INSTRUMENTATION OF NMR SPECTROMETER.pptx

the ppt describes the various parts of NMR spectrometer. the application of NMR spectrometer, the solvent requirements. it also explains about FTNMR

1. INSTRUMENTATION OF NMR SPECTROMETER 23-Feb-23 1 SUBMITTED TO: Dr. Rajesh R HOD & Professor Department of Pharmaceutical Analysis ABMRCP, Bengaluru SUBMITTED BY: S Kishor Kumar Singha ACP22PHCE005 Department of Pharmaceutics ABMRCP, Bengaluru

2. TABLE OF CONTENTS Introduction. NMR instrumentation. Continuous wave NMR. Fourier transform NMR. 23-Feb-23 2

3. INTRODUCTION: Two general types of NMR Spectrometer: a) Continuous Wave spectrometers, b) FT NMR spectrometers. Beginning in 1953 with the first commercial NMR spectrometer the early instruments used permanent magnets or electromagnets with fields of 1.41 T, 1.87 T, 2.20 T, or 2.35 T corresponding to 60 MHz, 80 MHz, 90 MHz, or 100 MHz, respectively, for proton magnetic resonance. NMR spectrometers are referred to as 300 MHz instrument or 500 MHz instruments depending upon the frequency of Radio frequency radiation used for resonance. 23-Feb-23 3

4. NMR INSTRUMENTATION: MAGNET FIELD LOCK SHIM COILS PROBE UNIT Sample holder RF oscillator Sweep generator RF receiver DETECTOR AMPLIFIER & RECORDER. 23-Feb-23 4

5. 1. MAGNET: It is used to suppl y principal part of the magnetic field Ho, which determines the Larmer frequency of any nucleus. The stronger the magnetic field, the better the line of separation of chemically shifted nuclei on the frequency scale. It should give homogenous magnetic field, i.e. the strength of the magnetic field should not change from point to point. 23-Feb-23 5

6. Contd. The magnet must be capable of producing a very strong magnetic field. Range from 60 MHz (1.4 T) to 700 MHz (16.4 T) and higher. The most powerful commercial NMR spectrometer is currently 1.2GHz (28.2 Tesla). Three types of magnet have been used in NMR Spectrometer: a) Permanent Magnet, b) Electromagnet, c) Superconducting Magnet. 23-Feb-23 6

7. A) Permanent Magnet: 23-Feb-23 7 Permanent magnets with magnetic field strengths of 0.7, 1.4, 2.1 T have been used in commercial continuous wave instruments. Oscillator frequencies for proton studies are 30, 60 and 90 MHz . They are highly temperature sensitive and require shielding. Because of field drift problems, they are not ideal for extended period for data accumulation. Field variation is not possible, as it is required, because different nuclei resonate at different magnetic field.

8. B) Electromagnet : 23-Feb-23 8 They are more effective than the permanent magnet because of possibility of field variation. They require power supply to produce magnetic field and cooling system to counter the heat generated from the electric power.

9. C) Super conducting magnet : 23-Feb-23 9 They are used in most modern high-resolution instruments. They can attain magnetic field as large as 23T, corresponding to proton frequency of 1GHz. The superconducting solenoid is made up of niobium-tin or niobium-titanium wire. To superconduct the solenoid, is bathed in liquid helium at temperature of 4K (-269.15⁰C). The helium Dewar flask is held in outer liquid nitrogen Dewar to decrease the evaporative loss of more expensive liquid helium. The sample probe is put in the bore along with a set of room temperature shim coils.

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12. 2. Field lock : In order to produce a high-resolution NMR spectrum of a sample there is need to homogeneous magnetic field. The field strength might vary over time due to aging of the magnet, movement of metal object near the magnet, and temperature fluctuations. Two types: a) External Lock System, b) Internal Lock System. 23-Feb-23 12

13. Contd. Field lock is necessary to prevent the magnetic field from drifting and keep it stable as possible. T he lock system is monitoring the resonance frequency of the signal in the lock channel; and if a shift is detected the magnetic field is adjusted accordingly so the resulting overlay of FIDs will display no shifted or broadened spectra. 23-Feb-23 13

14. Some examples: 23-Feb-23 14

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16. Types of lock system: External Lock System: In this an external reference nucleus is continuously irradiated at its resonance frequency and resultant NMR dispersion signal is continuously monitored. Internal Lock System: In this system a suitable reference such as TMS is added to the sample. The analytical sample nuclei and reference experience the same field. An automatic shim coil control continuously compensate for the several magnetic field gradient. 23-Feb-23 16

17. 3. Shim coils: 23-Feb-23 17 The purpose of shim coils on a spectrometer is to correct minor spatial inhomogeneities in the magnetic field. Through these coils current is adjusted until the magnetic field has the required homogeneity. Magnetic field produced by the shim coils cancel the small residual inhomogeneities in the main magnetic field.

