Comparison with U ≈ 50 μK of neon samples of different isotopic compositions

New comparisons are reported of the triple-point temperatures, T-tp, of seven neon samples with different Ne-22 amount concentrations, (22)x, in cryogenic cells sealed by INRIM, NMIJ, NPL and PTB. This work stems from a collaboration between those laboratories aiming at establishing the relationship between T-tp and (22)x. Several problems remained unsolved in the published results. The new measurements supply more thermal data of lower uncertainty, to help solve some of these problems. The detailed results from 35 meltings are reported, characterized by improved uncertainty, and include an analysis of calorimetric, thermometric and sample-related effects. A broad range of collected and processed information is analysed in detail in order to confirm expanded uncertainty levels of the order of 30 mu K for a single cell and 50 mu K for a comparison of pairs of cells. The quality of the INRIM results is then discussed. In most cases the uncertainty budget for the INRIM measurements, including analysis of chemical impurities and the hydrostatic head correction, reached the targeted improved uncertainty. Finally, the results are discussed and compared with previous studies in support of the future assignment by the CCT of an accurate and reliable relationship T-tp versus (22)x. The paper discusses various improvements regarding the influence of the thermal measurements on both the uncertainty of the relationship and the ability to recognize either possible isotopic fractionation during sample preparation or incorrect assignment of (22)x values to some measured samples.