NMR of 129Xe in the Gas Phase

Because of the collisions between the Xe atoms in the gas phase, the chemical shift of 129Xe in the gas phase is temperature and pressure dependent. Quantitatively, a relationship between the 129Xe chemical shift in ppm and the density of the gas has been found to exist [3]  

Materials at temperatures above the glass transition temperature (Tg) absorb Xe readily, therefore the fact that after sealing of the sample tube the density of the Xe gas could decrease with time when more Xe is absorbed by the material is neglected. For samples which absorb Xe slowly, like some polymers below their T, this effect may not be negligible. Also for temperature dependent measurements the variation of the coefficients in Equation 12.1 with temperature should be taken into account [4, 5, 6], but in view of the widths of the resonance lines investigated here as a function of temperature, the temperature dependence of the reference has also been neglected. 

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