Evaluating O2 : Ar, N2 : Ar, and 29,30N2 using membrane inlet mass spectrometry configured to minimize oxygen interference

Abstract

Membrane inlet mass spectrometry (MIMS) provides detailed measures of dissolved ^28,29,30N₂, O₂, and argon (Ar) for estimating important gas fluxes and concentrations in aquatic ecosystems. Previous studies demonstrated a large O₂ scavenging effect while using a MIMS, where varying concentrations of O₂ can affect measured N₂ : Ar because O₂ interacts with N₂ in the ion source to produce NO⁺ (m/z = 30), potentially decreasing the detected current for ^28,29N₂ and increasing the detected current for ³⁰N₂. A common solution is to use a muffle furnace heated to 600°C with a copper reduction column to reduce the concentration of O₂ to minimal levels and accurately measure ^28,29,30N₂. However, this solution eliminates the detection of O₂ in environmental samples, which is a major benefit of using a MIMS. We questioned whether the MIMS was sensitive enough to provide accurate O₂ estimates when using the furnace and whether the O₂ scavenging effect was real and consistent among MIMS. We conducted four separate experiments on three different MIMS to test the O₂ scavenging effect and the potential detection of O₂ when using a MIMS with furnace. The furnace removed ~ 99% of O₂, and O₂ scavenging had little to no detectable effect on N₂ : Ar and an unclear effect on ²⁹N₂ : ²⁸N₂, but increased ³⁰N₂ : ²⁸N₂. In most cases, accurate O₂ data could be retrieved despite using the furnace. The need for O₂ reduction may be limited to measuring accurate ³⁰N₂ : ²⁸N₂ in isotope pairing studies, but without substantial loss of MIMS measurements used to describe O₂ dynamics.