The value of τa for xenon atoms on glass surfaces at 300 K can be estimated to be ∼10−10 s from the expression τa = τ0exp(−E/kBT), where E = 0.12 eV is the desorption activation energy xenon on borosilicate glasses [34] and assuming τ0 = 10−12 s. Although none of the correlation times associated with these events are long enough to cause biexponential relaxation, it is possible however that strong xenon adsorption sites are present on the Pyrex surface. The prolonged

correlation GS-1101 ic50 times at these locations may lead to a violation of the extreme narrowing condition and thus to differential line broadening. An additional hint for surface interactions as the source for the satellite broadening is the differential

broadening between the two satellite transitions. Such differential broadening may be the result of paramagnetic – quadrupolar cross correlation that was observed recently by Jerschow and co-workers by 23Na NMR in the presence of paramagnetic contrast agents [74]. The only source for paramagnetism in the sample used for the spectra in Fig. 2 was on the Pyrex surface [75]. Other causes for differential line broadening may be CSA-quadrupolar cross-correlation effects during prolonged surface adsorption. PF 2341066 Alternatively, the lineshape may be inhomogeneously broadened by differences in EFG experienced by the xenon atoms in various parts of the container that were not averaged by gas diffusion at the gas pressures used. Although the precise mechanism of the satellite broadening remains speculative thus far, it likely originated from interactions with the Pyrex surface that were scaled down by exchange with the gas phase where the NMR signal was observed. A ‘scaling down’ of

surface effects also takes place for quadrupolar splitting that is on the order of 6 MHz on a Pyrex surface [35] but that is observed as a few Hz splitting 5-Fluoracil in vitro in the gas phase. Another distinctive feature shown in Fig. 2 is that thermally polarized 131Xe and hyperpolarized 131Xe signals were 180° out of phase with respect to each other while both 129Xe spectra possessed the same phase. This observation warrants a more detailed explanation. 131Xe is unique among the stable (i.e., non-radioactive) noble gas isotopes because it is the only isotope with a positive gyromagnetic ratio γ  . Therefore, according to Em   = −γmz  ℏB  0, the energy level Em   with the highest possible positive z-  quantum number, mz   = +3/2, constitutes the ground state for 131Xe. Vice versa, 3He, 21Ne, 83Kr and 129Xe have negative gyromagnetic ratios, and the respective ground state is the one with the most negative mz   quantum number. The sign of γ   determines the sign of the coherence generated by a 90° pulse ( H^rf-pulse,x=-γB0I^x) and thus can be important in magnetization transfer or coherence transfer NMR experiments.