
half-line width
2