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 (ΔE < kT) is violated for the nuclear Zeeman interaction ΔE = γB 0 h/(2π) 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 J IS , the polarization of spin I (short for 'investigated') can be determined from the asymmetry A S 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.
Vuichoud, B., Milani, J., Chappuis, Q., Bornet, A., Bodenhausen, G., & Jannin, S. (2015). Measuring absolute spin polarization in dissolution-DNP by Spin PolarimetrY Magnetic Resonance (SPY-MR) Dedicated to Stefan Schäublin, Alfred Höhener and Richard Ernst for their pioneering work. Journal of Magnetic Resonance, 260, 127–135. https://doi.org/10.1016/j.jmr.2015.09.006