Ft Nmr 512qe
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THE PULSED FOURIER TRANSFORM INSTRUMENT
Presented by:KRINA PATEL 1st M.PHARM Department of Pharmacology, Nargund college of pharmacy.
BANGALORE-85
-:CLASSIFICATION OF THE NMR SPECTROPHOTOMETERS:1. CONVENTIONAL/CONTINUOUS NMR SPECTROPHOTOMETER 2. PULSED FOURIER TRANSFORMS NMR SPECTROPHOTOMETER
-:COMPONENTS OF THE SPECTROPHOTOMETER:
1. A magnet to separate the nuclear spin energy state.
2. Two RF channels, one for the field/frequency stabilization and one to furnish RF irradiating energy.
3. A sample probe containing coils for coupling the sample with the RF field; it consists of Sample holder, RF oscillator, Sweep generator and RF receiver.
4. A detector to process the NMR signals.
5. A recorder to display the spectrum.
PULSE FT-INSTRUMENT:THE CONTINUOUS WAVE TYPE OF NMR SPECTROMETER OPERATES BY EXCITING THE NUCLEI OF THE ISOTOPE UNDER OBSERVATION ONE TYPE AT A TIME. In the case of H1 nuclei each distinct type of proton (phenyl, vinyl, methyl and so on) is excited individually ,and its resonance peak is observed and recorded, independently of all the others. As we scan, we look at first one type of hydrogen and then another, scanning until all of the types have come into resonance.
An alternative approach, common to modern, sophisticated instrument, is to use a powerful but short burst of energy called a pulse that excites all of the magnetic nuclei in the molecule simultaneously and all the signals are collected at the same time with a computer In an organic molecule for instance all of the H1 nuclei are induced to undergo resonance at the same time. An instrument with a 2.1-Tesla magnetic field uses a short (1-10microsec) burst of 90-MHz energy to accomplish this.
The source is turned on and off very quickly, Generating a pulse similar to that shown in figureA. According to a variation of the Heisenberg uncertainty principle, even though the frequency of the oscillator generating this pulse is set to 90MHz ,if the duration of the pulse is very short, the frequency content of the pulse is uncertain because the oscillator was not on long enough to establish a solid fundamental frequency. Therefore, the pulse actually contains a range of frequencies centered about the fundamental, as shown in figure-B. This range of frequencies is great enough to excite all of the distinct types of hydrogens in the molecule at once with this single burst of energy.
When the pulse is discontinued the excited nuclei begin to lose their excitation energy and return to the original state or relax. As each excited nucleus relaxes it emits EMR. Since the molecule contains many different nuclei ,many different frequencies of EMR are emitted simultaneously. This emission is called a free-induction decay (FID) signal. (Shown in figure-C) The intensity of FID decays with the time as all of the nuclei eventually lose their excitation. The FID is a superimposed combination of the all frequencies emitted and can be quite complex. We usually extract individual frequencies due to different nuclei by using a computer and a mathematical method called a Fourier-transform analysis.
The determine frequency is not exact frequency emitted by the methyl hydrogen. Due to the design of the instrument, the basic frequency of the pulse is not the same as the frequency of the acetone resonance. The observed FID is actually an interference signal between the RF source (300 MHz) and the frequency emitted by the excited nucleus where the wave length is given by:
In the other words these singles represents the difference in the two frequencies. Since the frequency of the pulse is known, we could readily determine the exact frequency. However we do not need to know it since we are interested in the chemical shift of those protons which is given by:
Which can be reduce to unit analysis: Showing that δacetone is the chemical shift of the protons of acetone from the position of the pulse, not for TMS. If we know the δTMS , the position of the TMS from the pulse , the actual chemical shift of this peak can be calculated by the adjustment.
We can plot this peak as a chemical shift on a standard NMR spectrum chart. (fig-c). The peak for acetone appears at about 2.1 ppm. We have converted the time-domain signal to a frequencydomain signal, which is the standard format for a spectrum obtained by a cw instrument.
Now consider the 1H FID from the ethyl phenyl acetate. This complex molecule has many types of hydrogens, and the FID is the superimposition of many different frequencies, each of which could have a different decay rate! A mathematical method called a fourier transform, however, will separate each of the individual components of this signal and convert them to frequencies. The Fourier transform breaks the FID into separe sine or cosine wave components. This procedure is too complex to be carried out by eye or by hand; it require a computer.
