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

Membrane inlet mass spectrometry (MIMS) provides detailed measures of dissolved N-28,29,30(2), O-2, and argon (Ar) for estimating important gas fluxes and concentrations in aquatic ecosystems. Previous studies demonstrated a large O-2 scavenging effect while using a MIMS, where varying concentrations of O-2 can affect measured N-2 : Ar because O-2 interacts with N-2 in the ion source to produce NO+ (m/z = 30), potentially decreasing the detected current for N-28,29(2) and increasing the detected current for N-30(2). A common solution is to use a muffle furnace heated to 600 degrees C with a copper reduction column to reduce the concentration of O-2 to minimal levels and accurately measure N-28,29,30(2). However, this solution eliminates the detection of O-2 in environmental samples, which is a major benefit of using a MIMS. We questioned whether the MIMS was sensitive enough to provide accurate O-2 estimates when using the furnace and whether the O-2 scavenging effect was real and consistent among MIMS. We conducted four separate experiments on three different MIMS to test the O-2 scavenging effect and the potential detection of O-2 when using a MIMS with furnace. The furnace removed similar to 99% of O-2, and O-2 scavenging had little to no detectable effect on N-2 : Ar and an unclear effect on N-29(2) : N-28(2), but increased N-30(2) : N-28(2). In most cases, accurate O-2 data could be retrieved despite using the furnace. The need for O-2 reduction may be limited to measuring accurate N-30(2) : N-28(2) in isotope pairing studies, but without substantial loss of MIMS measurements used to describe O-2 dynamics.