Evidence of a Systematic Deviation of the Isotopic Composition of Neon from Commercial Sources Compared with its Isotopic Composition in Air

Results of a study are reported, concerning the variation in isotopic composition of a limited number of neon samples of commercial origin and the resulting influence on the temperature of the triple point of this element. All seven neon samples investigated were found to contain more 22Ne than neon in air and that the 22Ne isotope amount fraction varied by as much as 0.2 % from sample to sample. This variation corresponds to a range of triple-point temperatures (Ttp) of more than 200 µK, much larger than the state-of-the-art uncertainty in the realization of this phase transition for metrological purposes. Deviations in the isotope amount fraction of 21Ne content were irrelevant, as far as their effect on Ttp is concerned, though they may be relevant in other fields of isotope studies. Neon isotope amount ratios at IRMM-Geel were obtained using the same measurement procedures and instruments as developed in the framework of the re-determination of the Avogadro constant and taking all significant sources of uncertainty into account. The repeatability of the ion current ratio measurements on individual samples was 5 10-5 relative. All uncertainty statements are made following the ISO/BIPM Guide on the Expression of Uncertainty in Measurements. Being these results unexpected, a more comprehensive study is following on a much wider range of samples of commercial origin.JRC.D.4-Isotope measurement