2CALIS doubling the sensitivity of CALIS for calibration of the rf field strength for indirectly observed nuclei.

A new set of pulse sequences, 2CALIS, that exhibit double sensitivity of the recent CALIS pulse sequences for accurate calibration of the rf field strength for an indirectly observed spin is introduced. The sensitivity gain is a result of not forming heteronuclear coherence transfer gradient echoes although they are excellent for artifact suppression. It is, however, demonstrated that the scheme in 2CALIS for suppression of non (13)C-attached proton magnetization is adequate for calibration of the (13)C rf field strength even on natural abundance samples. A 2CALIS version with Watergate applicable to biomolecules in aqueous solution is also presented and demonstrated both in (13)C natural abundance and on a (13)C, (15)N enriched protein sample.