To separate the temperature dependence of 79Br chemical shifts from their field dependence, it suffices to subtract the latter’s contribution monitored through the 13C resonance. As shown in Fig. 1 this permits one to recover an unequivocal linear temperature dependence of the 79Br chemical shift [14]. A least-squares analysis of the data yields the same slope −0.025 ± 0.002 ppm/K at both B0 = 9.4 and 18.8 T, with correlation coefficients close to 1. It is worth pointing out that in the temperature range probed in this work, the observed drift of B0 does not lead to any loss of spectral resolution, which would of course hamper monitoring of the chemical

shifts. Otherwise, shimming would be necessary before recording both 79Br and 13C spectra at each temperature. If a 15N chemical-shift thermometer were preferred, as described in Ref. [4], this would require a blend with another 15N labeled Epigenetics Compound Library clinical trial compound with a chemical shift that does not depend on the temperature. Fig. 2a shows plots of the 79Br chemical shift versus spinning frequency recorded for KBr in rotors with 1.3, 2.5, 3.2 and 4.0 mm diameter without any temperature regulation. In all cases the acquisition was not begun until the 79Br chemical shift had become stable. The constant 13C chemical shift of adamantane recorded with a 2.5 mm rotor is also included. The up-field shifts of the selleck products 79Br resonances may be attributed to increasing frictional heating of the sample

with increasing spinning frequencies and can be fitted by using polynomial functions included in the figure. The corresponding frictional heating of the sample shown in Fig. 2b for each type of rotor was calculated by using linear fits in Fig. 1 to convert shifts to temperatures. In the absence of Decitabine solubility dmso an active temperature control, we observed a ca. 20% increase in the line-width of the 79Br signal at the highest spinning frequencies employed in this work with different types of rotors. This is a strong indication of

inherent temperature gradients ranging from 3 to 5 °C within fully packed rotors. Increasing the flow of the gas to control the temperature can attenuate these gradients. The precise calibration of temperature gradients within the sample, mandatory for accurate determination of temperature-induced phase transitions and for the study of the activation of specific motional processes, would require the restriction of the sample to thin, disc-shaped regions, positioned at the center of the rotor and at its bottom and top ends. We have shown that a simple blend of KBr and adamantane powders can be used as a reliable chemical-shift thermometer to measure the sample temperature accurately in real time, even in unstable static fields. We presented a simple way to determine the accurate temperature dependence of the 79Br resonance after subtracting changes of resonance frequency due to changes of the static field, monitored by the 13C resonance of adamantane. We thank Nicolas Birlirakis for discussions.