Scale distortion from pressure baselines as a source of inaccuracy in triple-isotope measurements.

Rationale: Isotope ratio measurements have become extremely precise in recent years, with many approaching parts-per-million (ppm) levels of precision. However, seemingly innocuous errors in signal baselines, which exist only when gas enters the instrument, might lead to significant errors. These "pressure-baseline" (PBL) offsets may have a variety of origins, such as incoherent scattering of the analyte, isobaric interferences, or electron ablation from the walls of the flight tube. They are probably present in all but ultra-high-resolution instruments, but their importance for high-precision measurements has not been investigated.

Methods: We derive the governing equations for the PBL effect. We compare the oxygen triple-isotope composition of gases on three different mass spectrometers before and after applying a correction for PBLs to determine their effects. We also compare the composition of atmospheric O with that of several standard minerals (San-Carlos Olivine and UWG-2) on two high-precision mass spectrometers and compare those results with the differences reported in the literature.

Results: We find that PBLs lead to stretching or compression of isotopic variations. The scale distortion is non-mass-dependent, affecting the accuracy of triple-isotope covariations. The governing equations suggest that linear stretching corrections using traditional isotopic delta values (e.g., δ O) are rigorous for PBL-induced errors in pure gases. When the reference and sample gases are not comparable in composition or purity, however, a different correction scheme may be required. These non-mass-dependent errors are systematic and may have influenced previous measurements of triple-isotope covariations in natural materials.

Conclusions: Accurate measurements of isotopic variations are essential to biogeochemistry and for testing theoretical models of isotope effects. PBLs are probably ubiquitous, contributing to the interlaboratory disagreements in triple-isotope compositions of materials differing greatly in δ O values. Moreover, they may lead to inaccurate determination of triple-isotope compositions and fractionation factors, which has implications for isotopic studies in hydrology and biogeochemistry.