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May 12, 2013:
Graduation

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Development of Methods for Hyperpolarized NMR


Characterization of Hyperpolarized Spin Systems
Hyperpolarization can give rise to unexpected behavior of the spin system. In the case of spins polarized to negative polarization, spontaneous emission of multiple signals is observed. This behavior cannot be explained by the commonly used Bloch equations with radiation damping. Aa simple model explains the observed features of the signal, based on the replenishment of fresh negatively polarized sample during and after the sample injection process into the NMR coil. Modified Bloch equations include an exponentially decaying term describing the magnetization added to the system during the measurement. The functional form of this added term represents the simplest possibility for including an initially large and later decaying contribution of polarization. Nevertheless, the numerical solutions to these equations are capable of describing all of the general features of the multiple emissions in the 1H signal.

Pulse Schemes for Hyperpolarized NMR
Much of the power of NMR spectroscopy is realized by the ability to measure chemical shift correlations, which give information about molecular structure. Conventionally, chemical shift correlations are obtained from two- or multi-dimensional NMR experiments, where a indirect evolution time segment is incremented between successive scans. These experiments rely on the spin system to return to the equilibrium populations before every new, successive scan. When using hyperpolarized sample, a large amount of signal is achieved from one single NMR scan; however, the signal does not return to the polarized state after it has been converted into a coherence, and can be used only once. Often, a sample can be hyperpolarized only once, and conventional multi-dimensional NMR methods are thus not applicable.
We are developing alternative NMR techniques to obtain the maximum amount of information from a hyperpolarized sample. A robust method for determining two-dimensional heteronuclear chemical shift correlations is by off-resonance decoupling. A small number of scans can be recorded with a variable flip angle from a single hyperpolarized sample, while applying a decoupling field at different frequencies (see figure). The scaling of the apparent scalar coupling can then be used for determining the chemical shift of a coupled nucleus in reminiscence of an indirect spectral dimension.
Alternatively, multi-dimensional NMR experiments from single hyperpolarized samples can be acquired using a robust scheme that involves sequential acquisition of transients by means of small flip-angle excitation.

A Unique Correlation between Reactant and Product
Often, NMR spectroscopy is used to determine correlations between atoms through space. However, in exchange spectroscopy experiments, correlations between different chemical forms are measured. A powerful extension of this type of experiment makes use of the fact that in a hyperpolarized experiment, typically the only signals that are observable originate from the initially present magnetization.
By manipulating this magnetization, for example with a selective pulse, it is possible to track atoms through reactions. Here, the spin state of the initially inverted spin ~195ppm in 3-methylbenzophenone transfers to the reaction product in a Grignard reaction.

Rapid sample injection
To enable liquid state NMR spectroscopy of hyperpolarized analyte, where reactions and dynamic processes can take place, our experiments involve the thawing of the sample, followed by injection into an NMR spectrometer. We have developed a rapid sample injection system, which minimizes the loss of hyperpolarization during sample transfer and allows us to obtain high-resolution spectra of almost any small molecule.