Applying cavity ring-down spectroscopy for the measurement of dissolved nitrous oxide concentrations and bulk nitrogen isotopic composition in aquatic systems: Correcting for interferences and field application

Abstract

Laser spectroscopy is an emerging technology for measuring nitrous oxide (N 2 O) dynamics in the environment, but most studies have focused on atmospheric applications. We have coupled a commercially available cavity ring-down spectroscope (CRDS) (Picarro G5101-I isotopic N 2 O analyzer) to an air/water gas equilibration device to collect continuous in situ dissolved N 2 O molar concentration and bulk nitrogen isotopic (d 15 N-N 2 O) data. The d 15 N-N 2 O values measured by the CRDS unit were found to be significantly affected by changes in the mixing ratios of O 2 , CO, CH 4 , and CO 2 . There was also an effect of N 2 O mixing ratio on d 15 N-N 2 O. A series of equations was developed to correct for the matrix effect of O 2 and the spectral interference by CH 4 . Chemical traps effectively prevented interferences by CO and CO 2 . The maximum corrections required for N 2 O mixing ratio and O 2 matrix effects, were 1& (at a mixing ratio of 1.2 ppmv), and 11& (at 0% O 2 content), respectively. The CH 4 correction only became important at mixing ratios greater than 500 ppmv (>0.5&). Measurements of N 2 O molar concentration and d 15 N-N 2 O from the CRDS isotopic N 2 O analyzer were similar to those measured with isotope ratio mass spectrometry. We demonstrated the utility of the laser-based system with field deployments in three estuarine tidal creeks in subtropical Australia. Future work in this field should focus on the application of the laser-based system to the measurement of N 2 O isotopologues in aquatic habitats, allowing for further constraints to be placed on the pathways of N 2 O cycling in aquatic system.

Continuous measurements of nitrous oxide isotopomers during incubation experiments

“…This difference has a significant effect on the spectroscopy. We find a linear dependence similar to the one found by Erler et al (2015). The presented data .…”

An automated, laser‐based measurement system for nitrous oxide isotope and isotopomer ratios at nanomolar levels

“…The coupling of a purge‐and‐trap autosampler with a cavity ring‐down spectrometer provides automatic and high‐throughput N 2 O isotope/isotopomer measurements at nanomolar levels, which is a significant reduction in sample size requirement compared with existing laser‐based methods. The coupling of PT not only achieves automation, but also allows chemical conversion and physical separation to be performed to minimize interfering compounds such as CO 2 , CO and O 2 during isotopic and isotopomeric analyses, thus negating the need for complex correction schemes that previous studies have needed . To this end, the PT‐CRDS technique is certainly a viable alternative for the investigation of N 2 O cycling across large oxygen gradients in natural waters .…”

“…Such a cryo‐trapping temperature allows N 2 O to be concentrated (N 2 O boiling point is −89°C) whereas O 2 in the samples passes through without being trapped (O 2 boiling point is −183°C). To this end, we eliminate the interference of varying N 2 O:O 2 mixing ratios on the isotopic analysis of aquatic samples . To minimize atmospheric and inter‐sample contamination, the SSIM performs sequential vacuuming and flushing with N 2 O‐free carrier gas inside the gas transfer line and sample chamber (see Figure for valve timing and functions).…”

“…Until recently, the main disadvantages of CRDS instruments have been the relatively large sample size requirement (20–130 nmol‐N 2 O) and the lack of automatic sample preparation. To this end, researchers who attempted to utilize laser‐based N 2 O systems for aquatic nitrogen and oxygen isotopic analyses were limited to (1) extraction of dissolved N 2 O using a gas equilibration device under continuous‐flow mode or (2) conversion of samples with high nitrite or nitrate concentrations to N 2 O via chemical or biological methods . We have now successfully coupled an automated purge‐and‐trap module to a CRDS system (referred to as PT‐CRDS), and we demonstrate the ability to conduct automated and high‐throughput measurements of N 2 O concentrations and isotope/isotopomer signatures from discrete seawater samples at much lower N 2 O concentrations, while maintaining accuracy and precision that are similar to those obtained by GC (N 2 O concentration measurements) and GC/IRMS (isotope/isotopomer measurements).…”

Estuaries as Sources and Sinks of N2O Across a Land Use Gradient in Subtropical Australia

“…Instrument stability was confirmed prior to surveys using N 2 (0 ppm N 2 O) and N 2 O (28 ppm) gasses (no drift detected). There was an ~10‐min lag time for N 2 O measurements (Erler et al, ; Maher et al, ) and, due to variable equilibration times, an additional 20‐min lag for 222 Rn (Santos et al, ). Timestamps were used to correct for lags.…”