where P HP I , P HP S , P th I and P th S are the polarizations of the 1 H and 13 C spins in the hyperpolarization experiment and those of the thermal-reference experiment, respectively. To estimate the 1 H and 13 C polarization of the analyte that could be expected from our d DNP experiments, we performed a similar d DNP experiment on [1-13 C]sodium acetate injected into a Bruker Fourier 80 benchtop spectrometer operating at 1.88 T. We found 1 H and 13 C polarizations were 6 % and 20 %, respectively (see Fig. ). In ZULF experiments, these polarization levels would translate into an enhancement of ε = 51000 with respect to prepolarization at 2 T according to Eq. 3 (provided the 13 C spins are efficiently inverted prior to dissolution to yield P [ 13 C] = -20 %). Such enhancement can be expected in the d DNP-ZULF experiment. However, our best-performing experiment yielded an enhancement of ε = 11000, less than a quarter of the expected 51000 (see Sec. IV). Figure shows the signal enhancement of d DNP-ZULF experiments with respect to prepolarization at 2 T, calculated with Eq. 3. The result of our best-performing experiment is represented by the white bar. As both 1 H and 13 C polarization contribute to the enhancement, the value of ε = 11000 cannot be represented as a point on the plane but rather as a linear combination of 1 H and 13 C polarization. For example, if the 1 H polarization was P [ 1 H] = 0 %, the 13 C polarization of the 11000-enhanced signal would be P [ 13 C] = -6.7 %. The enhancement which would be achieved with the polarization levels of a state-of-the-art d DNP apparatus as reported in reference is shown in Fig. as a long-term goal for d DNP-ZULF NMR.