Decoupler, calibration field strength

Bax A (1983b) A simple method for the calibration of the decoupler radiofrequency field strength. J Magn Reson 52 76-80... 

Solid state 13C NMR measurements were performed on a JEOL GSX-270 spectrometer operating at 67.8 MHz equipped with a CP/MAS accessory. The field strength of the H decoupling was 1.2 mT, contact time 2 ms, repetition time 5s, and spectral width 27.0 kHz. 8K data points were used. Samples was placed in a cylindrical rotor and spun at about 4 kHz. The nC chemical shifts were calibrated indirectly through the adamantane peak(29.5 ppm relative to tetramethylsilane, TMS, (CH3)4Si). 

Figure 3.56. Calibration of the homonuclear decoupling field strength via the Bloch-Siegert shift. The decoupler offset from the unperturbed resonance was 47.5 Hz causing a shift of 5.5 Hz, indicating the mean if field to be 23 Hz.

The radiofrequency field strengths must be carefully calibrated, preferably using the 2D experiment described by Nielsen et al. [15]. Strong radiofrequency fields are most convenient because they increase the efficiency of the multiple-pulse homonuclear decoupling, while high stability of the power and phases of the radiofrequency pulses is crucial. 

Figure 3.69. Calibration of the heteronuclear decoupler field strength via off-resonance proton decoupling. From the reduced splitting of the C- H doublet, the proton rf field strength is calculated to be 2.2 kHz, a typical value for broadband proton decoupling.

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