Baselines Matter: Mass Spectrometric Assessments of Biological O₂ Supersaturation (ΔO₂:Ar) Benefit from Two-Point...

Membrane-inlet mass spectrometry (MIMS) based assessments of ΔO₂:Ar to estimate marine productivity are becoming a widely used tool in biogeochemistry. Especially continuous ship-borne surveys of dissolved gases allow for high spatial and temporal resolution in the analysis of surface ocean net community productivity. Depending on instrument configuration and architecture, however, measurements may be afflicted with substantial detection baselines for each analyzed gas. We hypothesized that ignoring these baselines (as seems to be common practice) can considerably affect the outcomes of the measurements. Using MIMS data from two cruises, we assessed the impact of calibration procedures and different pressure regimes, i.e., gas loads, on the ΔO₂:Ar ratios and analyses of net community productivity. We compared conventional ratio-based one-point calibration approaches with two-point calibration approaches that include baselines and either calibrate for the equilibrium ratio of O₂:Ar, or calibrate for the individual gas concentrations. Our data show that higher gas loads increase apparent signal heights, but also disproportionately increase the individual gas-associated baseline levels, thereby reducing instrument sensitivity while increasing a potential bias if data is used without correction. Depending on the adjusted gas loads, severe underestimations of final ΔO₂:Ar values by factors of ca. 1.4 and 4 were calculated when disregarding baselines, emphasizing the critical importance of baseline determinations in MIMS-based methods.