A method to induce and assess isotopic equilibrium of oxygen

As part of the preparation of a primary isotopic gas standard (PIGS) for oxygen, a study was carried out to assess the feasibility of developing a method for the preparation of a gravimetrically prepared isotope mixture of oxygen. Enriched 18O2 and O2 of natural isotopic composition were mixed gravimetrically. Several procedures were investigated to attain isotopic equilibrium of this mixture, i.e., the isotopic species distribution obeying the statistical probability. Pt and Ag powder were used as catalysts. Mass spectrometric measurements were carried out on the mixture before and after isotopic equilibration. The observed values for the isotope ratios J17/160 and J18/160 were compared to computed values, using standard equations, for non-equilibrated mixing and for isotopic equilibrium. As a result, isotopic equilibration of the oxygen mixture was achieved. A method is proposed to assess isotopic equilibrium via a ratio consistency factor χ(iO), which is a measure of the closeness of the observed ion current ratios for the prepared mixture to the computed ion current ratios for ideal isotopic equilibrium. In this study, the value of χ(18O)=0.0006 (21) for the prepared mixture, with combined uncertainty uc given in parentheses applying to the last two digits (χ=0 for complete equilibration). Preliminary conversion factors for oxygen ratio measurements Kconv=0.997 (54) for n(18O)/n(16O) and Kconv=0.990 (28) for n(17O)/n(16O) were derived from the comparison of the observed ion current ratios with the gravimetrically prepared isotope amount ratios. As a result of this study, the main areas of improvement in view of the preparation of a final gravimetric oxygen isotope mixture were exposed and discussed thoroughly.