To measure the degree of polarization induced by the DNP process, one possibility is to calculate the ratio of the microwave OFF signal to microwave ON signal (neglecting depolarization under MAS conditions). While the ON signal is easy to measure since it typically a much better signal-to-noise ratio, measuring the OFF signal can often be challenging and the quality of the OFF signal will determine the error.
Measuring the NMR signal intensities of strongly coupled spin systems is a direct measure that does not require measuring the off signal. And a very elegant way is described in this paper from 2013:
Lau, Justin Y. C., Albert P. Chen, Yi-Ping Gu, and Charles H. Cunningham. “A Calibration-Based Approach to Real-Time in-Vivo Monitoring of Pyruvate C1 and C2 Polarization Using the J_CC Spectral Asymmetry #DNPNMR.” NMR in Biomedicine 26, no. 10 (October 2013): 1233–41.
A calibration-based technique for real-time measurement of pyruvate polarization by partial integral analysis of the doublet from the neighbouring J-coupled carbon is presented. In vitro calibration data relating the C2 and C1 asymmetries to the instantaneous C1 and C2 polarizations, respectively, were acquired in blood. The feasibility of using the in vitro calibration data to determine the instantaneous in vivo C1 and C2 polarizations was demonstrated in the analysis of rat kidney and pig heart spectral data. An approach for incorporating this technique into in vivo protocols is proposed.
A similar approach was used later by Vuichoud et al. analyzing the asymmetry of signal intensities of 2H Pake patterns. The same technique has been used in the Polarized Target Community to measure absolute target polarizations as initially described by Hamada et al.
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