18. 4. Probe: It is a sensing element of the spectrometer system, in which the sample holder is placed. The sample is placed in the probe, which contains the transmitter and receiver coils and a spinner to spin the tube. It is used to excite and detect the magnetization in radio-frequency of sample. For maximum sensitivity, a fixed frequency probe is needed (mean: a separate probe is required for each nucleus like1H, 13C, 19F). 23-Feb-23 18

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20. A) Sample holder The sample holder in NMR is normally tube-shaped and is therefore called the sample tube. The tube must be transparent to RF radiation, durable, and chemically inert. Glass or Pyrex tubes are commonly used. These are sturdy, practical, and cheap. 23-Feb-23 20

21. Contd. It consist of 5mm outer diameter and 7.5cm long glass tube containing 0.4 mL of liquid. This type of tube is used for obtaining s pectra of bulk samples and solutions. A spinner is provided to spin the tube at several hundred rotations per minute in order to give homogeneity. 23-Feb-23 21

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23. B) Radio Frequency Oscillator/ Generator: In order to irradiate the sample with EM radiation a highly stable crystal controlled transmitter is imposed at right angles to the applied magnetic field. This is done so that the applied radiofrequency field should not change the effective magnetic field in the process of irradiation. To achieve maximum interaction of the RF radiation with the sample the oscillator coil is wound around the sample cell. It can apply radiofrequency of 60MHz, 100MHz, 400MHz, 1GHz depending on the capacity of the instrument. 23-Feb-23 23

24. C) Sweep Generator: It is used to control the magnetic field strength ( also known as the frequency) applied to the sample. The sweep generator produces a continuously changing magnetic field which scans across the NMR spectrum of the sample. This allows the spectrometer to acquire the NMR signals from the sample over a range of frequencies and build a complete NMR spectrum. For a nucleus to resonate, the precession frequency should become equal to the frequency of the applied RF radiation. This can be achieved by : 1) Field Sweep method, 2) Frequency Sweep method. 23-Feb-23 24

25. 1) Field Sweep method: There is a relationship between the resonance frequency of the nucleus and the strength of the magnetic field in which the sample is placed. If the RF frequency is constant in order to bring their resonance the precision of the nucleus is to be changed by changing the applied magnetic field. 23-Feb-23 25

26. 2) Frequency Sweep method: This method is used to resonate the nucleus. If the applied magnetic field is kept constant, the precision frequency is fixed. In order to bring about the resonance, the frequency of the RF radiation should be changed so that it becomes equal to the resonance frequency. 23-Feb-23 26

27. D) Radio Frequency receiver: A few turns of wire wound tightly around the sample tube. This receiver coil is perpendicular to both external magnetic field & transmitter coils. When RF radiation is passed through the magnetized sample, the resonance occurs, which cause the current voltage across the coil to drop. The NMR signals in the time domain must be converted to a frequency domain spectrum by application of a Fourier transformation. 23-Feb-23 27

28. 5) Detector: In the detector system the high frequency radio signal is first converted to audio frequency signal, which is much easier to digitalize. The signal from the RF amplifier is made of 2 components: a) carrier signal: which has the frequency of the oscillator producing it. b) a superimposed NMR signal from the analyte. 23-Feb-23 28

29. Contd. When the radiofrequency radiation is passed through magnetized sample two phenomenon namely absorption and dispersion may occur. The detector should be capable of separating absorption signal from dispersion signal. There are two ways of detecting the NMR phenomenon: a) single coil detection ( radiofrequency bridge), b) crossed coil detection (Nuclear detection). 23-Feb-23 29

30. a) Single Coil detection: Single coil probe has one coil that not only supplies the RF radiation to the sample but also serves as part of the detector circuit for the NMR absorption signal. In this method coil used for surrounding the sample serves as both transmitter and receiver coil. To detect the resonance absorption and to separate the NMR signal from the imposed RF field, a RF bridge is used. The applied signal is balanced against the received signal and the resonance signal is recorded as an out of balance e lectromagnetic field which may be amplified and recorded. At the resonance the imbalance generated in the coil is by virtue of developing magnetization of the sample. 23-Feb-23 30

