NMR 2- instrumentation

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NMR Instruments 
(a) Magnet, 
(b) radiofrequency source & detector 
(C) data handling/display 
 
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• Magnets: electromagnets up to 100 MHz or 
superconducting magnets up to 600 MHz. 
• Use shim coils to ensure magnetic field 
homogeneity + spinning sample tube. 
• Source of irradiation – radio frequency 
antenna 
– Older instruments were continuous wave i.e. 
held magnetic field at constant Bo and 
varied the frequency until protons came 
into resonance. 
– Modern instruments –pulsed FT 
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CLASSICAL INSTRUMENTATION 
typical before 1960 
field is scanned 
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A Simplified 60 MHz 
NMR Spectrometer 
Transmitter 
Receiver 
Probe 
hv 
S N 
 RF 
Detector 
Recorder 
RF (60 MHz) 
Oscillator 
~ 1.41 Tesla 
(+/-) a few ppm 
absorption 
 signal 
MAGNET MAGNET 
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Fortunately, different types of protons precess at 
different rates in the same magnetic field. 
N 
S 
CH2 C
O
CH3
59.999995 MHz 
59.999820 MHz 
59.999700 MHz 
hn 
60 MHz 
To cause absorption 
of the incoming 60 MHz 
the magnetic field strength, 
Bo , must be increased to 
a different value for each 
type of proton. 
Bo = 1.41 Tesla 
Differences are very small, 
in the parts per million range. 
EXAMPLE: 
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scan 
HIGH 
FIELD 
LOW 
FIELD 
UPFIELD DOWNFIELD 
IN THE CLASSICAL NMR EXPERIMENT THE INSTRUMENT 
 SCANS FROM “LOW FIELD” TO “HIGH FIELD” 
NMR CHART 
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CH2 C
O
CH3
EACH DIFFERENT TYPE OF PROTON COMES AT A 
DIFFERENT PLACE - CAN TELL HOW MANY 
DIFFERENT TYPES OF HYDROGEN THERE ARE 
NMR Spectrum of Phenylacetone 
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MODERN INSTRUMENTATION 
PULSED FOURIER TRANSFORM 
 TECHNOLOGY 
requires a computer 
FT-NMR 
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• all frequencies irradiated simultaneously by short ( 
seconds) powerful pulse of rf. After pulse the signal 
returning to the detector where it is picked up. This 
dies away exponentially as the system relaxes. This 
relaxation signal is recorded as a free induction decay 
or FID signal. 
 
• The FID is digitised using an ADC. A processor 
performs a fourier transform (FT) to produce the 
individual the individual component. The FT allows us to 
plot the signal as either an interferogram in the time 
domain or more usually in the frequency domain. 
 
 
Pulsed Fourier Transform (FT) nmr 
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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 
(v1 ..... vn) 
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FREE INDUCTION DECAY 
 
v1, v2, v3 have different half lives 
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COMPOSITE FID 
“time domain“ spectrum 
n1 + n2 + n3 + ...... 
time 
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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 
FID NMR SPECTRUM 
DOMAINS ARE 
MATHEMATICAL 
TERMS 
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 Composite FID is Transformed into a 
classical NMR Spectrum : 
CH2 C
O
CH3
“frequency domain” spectrum 
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 COMPARISON OF 
CW AND FT TECHNIQUES 
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CONTINUOUS WAVE (CW) METHOD 
The magnetic field is “scanned” from a low field 
strength to a higher field strength while a constant 
beam of radiofrequency (continuous wave) is 
supplied at a fixed frequency (say 100 MHz). 
Using this method, it requires several minutes to plot 
an NMR spectrum. 
THE OLDER, CLASSICAL METHOD 
SLOW, HIGH NOISE LEVEL 
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PULSED FOURIER TRANSFORM 
 (FT) METHOD 
The excitation pulse, the data collection (FID), and 
the computer-driven Fourier Transform (FT) take 
only a few seconds. 
Most protons relax (decay) from their excited states 
very quickly (within a second). 
The pulse and data collection cycles may be repeated 
every few seconds. 
Many repetitions can be performed in a 
very short time, leading to improved signal ….. 
FAST 
LOW NOISE 
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• with FT nmr we can perform multiple recording of 
each spectrum and these FID’s can be added 
together. The spectrum noise will average to zero 
while the true spectrum signals add coherently. 
 
• The signal/noise ratio 
 
 
signals of#s/n 
IMPROVED SIGNAL-TO-NOISE RATIO 
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By adding the signals (FID’s) from many pulses together, 
The signal strength may be increased above the noise level. 
IMPROVED SIGNAL-TO-NOISE RATIO 
noise 
signal 
add many 
pulses 
noise is random 
and cancels out etc. 
1st pulse 
2nd pulse 
nth pulse 
enhanced 
signal 
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• In practice for proton nmr a minimum 
of 10 FID’s will be recorded before 
processing while with 13C nmr several 
hundred FID’s will be recorded 
IMPROVED SIGNAL-TO-NOISE RATIO