LowCOST-HSQC variants for fast pulsing high $\omega_1$-resolved 2D-experiments

The LowCOST-HSQC is a sensitivity-enhanced HSQC version that retains unused proton polarization for subsequent scans in correlations to low natural abundance nuclei like 13C or 15N. Together with fast polarization distribution via isotropic mixing, it allows the acquisition of fast pulsing 2D-experiments. We give a detailed introduction and comparison of three possible INEPT-type transfer elements for the LowCOST approach -- the original LowCOST, the ZIP, and a specific TIG-BIRD element -- and evaluate various variants of the LowCOST-HSQC.