Our isotopic calibrations relative to primary reference materials relied on off-line methods whereby all listed organic RMs were quantitatively combusted overnight at 800 ºC in sealed ‘quartz’ ampoules to produce 100 % yields of carbon dioxide, nitrogen and water.
Water was quantitatively reduced to hydrogen via reaction with uranium metal. Each gas was cryogenically purified, collected separately, and measured isotopically in manual dual-inlet mode against gases derived from primary RMs. In contrast, on-line oxidative and reductive interfaces cannot guarantee 100 % yields of product gases.
Our laboratory’s comparison of on-line, TCEA-based δ2H values of aromatic coumarin and numerous n-alkanes indicated that coumarin does not perfectly fall on the correlation line comparing off-line versus on-line δ2H values. We surmise that aromatic hydrogen in coumarin has a slightly different response to TCEA pyrolysis conditions. Aromatic structures like coumarin may have a tendency to rapidly attach themselves to the vitreous carbon granules in the hot reactor before liberating all hydrogen atoms.
An incomplete hydrogen yield may entail some isotope fractionation that likely depends on temperature, flow rate, presence of heteroatoms N and O, and reactor conditioning. Our observation underscores the need to optimize the match of reference materials to analytes.
The following is a short listing of available reference materials for EA applications with specific comments.