Nuclear Magnetic Resonance Spectrometers

The NMR techniques rapidly grow into one of the leading technique for molecular identification. As its applications growth in chemical research, biochemistry, pharmaceutical chemistry, polymer science, petroleum research, agricultural chemistry and medicine, due to its unique results.

Some basic facts about NMR:-

The NMR signal is a natural physical property of the certain atomic nuclei but it can only be detected with an external magnetic field. Most modern NMR spectrometers utilize a magnet fabricated from superconducting materials and the magnet winding is cooled with liquid helium. The sample for NMR analysis remains at room temperature. Each isotope of each element in the Periodic Table has a different NMR signal frequency. The most commonly observed nucleus is hydrogen (1H) as found in water (H2O), although the NMR signal from many other nuclei can be observed with the appropriate hardware.

The NMR signal for most spectrometers is in the radio frequency part of the electromagnetic energy spectrum and NMR spectrometers are referred to by the proton NMR frequency, e.g. 60MHz, 300MHz or 500MHz. In most high-field superconducting magnet NMR systems, the Radio Frequency (RF) signal is "pulsed" in short bursts thus the name "Pulsed" NMR. Alternatively, the NMR signal can be observed by sweeping the magnetic field with continuous RF, thus the name "Continuous Wave" NMR or "CW" NMR. Continuous wave NMR is usually found on low-field permanent magnet NMR spectrometers. Pulsed NMR systems can do many more types of NMR experiments than CW NMR systems, however the pulsed NMR signal must be processed on a computer with a Fourier Transform (FT) to be interpreted, thus the name "FT NMR." Transforms other than the Fourier Transform can also be used.

The power of NMR is that for most nuclei the NMR frequency changes very slightly with different molecular environment or chemical bonding. These shifts are very small, on the order of 1 part in 10 9. This is referred to as "Chemical Shift." Tables of chemical shifts are tabulated and used to determine molecular structure. Combining NMR chemical shift information with other NMR information such as peak integration, coupling constants and Nuclear Overhauser Enhancement (NOE) can result in complete three-dimensional molecular structures of molecules in solution without using X-ray crystallography.

NMR samples are usually liquid solutions contained in glass tubes. NMR solution sample volume ranges from 50ul to 5ml depending on the NMR probe. Sample concentrations of solute for 1H NMR are usually in the range of 100ug to 5g, with 10 to 50mg being typical. NMR is not a chemical trace analysis technique. The NMR signal of solids and gases can be recorded but to measure solid-state NMR additional specialized hardware is required.

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