It is the mathematical operation in which the complex
waveform can be broken-down into simple mathematical
It is the mathematical operation required to convert a time
domain spectrum to frequency domain spectrum (or vice versa).
Following an adequate S/N ratio, digital data must be
transformed into the frequency data.
A computer is essential to solve these complex equations
CONTINUOUS WAVE NMR (CW-NMR
In continuous wave NMR (CW-NMR), the sample is continuously
irradiated with a frequency while the magnetic field is varied and the
spectrum is a recording of which magnetic fields caused the sample to
absorb RF (happens when the Larmor frequency)
DEMERITS OF CW NMR
Conventional NMR is not sensitive.
Development of good spectra in microgram quantities is
Time consuming takes- 100-1000 times longer to record a
scan relative to FT-NMR
Some times it is impossible
FTNMR or pulse NMR, the sample is irradiated periodically
with brief, highly intense pulses of radio- frequency radiation,
following which the free induction decay signal - a characteristic
radio- frequency emission signal stimulated by the irradiation – is
recorded as a function of time.
The frequency- domain spectrum can be obtained by a
Fourier transform employing a digital computer
The spectral range is not scanned
Stimulate all transitions simultaneously
Each of N increments is exposed to a field
for a very short time(10μsec
Sample irradiated by a pulse of RF radiation containing all
the frequencies over the 1H range (fixed field).
Relaxation by emitting radiation: signal- Free Induction
FID signals contains the vector-sum of the responses from
all the spins
A time domain spectrum is obtained
Fourier transformed into a frequency domain
– A radio transmitter coil that produces a short powerful pulse
of radio waves
– A powerful magnet that produces strong magnetic fields
Magnets and probes are similar to those of continuous wave
– The sample is placed in a glass tube that spins so the test
material is subject to uniform magnetic field.
– A radio receiver coil that detects radio frequencies emitted as
nuclei relax to a lower energy level
– A computer that analyses and record the data
Power of the RF pulse
The intensity of the signal detected in pulsed NMR is a function
of the power of RF pulse used for excitation
Suitable RF power and pulse width cause magnetization to rotate
by 90⁰ pulse.
Relaxation process occurs
The magnitude of the magnetization decreases with time.
The resulting signal is known as Free Induction Decay(FID)
ii) Pulse duration and recycling time
All precessional frequencies within the effective band width of
the pulse are excited.
The extent is inversely proportional to the duration of pulse in
the time domain.
The broader the pulse spectral region, the shorter is the pulse
Detects the decay of magnetization with respect to time.
The FID corresponding to absorption of a single frequency
spectrum is a simple exponentially decaying sine wave.
The FID, modulated by all the frequencies, consists of a set of
interfering wave forms along with noise.
FID related with time is called time domain spectrum.
Homogeneity of the sample
Stability of the magnetic field
Presence of paramagnetic substances in the sample
Chemical exchange of nuclei
Digitized by employing an analogue to digital converter
It is collected as an array of integers in a computer
The interval between points at which the FID is sampled
It shows the max. frequency that can be measured in the
Spectral width (Sw) =1/2 x Δt
4) Signal to Noise ratio (S/N)
Measures the efficiency of an instrument to distinguish between the
signals and electronic noise
S/N = mean / standard deviation
It is a function of variables such as instrumental and nature of the
S/N ratio depends on
The strength of the applied magnetic field
S/N α [ H0 ]3/2
The more intense the magnetic field, the better will be S/N
S/N = average signal amplitude/ root mean square (RMS) noise
RMS noise = average peak to peak noise 2.5
to noise ratio (S/N) enhancement
For PMR spectra, S/N enhancement is required in case of
micro amounts sample. In NMR of other nuclei, enhancement is
commonly employed since the sensitivity of NMR experiments
may drastically reduced.
1. Appropriate instrument design
2. Signal averaging to isolate the signal from noise.
3. By FT using filtering techniques.
Advantages of FT instruments
Jaquinot advantage (few optical elements)
Resolving power (reproducibility)
Data obtain in one sec
ADVANTAGES OF FT NMR
Dramatic increase in the sensitivity of NMR measurements
Has widespread applications esp. for 13C NMR, 31P NMR and 19F
NMR giving high signal to noise ratio facilitating rapid scanning
Can be obtained with less than 5 mg of the compound
The signals stand out clearly with almost no electronic
Used in engineering, industrial quality control and medicine
MRI is most prominent FT NMR applications
Instrumental methods of Chemical Analysis by H. Kaur, 4th
Edition, page no: 421-446
Instrumental Analysis by skoog.
Instrumental analysis by willard