Using the heteronuclear Bloch-Siegert shift of protons for B calibration of insensitive nuclei not present in the sample.

Indirect rf field calibration using the heteronuclear Bloch-Siegert shift is presented. This method is useful for calibrating ω = -γB for the rf channels of small volume fast-spinning probes on which direct rf calibration is practically inconvenient or difficult for insensitive low-γ nuclei. Proton signals are observed for the rf calibration of the insensitive nuclei without requiring their presence in the sample. For a linearly modulated rf field, the heteronuclear Bloch-Siegert shift is given by, Δω=ωω/ω-ω, where ω and ω are the Larmor and irradiation frequencies, respectively. A short protocol using full-echo acquisition of protons is described for measurement of the phase change induced by the Bloch-Siegert shift. The calibration procedure is validated by a comparison with direct C calibration and demonstrated for N rf field measurement of a 0.75 mm 100 kHz triple-resonance magic-angle spinning probe.