Hyperpolarization methods

In another class of experiments, hyperpolarized states are generated by spin-sensitive chemical reactions. These include para-hydrogen-induced polarization (PHIP) [3-5] and chemically induced dynamic nuclear polarization (CIDNP) [6-8]. The latter involves non-equilibrium nuclear spin state populations that are produced in chemical reactions that proceed through radical pair intermediates. CIDNP s applicability has been focused towards the study of chemical reactions and the detection of surface exposed residues in proteins [9], but has so far remained limited to specialized chemical systems. 

Another method of polarizing nuclear spins is dynamic nuclear polarization (DNP), whereby the comparably large electron spin polarization (see Fig. 1) is transferred to nuclear spins by saturating the electron resonance. DNP is almost as old as NMR spectroscopy, building on the aforementioned theoretical work by Overhauser [21] who predicted what is today known as the Overhauser effect (OE). DNP was soon after demonstrated experimentally by Carver and Slichter [22, 23]. The enhancement, s, that can be obtained by DNP is determined by the gamma ratio 7e/7n which is 660 for protons and 2,625 for 

DNP in the solid state, via the solid effect (SE) can be traced back to Potmd [24], Abragam [25] and Jeffries [26]. Borghini described DNP in a spin temperature model [27, 28], which led to the discovery of thermal mixing (TM) [29]. In 1963 [30, 31], the cross effect (CE) was described, and later verified by Hwang, Hill [32, 33] and Wollan [34, 35]. In the following years, major work was carried out by Wind and coworkers [36]. Hausser and Stehlik derived the framework for DNP in 

The focus of this review is on DNP methods with applications in liquids. This includes dissolution DNP, which is of course strictly a solid-state polarization method although targeted towards NMR spectroscopy or imaging in the liquid state. 

---

These limitations of sensitivity and contrast have led researchers to develop hyperpolarization methods, where the nuclear polarization is greatly increased by the manipulation of spin states. Currently, the most general and popular hyperpolarization method is dynamic nuclear polarization (DNP), where the much higher polarization of an unpaired electron is transferred to the nuclei of interest. DNP was originally... 

While H NMR and NMR spectroscopy were proved to be useful tools for the online monitoring of polymerization reactions, their limitations, such as the intrinsic low sensitivity of the measurements and the time-consuming signal averaging, the use of expensive deuterated solvents ( H NMR) that limits their application in industrial processes, led to the development of the so-called hyperpolarization methods which overcome the lack of sensitivity of NMR... 

Kuhn, L. (ed.) (2013) Hyperpolarization Methods in NMR Spectroscopy, Topics in Current Chemistry, Springer-Verlag Berlin Heidelberg, vol. 338. 

Without doubt, hyperpolarization methods have entered the arena of biomedical NMR, although more work is needed to provide practical implementations. Considering that the first DNP experiment was carried out almost half a century ago, it is astonishing that many polarization mechanisms remain poorly understood, leaving exiting opportunities for further work. 

Up to now, neither this method nor STARTMAS has been used by researchers other than their authors, especially because they are subjected to many imperfections of the pulse sequence. Still, it may be anticipated that they will open up new possibilities in a variety of applications, including studies on unstable systems, in-situ high-temperature experiments, hyperpolarized solids, or measurements on very slowly relaxing spins. 

New DNP methods should be able to overcome the limitation of the very short lifetime of the hyperpolarization, and should be integrated into the currently available solid-state and liquid-state NMR experiments. 

---