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About DNP
The first suggestion on how to increase a nuclear polarisation beyond Boltzmann equilibrium was made by Overhauser in 1953. The Overhauser effect results in an increase of the signal from one type of spins I, when the resonance of another type of spins S is saturated. It requires a time-dependent coupling between I and S, but the average coupling may be zero. The original Overhauser effect is based on hyperfine coupling between spins of conduction electrons and nuclear spins in a metal, but the same principle holds for free radicals in liquid solution. In principle, therefore, it is possible to enhance the NMR signal from a (liquid) solvent or solute by adding a radical, and saturating the ESR transition. However, since the effect is based on coupled relaxation equations,- in the context of the nuclear Overhauser effect often called the Solomon equations -, its efficiency is field-dependent and decreases sharply when going to the field values common in modern NMR spectrometers. One proposed solution is the development of a "shuttle spectrometer", where the sample will be repeatedly shuttled between a relatively low field for ESR saturation, and the high NMR observation field. See http://www.bio-dnp.uni-frankfurt.de/. In a stricter sense, Dynamic Nuclear Polarisation refers to transfer of polarisation in solids from electron spins on spatially fixed paramagnetic centers to the nuclear spins. These methods were developed from 1957 onwards, mainly in Saclay and Berkeley, and had as motivation the development of nuclear-polarised targets for particle physics. The two principal processes for polarisation transfer are the "solid effect" and "thermal mixing". In both cases the ESR line is irradiated outside its center; for the solid effect even completely outside the line. The solid effect is a true, coherent, polarisation-transfer process, while thermal mixing is based on incoherent relaxation processes. In most polarised targets the sample material is a glassy frozen solution of stable radicals at concentrations of 10-20 mM; thermal mixing is expected to dominate, except perhaps for protons. The development of DNP as a specific tool for sensitivity enhancement ("hyperpolarisation") in NMR was started in the 1980's in Delft, combining DNP and 13C MAS-NMR in a 0.33 T field. That research was focused on the study of coals, and used the radicals naturally present in such materials. More general DNP/MAS techniques for target-type samples and in rather higher fields have been developed at MIT, see http://web.mit.edu/fbml/cmr/griffin-group/. The extension to liquid samples was developed some five years ago by a group working at Amersham Health (now GE Healthcare) in Malmö, who discovered that it is actually possible to transform a frozen-beads sample into a polarised liquid solution at room temperature by dissolving it rapidly in superheated water. They have shown several interesting applications in the MRI/MRS field, and the method, now usually called dissolution-DNP, has attracted very wide interest. The original publication is: Ardenkjaer-Larsen JH, Fridlund B, Gram A, Hansson G, Hansson L, Lerche MH, Servin R, Thaning M, Golman K. Increase in signal-to-noise ratio of > 10, 000 times in liquid-state NMR. Proc Nat Acad Sci USA 2003; 100; 10158-10163. and their instrumentation is described in: Wolber J, Ellner F, Fridlund B, Gram A, Jóhannesson H, Hansson G, Hansson LH, Lerche MH, Månsson S, Servin R, Thaning M, Golman K, Ardenkjær-Larsen JH. Generating highly polarized nuclear spins in solution using dynamic nuclear polarization. Nucl Instr Methods A 2004; 526: 173-181