Evaluating O2:Ar, N2:Ar, and 29,30N2 using Membrane Inlet Mass Spectrometry configured to minimize oxygen interference

Authors: BOEDECKER, ASHLYNN - Baylor University; Taylor, Jason; TAPPENBECK, TYLER - University Of Montana; HALL, ROBERT - University Of Montana; ROBBINS, CALEB - Baylor University; SCOTT, JEFFERSON - Baylor University

Submitted to: Limnology and Oceanography Methods
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/2014
Publication Date: N/A
Citation: N/A

Interpretive Summary: Measuring dissolved nitrogen, oxygen, and argon gas in aquatic ecosystems with Membrane Inlet Mass Spectrometry (MIMS) has become a major tool for scientists studying removal of excess nitrogen from lakes, rivers and oceans. Previous work has described technical issues with interference of oxygen in making accurate measures of nitrogen gas. Scientists from three different institutions tested how a common method for removing oxygen influences scientists ability to accurately quantify dissolved nitrogen gas. We found no effect on nitrogen gas measurements suggesting that oxygen interference is not a problem using the most recent versions of MIMS. Additionally, we report for the first time that despite removing 99% of O2, MIMS configured for O2 removal is sensitive enough to still estimate O2 concentrations based on residual gases in samples. These results greatly simplify measurements of dissolved N2 gas, potentially increasing the efficiency at which scientists can use this technology in research studying denitrification, a major process for removal of excess nitrogen from impacted freshwater and coastal ecosystems

Technical Abstract: estimating important nutrient fluxes and concentrations in aquatic ecosystems. Previous studies demonstrate a large O2 scavenging effect when using MIMS, where varying concentrations of O2 can affect measured N2:Ar because O2 interacts with N2 in the ion source to produce NO+ (mass 30), potentially decreasing the detected current for all isotopic forms of N2 and possibly increasing the detected current for 30N2. A common solution is to use a muffle furnace heated to 600°C with a copper reduction column (MIMS with furnace) 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 MIMS. We asked whether the MIMS may be sensitive enough to provide accurate O2 estimates even when using the furnace and whether the O2 scavenging effect is 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 oxygen. 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 because even of 1% of the O2 remained, there was enough signal to measure it environmental variation. 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 in aquatic ecosystems.