Measuring Dissolved Methane in Aquatic Ecosystems Using An Optical Spectroscopy Gas Analyzer
Summary
This study demonstrates an approach to measure methane gas concentrations in aqueous samples using portable optical analyzers coupled to an injection chamber in a closed loop. The results are similar to conventional gas chromatography, presenting a practical and low-cost alternative particularly suitable for remote field studies.
Abstract
Measuring greenhouse gas (GHG) fluxes and pools in ecosystems are becoming increasingly common in ecological studies due to their relevance to climate change. With it, the need for analytical platforms adaptable to measuring different pools and fluxes within research groups also grows. This study aims to develop a procedure to use portable optical spectroscopy-based gas analyzers, originally designed and marketed for gas flux measurements, to measure GHG concentrations in aqueous samples. The protocol involves the traditional headspace equilibration technique followed by the injection of a headspace gas subsample into a chamber connected through a closed loop to the inlet and outlet ports of the gas analyzer. The chamber is fabricated from a generic mason jar and simple laboratory supplies, and it is an ideal solution for samples that may require pre-injection dilution. Methane concentrations measured with the chamber are tightly correlated (r2 > 0.98) with concentrations determined separately through gas chromatography-flame ionization detection (GC-FID) on subsamples from the same vials. The procedure is particularly relevant for field studies in remote areas where chromatography equipment and supplies are not readily available, offering a practical, cheaper, and more efficient solution for measuring methane and other dissolved greenhouse gas concentrations in aquatic systems.
Introduction
Ecosystems at the terrestrial-aquatic interphase, like wetlands, lakes, reservoirs, rivers, and creeks, are important sinks and sources of greenhouse gases (GHG) like carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)1,2. CH4, specifically, is produced during anaerobic respiration in the saturated pore spaces of sediment pores. Once it is produced, a fraction is oxidized and transformed to CO2, while the rest will eventually diffuse through the water column and vegetation or burst out into bubbles3. The concentration of CH4 in the water saturating the sediment pores (i.e., porewater) at a given time offers a glimpse into the balance between CH4 produced, consumed, and transported4. When measured over vertical profiles or time, porewater concentration also allows for identifying zones more active in CH4 production and consumption and their seasonal variation.
Traditionally, the methods to determine the concentration of GHG from porewater in ecosystems involve processing water samples collected in the field to equilibrate gases in a created headspace. Then, the headspace is analyzed through gas chromatography to determine the concentrations5. While this method is widely applied in ecological studies, it requires bench-top gas chromatography-flame ionization detection (GC-FID) systems that entail allocating conventional lab space and a high degree of expert knowledge to calibrate and operate (for example6). It also requires the use of specialized consumables, such as large tanks of carrier gasses (i.e., Nitrogen (N2) and Helium (He)), which are not readily available in remote locations. These requirements and the associated logistics of sample transport to the lab may constrain sampling design and, in some cases, limit the study's scope when chromatography equipment is unavailable.
This study aimed to develop an alternative method to measure dissolved greenhouse gas concentrations from headspace samples of aqueous solutions using portable optical spectroscopy-based gas analyzers. This type of optical gas analyzer is a cost-effective alternative to standard GC-FID systems, and its portability makes it an ideal choice for fieldwork applications. Portable optical spectroscopy-based gas analyzers produce high-frequency gas concentration measurements (i.e., ~ 1 s-1) with 2 – 5 s response times, depending on the brands and models. These instruments are designed and marketed primarily for determining gas fluxes from GHG-emitting surfaces like soils, water, and vegetation7,8,9. Optical analyzers allow flux calculation from continuous concentration measurements in non-steady state headspace chambers deployed over the emitting surfaces of interest. In their regular intended use with surface chambers, the high-frequency measurements of the rate of change in concentrations observed in the chamber and known chamber dimensions, pressure, and temperature allow for interpretation of those data as for the rate of emission (or uptake) per surface area (i.e., surface fluxes)10. However, portable gas analyzers are neither equipped nor optimized for dissolved concentrations in aqueous media, necessitating additional adaptations and interpretations for that type of analysis.
