The adiabatic approach for transferring PHIP at ULF was investigated in experiments according to protocol 3 by measuring the transfer efficiency as a function of field sweep duration. Previous studies indicated that a field sweep from zero to a few µT provides a higher C polarization than when inverting the field from negative to positive fields . Thus, in our field sweep study the initial field was set to 50 nT and the final field to 2 T. For 13 C2-maleic acid, this ensures a passage through the LAC region, selectively transferring proton polarization to the C nucleus. Both experiment and calculation indicate that even a rather fast field sweep, faster than 1s, provides maximum transfer efficiency (Fig. .a). A longer sweep results only in a slow decrease in observed C polarization caused by relaxation. The highest C polarization obtained in this type of experiment was P( 13 C) = 6.5%, more than a four-fold increase compared to fixed field experiments, and more than two-fold compared to coherent transfer experiments. Meanwhile, theoretical considerations predict almost equal relative levels of polarization for coherent transfers (P( 13 C) = 84 %) and adiabatic field sweeps (P( 13 C) = 82 %). The origin of this discrepancy between simulations and experiment must again be sought in the lack of robustness of the coherent transfer method and its sensitivity to real-world experimental imperfections, in contrast to the intrinsic robustness of the adiabatic approach. For 13 C1-maleic acid, field sweeping through ULF does not produce any appreciable C polarization, neither in experiment, nor in simulations (Fig. .a). This is the result of the relatively broad LAC region for C in this isotopologue, caused by the large (JCH = 168 Hz) coupling to the nearest proton. A possible remedy here would be a sweep through a broader range of fields, but this would inevitably lead to severe losses of polarization due to rapid relaxation of the considered C nucleus.