Gas Phase NMR
Chapter 9: High-resolution Spectra in PHIP
Published:09 Feb 2016
Rodolfo H. Acosta, Ignacio Prina, Lisandro Buljubasich, 2016. "High-resolution Spectra in PHIP", Gas Phase NMR, Karol Jackowski, Michał Jaszuński
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Nuclear Magnetic Resonance (NMR) is one of the most powerful analytical techniques used for materials characterization at a microscopic level. The application of NMR in science and technology includes chemistry, biology, food research and quality control, environmental studies of plants and soils. Determination of pore structures has a great impact in the oil industry and medicine. Additionally Magnetic Resonance Imaging (MRI) is perhaps the most powerful diagnosis technique used in medicine in modern days. Despite all the power of NMR, there is a major drawback in its application that is the poor inherent sensitivity of the signals that can be detected. This fundamental insensitivity originates from the minuscule size of nuclear magnetic moments, which results in an exceedingly small equilibrium nuclear spin polarization even in high magnetic fields. Traditionally, NMR has dealt with excitation and detection of nuclear spin angular momentum in systems in thermal equilibrium with an external static magnetic field. The intensity of the NMR signal is proportional to the population difference of quantum states, which is driven by the difference in energy levels and is given by γB0/kBT, where γ is the nuclear gyromagnetic ratio, B0 is the external magnetic field intensity, T the absolute temperature and kB Boltzmann's constant. The excess population can be described by the polarization P. For instance, for spin-1/2 nuclei, P is the population difference between the two energy states over the whole spin population. A sample of water at room temperature, placed in a magnetic field of 4.7 T has a polarization for 1H that amounts to P=1.6×10−5. This amounts to a population difference of only 1 in 62 500 for protons. The most common strategies to overcome this small polarization are the use of higher magnetic fields or low temperature probes. However, in many applications temperature is not a variable, as for instance in physiological studies.