31. Schematic diagram: 23-Feb-23 31

32. b) Crossed Coil detection: In this method there are separate transmitter and receiver coils are used. These two coils are fixed at right angle to each other as well on to the direction of external magnetic field. This energy transfer (induction) is influenced by magnetic nuclei in a sample placed between the two coils, as long as the RF current in the first coil was in resonance with nuclear transition. 23-Feb-23 32

33. Schematic diagram: 23-Feb-23 33

34. Solvents handling in NMR spectrometer: Ideal requirements of Solvent : The ideal solvent should contain no protons. It should be inert, volatile, magnetically neutral. It should be inexpensive. Deuterated solvents are used in NMR for two reasons. First, deuterium provides a signal on which to lock the spectrometer frequency, thus enhancing long-term stability. Second, it dramatically reduces the proton signal that arises from the solvent, which would otherwise overwhelm the signal of the targete d compound. 23-Feb-23 34

35. Contd. Some of the solvents used are: Deutero Chloroform (CDCl3). Deutero Dimethyl Sulfoxide (D6-DMSO). Deutero Methanol (CD3OD). Deutero Water (D2O). Deutero Benzene (C6D6). Carbon Tetrachloride (CCl4). CDCl3 is a most useful NMR solvent. It can dissolve compounds of reasonably varying polarity, from nonpolar to considerably polar, and the small residual CHCl3 signal at 7.27 ppm seldom causes a problem. CDCl3 can easily be removed by ‘blowing off’ , since it is volatile should recovery of the sample be necessary. 23-Feb-23 35

36. SCHEMATIC REPRESENTATION OF CONTINUOUS WAVE NMR SPECTROMETER 23-Feb-23 36

37. WORKING: IN CW NMR, the magnetic field is varied and the RF is kept constant. The sample is dissolved in solvent containing no interfering protons usually, CDCl3 and small amount of TMS to serve as standard. As the magnetic field increases the precessional frequency of all proton increases. When the precessional frequency of a given type of proton reach a certain frequency, it is said to be in resonance. 23-Feb-23 37

38. Contd. The two coils can be adjusted to vary the applied magnetic field slightly by passing current through them, causing each chemically different proton to come into resonance sequentially which can be recorded in the form of peaks/signals. Instruments which vary magnetic field in a continuous fashion scanning from the downfield end to upfield end of the spectrum are called continuous wave instruments. 23-Feb-23 38

39. SCHEMATIC DIAGRAM OF FT NMR SPECTROMETER: 23-Feb-23 39

40. WORKING: FT NMR use 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 computer. For e.g In an organic molecule, all of the H1 nuclei are induced to undergo resonance at same time. The pulse contains range of frequencies centered about the hydrogen in the molecule at once this signal burst of energy. When the pulse is discontinued the excited nuclei begin to lose their excitation energy and return to their original state. As each excited nucleus relaxes it emits EMR. 23-Feb-23 40

41. WORKING: Since the molecule contains many different nuclei many different frequencies of EMR are emitted simultaneously. This emission is called Free Induction Decay (FID) signal. The intensity of FID decays with the time as all of the frequencies emitted and it can be quite complex. We usually extract individual frequencies of different nuclei by using a computer and a mathematical method called Fourier Transform Analysis. 23-Feb-23 41

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43. Advantages of FT NMR: They are more sensitive and can measure weaker signals. The pulsed FT-NMR is much faster as compare to CW NMR. FT-NMR can be obtained with less than 0.5mg of sample. This is important in the biological chemistry, where on μ g quantities of material may be available. FT-NMR also gives improves spectra for sparingly soluble compounds. 23-Feb-23 43

44. REFERENCES: https://wwwL2.chemistry.msu.edu/faculty/reusch/virttxtjml/spectrpy/nmr/nmr1.html NMR Spectroscopy for Medical and Dental Applications, Zia et al. http://dx.doi.org/10.1055/s-0039-1688654 Essential Practical NMR for Organic Chemistry, S. A. Richards and J. C. Hollerton . Spectrometric Identification of Organic Compounds, Robert M. Silverstein, Francis X. Webster, David J. Kiemle and David L. Bryce. Principles of Instrumental Analysis, Douglas A. Skoog, E. James Holler and Stanley R. Crouch. www.slideshare.net 23-Feb-23 44