COMPONENTS OF FT-NMR
A simplified form of the block diagram showing the instrument components of a typical Fourier transform NMR spectrometer. The central component of the instrument is a highly stable magnet in which the sample is placed. The sample is surrounded by the transmitter/receiver coil. A crystal controlled frequency synthesizer having an output frequency of Vc produces radio-frequency radiation. This signal es into a pulsar switch and power amplifier, which creates an intense and reproducible pulse of RF current in the transmitter coil. The resulting fid signal is picked up by the same coil which now serves a as receiver. The signal is then amplified and transmitted to a phase sensitive detector. The detector circuitry produced the difference between the nuclear signals Vn and the crystal oscillator output Vc which leads to the low frequency time-domain signal as shown in the fig. This signal is digitalizea Fourier transform program and other data analysis software. The output from this program is plotted giving a frequency domain spectrum.
ADVANTAGES OF FT-NMR
FT-NMR is more sensitive and t can measure weaker signals. The pulsed FT-NMR is much faster (seconds instead of min) as compared to continuous wave NMR FT-NMR can be obtained with less than 0.5 mg of compound. This is important in the biological chemistry, where only μg quantities of the material may be available. The FT method also gives improved spectra for sparingly soluble compounds. Pulsed FT-NMR is therefore especially suitable for the examination of nuclei that are magnetic or very dilute samples. The signals standout clearly with almost no electronic background noise.
-:DIFFERENCE BETWEEN CONTINUOUS & SPECTROPHOTOMETER: 1. 2.
3. 4.
C.W.NMR Permanent & elctromagnets are more commonly used. Operates by exciting nuclei of the isotope under observation one type at a time . 5-10 minutes are required to scan the spectrum. Peak generated by C.W. NMR have ringing effect and it occurs due to the excited nuclei do not have time to relax back to their equilibrium state before the field and the pen of instrument have advanced to a new peak.
1.
2.
3.
4.
FT-NMR Super conducting magnets that have high filled strength are used. Use of powerful but short burst of energy called pulse that excities all nuclei in the molecule simultaneously. The pulse duration may be approximately 1-10 microsecond at a burst 90 MHz energy. When the instrument is switched off the nuclei under goes process FID. Ringing is not seen.
REFERENCES 1.Willard H.H, Meritt Jr.L.L, Dean J.A, Settle Jr.F.A, Instrumental methods of analysis, 7th edition.427- 438. 2. Donald.L.P, Lampman G.M, George S.K, Introduction to spectroscopy, 3rd edition.113-116. 3. Skoog D.A, Holler F.J, Nieman T.A, Principles of instrumental analysis, 5th edition.470-473. 4. Silverstein R.M, F.A, Spectrometric identification of organic compounds, 6th edition.148-150.
3/22/2012
92 24
Presented by:KRINA PATEL 1st M.PHARM Department of Pharmacology, Nargund college of pharmacy.
BANGALORE-85
-:CLASSIFICATION OF THE NMR SPECTROPHOTOMETERS:1. CONVENTIONAL/CONTINUOUS NMR SPECTROPHOTOMETER 2. PULSED FOURIER TRANSFORMS NMR SPECTROPHOTOMETER
-:COMPONENTS OF THE SPECTROPHOTOMETER:
1. A magnet to separate the nuclear spin energy state.
2. Two RF channels, one for the field/frequency stabilization and one to furnish RF irradiating energy.
3. A sample probe containing coils for coupling the sample with the RF field; it consists of Sample holder, RF oscillator, Sweep generator and RF receiver.
4. A detector to process the NMR signals.
5. A recorder to display the spectrum.
PULSE FT-INSTRUMENT:THE CONTINUOUS WAVE TYPE OF NMR SPECTROMETER OPERATES BY EXCITING THE NUCLEI OF THE ISOTOPE UNDER OBSERVATION ONE TYPE AT A TIME. In the case of H1 nuclei each distinct type of proton (phenyl, vinyl, methyl and so on) is excited individually ,and its resonance peak is observed and recorded, independently of all the others. As we scan, we look at first one type of hydrogen and then another, scanning until all of the types have come into resonance.
An alternative approach, common to modern, sophisticated instrument, is to use a powerful but short burst of energy called a pulse that excites all of the magnetic nuclei in the molecule simultaneously and all the signals are collected at the same time with a computer In an organic molecule for instance all of the H1 nuclei are induced to undergo resonance at the same time. An instrument with a 2.1-Tesla magnetic field uses a short (1-10microsec) burst of 90-MHz energy to accomplish this.
The source is turned on and off very quickly, Generating a pulse similar to that shown in figureA. According to a variation of the Heisenberg uncertainty principle, even though the frequency of the oscillator generating this pulse is set to 90MHz ,if the duration of the pulse is very short, the frequency content of the pulse is uncertain because the oscillator was not on long enough to establish a solid fundamental frequency. Therefore, the pulse actually contains a range of frequencies centered about the fundamental, as shown in figure-B. This range of frequencies is great enough to excite all of the distinct types of hydrogens in the molecule at once with this single burst of energy.
