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Using the heteronuclear Bloch-Siegert shift of protons for B1 calibration of insensitive nuclei not present in the
Ivan Hung1, Peter Gor'kov1, Zhehong Gan1
1National High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, FL 32310, USA.
Abstract:
Indirect rf field calibration using the heteronuclear Bloch-Siegert shift is presented. This method is useful for calibrating ω1 = -γB1 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, ΔωBS=ω0ω12/ω02-ωirr2, where ω0 and ωirr 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 13C calibration and demonstrated for 14N rf field measurement of a 0.75 mm 100 kHz triple-resonance magic-angle spinning probe.
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