WELCOME

FOURIER-TRANSFORM

NUCLEAR MAGNETIC RESONANCE

(FT-NMR)

RAHUL B SM PHARM PART 1PHARMCEUTICAL CHEMISTRY

CONTENTS

Introduction

Fourier transformation

Components of FT NMR

Advantages

Reference

NMR

FOURIER-TRANSFORM

It is the mathematical operation in which the complex waveform can be broken-down into simple mathematical operations.

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

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Time

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Time

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Frequency

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)

CONTINUOUS WAVE NMR (CW-NMR

DEMERITS OF CW NMR

Conventional NMR is not sensitive.

Development of good spectra in microgram quantities is

difficult

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

FT-NMR

The spectral range is not scanned continuously

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 Decay (FID)

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 spectrum

FT-NMR INSTRUMENTATION

FrequencySynthesizer

Pulse Switch

RF Transmitter

RF Amplifier

Phase SensitiveDetector

Time SweptComputer and

FourierTransform

Output(Video/Hard Copy)

S N

– 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 instruments

– 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

1. SAMPLE STIMULATION

i. 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 90 pulse.⁰

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

2)DETECTOR 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.

Other factors

Homogeneity of the sample

Stability of the magnetic field

Presence of paramagnetic substances in the sample

Chemical exchange of nuclei

3) DIGITIZATION

Digitized by employing an analogue to digital converter

(ADC)

It is collected as an array of integers in a computer

The interval between points at which the FID is sampled

as Δt.

It shows the max. frequency that can be measured in the

FID.

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

sample.

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

5Signal 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 design2. Signal averaging to isolate the signal from noise.3. By FT using filtering techniques.

S/N = average signal amplitude/ root mean square (RMS) noiseRMS noise = average peak to peak noise 2.5

ADVANTAGES

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

background noise

Used in engineering, industrial quality control and medicine

MRI is most prominent FT NMR applications

REFERENCES:

1. Instrumental methods of Chemical Analysis by H. Kaur, 4th

Edition, page no: 421-446

2. Instrumental Analysis by skoog.

3. Instrumental analysis by willard