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Atomic nuclei (some of the nuclear isotopes) possess magnetic moments (associated with their spin quantum number value) similar to the familiar bar magnets. The nuclear magnetic moments are tiny compared to the value of the moments of the classical bar magnets. Just as much as the bar magnets turn and orient in an external magnetic field, the nuclear magnets in presence of externally applied magnetic fields align themselves with respect to the magnetic field direction. At atomic dimensions the quantization criteria apply and there are only specified fixed orientations of the nuclear magnetic moments permissible with respect to the magnetic field direction. Also, resonance frequency can be specified which is related by an equation to the strength of the External Magnetic Field. And, by applying an electro-magnetic radiation [EMR] of appropriate resonance frequency (mostly in the radio-frequency range of the EMR spectrum) transitions can be induced between the different orientations of the nuclear spin which is usually detectable in a sample which is an ensemble of such spins. This phenomenon is the Nuclear Magnetic Resonance.
This phenomenon proved itself to be capable of revealing the nature of nuclear environments in molecules (chemical compounds) because of the changes in the electronic structures due to the bonding criteria for the atoms forming the molecules. These are essentially the variations in the resonance frequencies due to electron circulations within molecules. And these variations called ‘Chemical Shifts’ are in the order of parts per million of the applied field/frequency. Thus if proton nuclei has a characteristic resonance frequency of 300MHz corresponding to a applied magnetic field of 7.05 Tesla, then the total range for the variation of the proton resonance frequency due to differences in molecular electron circulations (the Chemical shift range) is 10ppm. This corresponds to a total variation of 3 KHz in 300 MHz (since 1ppm=300Hz). This implies a stringent stability criterion for the Magnetic field and RF frequency sources and the required ratio must be also maintained to the same accuracy to obtain reliable readout parameters from the spectrum obtained from spectrometers. This is the requirement of field-frequency lock in NMR spectrometers. With that good stability ensured, the magnetic field must be shimmed to get high homogeneity of the field in the sample region. By such techniques a reproducibility of the chemical shift to the accuracy of 0.0001ppm seems possible with the current generation of spectrometers.
1cc of water contains proton spins of the order of 10 exponentiated to 22 spins and the actual sample of water in the detectable region of nmr-probe would contain about 10 exponentiated to 21 spins corresponding to 100 microliter of water solvent. A typical spectrometer of the 300MHz frequency can detect conveniently a spin count of 10 exponentaited to 18 which amounts to volumes in a few ‘microliter’ ranges. But the present generation of Spectrometers at as much high field as corresponding to 900MHz can be sensitive enough detect 10 exponentiated to 11 spins which in terms of sample volume in the ‘pico liter’ range. All this is due to the advances in instrumentation on the rf detection side during the continuous wave mode of NMR detection and subsequently and the improvements in tuning of sample coils simultaneously used for the transmitter and receiver purposes with High Power [up to 3KW peak power for solid samples] pulsing detecting the response possibly in the range of 10 microvolts induced RF in the coil due to NMR induction in pulsed NMR detection. Upto 100MHz proton resonance frequency, Electromagnets (23 KG) can be used but for fields higher than this value Supercon Magnet Systems (with superconducting current carrying elements) are necessary. The possibility of realizing superconducting magnet systems has brought about a total revolution in what was possible by NMR Spectroscopic Technique.
Further as the instrumentation was thus improving as described above, the theoretical understanding of the NMR phenomena and the insights gained paved the way for devising altogether new spectroscopic techniques characterizing the NMR phenomena and experimentation proved NMR to be even more promising as the potential spectroscopic tool required for solving molecular structures of increasing size as in the biologically significant functions. These all could be effectively interfaced to realize automated systems for computer controlled spectrometer settings for experiments, acquiring data processing and transmitting the spectrometer outputs at appropriate stages to computer terminals where the data processing can be according to the user specifications at that user’s own options, but the spectral data are acquired from a Central NMR Facility hosting spectrometers of a variety of specifications assorted from the Manufacturer’s assembly-line outputs.
Materials Used and Prepared for the Seminar
The powerpoint files can be downloaded at this links
1. Introductory de facto NMR instrumentation
2. Description NMR Phenomena for the Instrumentation Criteria
3. Nmr phenomenal events and the Instrumentation
4. Magnetic Resonance Images and Instruments
5. NMR Instruments Appearances and Site Configurations
Link to the Web page of Dr..Aravamudhan with a Course Material for Magnetic Resonance
Click here and display a webpage with a list of other websites built by this author
A hotlink to display a page with synopsis of Ph.D. thesis of Aravamudhan
Alink for an Elementary Article on NMR
An OUTLINE for activity at U.S.I.C.