Leveraging previous demonstrations of the use of optical analyzers to determine concentrations in discrete samples from headspaces8, we designed a small, closed chamber (i.e., no emitting surfaces) that connects to the analyzer in a closed loop. The change in concentrations after the injection of the headspace gas subsample, followed by dilution calculations, allows for determining the original headspace's concentrations. The precision of this approach was evaluated by comparing its results to those obtained through GC-FID in the same samples. The method is further demonstrated through a use case that analyzed the vertical profiles of CH4 in porewater samples collected from experimental sites in a freshwater marsh in Louisiana.
Protocol
Representative Results
Discussion
This study demonstrated the applicability of portable optical spectroscopy-based gas analyzers coupled to a custom-made injection chamber to analyze headspaces created from water samples. The demonstration focused on CH4, but the protocol could be applied to analyzing other relevant GHGs like CO2 and N2O8. The goal was to expand on previous systematic assessments of these closed-loop systems that represent an alternative analytical platform to conventional gas chro…
Offenlegungen
The authors have nothing to disclose.
Acknowledgements
This work was funded through DOE awards DE-SC0021067, DE-SC0023084, and DE-SC0022972. The porewater concentration data of the sampled sites at the marsh is publicly available at ESS-DIVE Data Archive (https://data.ess-dive.lbl.gov/view/doi:10.15485/1997524 , accessed on June 21, 2024)
Materials
| 1/4 in. I.D. x 3/8 in. O.D. Clear Vinyl Tubing | Home Depot | SKU # 702098 | Use to couple stopcocks and tubing connected to the instrument. Two short pieces (~4 cm). |
| 5/16 – 5/8 in. Stainless Steel Hose Clamp | Everbilt | 6260294 | Use to secure tubing connecting the stopcock valves and tubing connected to the instrument. |
| Crack-Resistant Teflon PFA Semi-Clear Tube for chemicals, 5/32" ID, 1/4" OD | McMaster-Carr | 51805K86 | Use to connect the injection chamber to the inlet and outlet ports of the instrument. We used two 0.68 m-long tubing in our experiment. |
| Drill with titanium step drillbit | Multiple companies | Use to drill the holes for septum and stopcocks in the jar's metallic lid. | |
| Gay butyl septum (stopper) | Weathon Microliter | 20-0025-B | Use as injection port and as vial septum (if compatible). |
| Headspace vials 20ml (23x75mm), Clear, Crimp Rounded Bottom | Restek | 21162 | Use to store the headspace sample. |
| Heavy Duty Steel Bond Epoxy GorillaWeld | Gorilla | 4330101 | Use to glue stopcock valves and septum to the jar's metallic lid. |
| Hypodermic Needles | Air-Tite Products Co. | N221 | Use to extract water from field vials, inject heaspace sample in vial and inject subsample to the injection chamber. |
| Mason jar (12 oz) | Ball, Kerr, Jarden | Larger or smaller chamber volumes can be chosen depending on sample concentrations. | |
| Optical spectroscopy-based gas analyzer | Multiple companies | Picarro G4301, Licor 7810, Licor 7820, ABB GLA131-GGA | These are some specific examples of analyzers that could be coupled to the injection chamber. We recognize that it is not an extensive list and other optical spectroscopy analyzers may also be suitable for the method. |
| Stopcock valve | DWK Life Sciences | 420163-0001 | Keep the valves open during normal operation. |
| Syringe (2.5 mL) | Air-Tite Products Co. | R2 | Use to extract subsamples from the headspace vials and inject them in the injecion chamber for analysis. |
| Syringe (30 mL) | Air-Tite Products Co. | R30HJ | Use to create headspace for gas analysis. |
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