Establish the actual isotope enrichment of the WOx marker films

Determine the actual 18O enrichment of the WOx marker films rather than relying solely on the nominal enrichment of the growth gas, in order to establish their absolute isotope fractions and assess absolute accuracy of the interleaved-pulse reconstruction.

Background

The WOx films were fabricated using nominal marker fractions of 12.5%, 16.7%, 25%, and 50%, calculated from the isotope composition of the oxygen supplied during growth under the assumption that the 18O2 feed was isotopically pure. The measurements therefore test linearity by comparing interleaved and short-phase results, rather than testing absolute accuracy against independently known isotope fractions.

Because the actual enrichment in the deposited films may differ from the nominal gas composition, the measured isotope ratios cannot be assigned deviations from an independently established expected value. Determining the actual enrichment would enable an absolute accuracy assessment for the method at high minor-isotope fractions.