Vuichoud, B., et al., Measuring absolute spin polarization in dissolution-DNP by Spin PolarimetrY Magnetic Resonance (SPY-MR). J. Magn. Reson., 2015. 260: p. 127-135.
Dynamic nuclear polarization at 1.2 K and 6.7 T allows one to achieve spin temperatures on the order of a few millikelvin, so that the high-temperature approximation ( Δ E < kT) is violated for the nuclear Zeeman interaction Δ E = γB0h/(2 π ) of most isotopes. Provided that, after rapid dissolution and transfer to an NMR or MRI system, the hyperpolarized molecules contain at least two nuclear spins I and S with a scalar coupling JIS, the polarization of spin I (short for ‘investigated’) can be determined from the asymmetry AS of the multiplet of spin S (short for ‘spy’), provided perturbations due to second-order (strong coupling) effects are properly taken into account. If spin S is suitably discreet and does not affect the relaxation of spin I, this provides an elegant way of measuring spin polarizations ‘on the fly’ in a broad range of molecules, thus obviating the need for laborious measurements of signal intensities at thermal equilibrium. The method, dubbed Spin PolarimetrY Magnetic Resonance (SPY-MR), is illustrated for various pairs of 13 C spins (I, S) in acetate and pyruvate.