When the pulse is discontinued the excited nuclei begin to lose their excitation energy and return to the original state or relax. As each excited nucleus relaxes it emits EMR. Since the molecule contains many different nuclei ,many different frequencies of EMR are emitted simultaneously. This emission is called a free-induction decay (FID) signal. (Shown in figure-C) The intensity of FID decays with the time as all of the nuclei eventually lose their excitation. The FID is a superimposed combination of the all frequencies emitted and can be quite complex. We usually extract individual frequencies due to different nuclei by using a computer and a mathematical method called a Fourier-transform analysis.
The determine frequency is not exact frequency emitted by the methyl hydrogen. Due to the design of the instrument, the basic frequency of the pulse is not the same as the frequency of the acetone resonance. The observed FID is actually an interference signal between the RF source (300 MHz) and the frequency emitted by the excited nucleus where the wave length is given by:
In the other words these singles represents the difference in the two frequencies. Since the frequency of the pulse is known, we could readily determine the exact frequency. However we do not need to know it since we are interested in the chemical shift of those protons which is given by:
Which can be reduce to unit analysis: Showing that δacetone is the chemical shift of the protons of acetone from the position of the pulse, not for TMS. If we know the δTMS , the position of the TMS from the pulse , the actual chemical shift of this peak can be calculated by the adjustment.
We can plot this peak as a chemical shift on a standard NMR spectrum chart. (fig-c). The peak for acetone appears at about 2.1 ppm. We have converted the time-domain signal to a frequencydomain signal, which is the standard format for a spectrum obtained by a cw instrument.
Now consider the 1H FID from the ethyl phenyl acetate. This complex molecule has many types of hydrogens, and the FID is the superimposition of many different frequencies, each of which could have a different decay rate! A mathematical method called a fourier transform, however, will separate each of the individual components of this signal and convert them to frequencies. The Fourier transform breaks the FID into separe sine or cosine wave components. This procedure is too complex to be carried out by eye or by hand; it require a computer.
COMPONENTS OF FT-NMR
A simplified form of the block diagram showing the instrument components of a typical Fourier transform NMR spectrometer. The central component of the instrument is a highly stable magnet in which the sample is placed. The sample is surrounded by the transmitter/receiver coil. A crystal controlled frequency synthesizer having an output frequency of Vc produces radio-frequency radiation. This signal es into a pulsar switch and power amplifier, which creates an intense and reproducible pulse of RF current in the transmitter coil. The resulting fid signal is picked up by the same coil which now serves a as receiver. The signal is then amplified and transmitted to a phase sensitive detector. The detector circuitry produced the difference between the nuclear signals Vn and the crystal oscillator output Vc which leads to the low frequency time-domain signal as shown in the fig. This signal is digitalizea Fourier transform program and other data analysis software. The output from this program is plotted giving a frequency domain spectrum.
ADVANTAGES OF FT-NMR
FT-NMR is more sensitive and t can measure weaker signals. The pulsed FT-NMR is much faster (seconds instead of min) as compared to continuous wave NMR FT-NMR can be obtained with less than 0.5 mg of compound. This is important in the biological chemistry, where only μg quantities of the material may be available. The FT method also gives improved spectra for sparingly soluble compounds. Pulsed FT-NMR is therefore especially suitable for the examination of nuclei that are magnetic or very dilute samples. The signals standout clearly with almost no electronic background noise.
-:DIFFERENCE BETWEEN CONTINUOUS & SPECTROPHOTOMETER: 1. 2.
3. 4.
C.W.NMR Permanent & elctromagnets are more commonly used. Operates by exciting nuclei of the isotope under observation one type at a time . 5-10 minutes are required to scan the spectrum. Peak generated by C.W. NMR have ringing effect and it occurs due to the excited nuclei do not have time to relax back to their equilibrium state before the field and the pen of instrument have advanced to a new peak.
1.
2.
3.
4.
FT-NMR Super conducting magnets that have high filled strength are used. Use of powerful but short burst of energy called pulse that excities all nuclei in the molecule simultaneously. The pulse duration may be approximately 1-10 microsecond at a burst 90 MHz energy. When the instrument is switched off the nuclei under goes process FID. Ringing is not seen.
REFERENCES 1.Willard H.H, Meritt Jr.L.L, Dean J.A, Settle Jr.F.A, Instrumental methods of analysis, 7th edition.427- 438. 2. Donald.L.P, Lampman G.M, George S.K, Introduction to spectroscopy, 3rd edition.113-116. 3. Skoog D.A, Holler F.J, Nieman T.A, Principles of instrumental analysis, 5th edition.470-473. 4. Silverstein R.M, F.A, Spectrometric identification of organic compounds, 6th edition.148-150.
3/22/2012
92 24
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