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ABB methane analyzer Accurate syringe-sample CH₄ measurements in peatlands

Modified laser absorption spectrometer permits syringe-sample CH₄ measurements in peatlands

Measurement made easy

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Overview

Peatlands represent a large source of atmospheric methane, a greenhouse gas with the potential for a much greater impact on climate change than carbon dioxide.

Measurements of methane emissions from peatlands are necessary to promote an understanding of their environmental consequences.

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Problem

Typically chambers inserted throughout the peat accumulate methane over time. Researchers insert permanent collars into the peat to a depth of 5 to 20 cm and, when it comes time to measure methane flux, they fit a gas-tight seal to the collar.

The chamber accumulates methane for 20 to 40 minutes. The methane concentration within the chamber is measured at several intervals. The rate of increase in methane concentrations gives a direct measure of methane flux.

On-site cavity-enhanced spectrometer analyzers represent a convenient alternative to sending samples in syringes back to a laboratory for analysis. But most of these analyzers contain a pump that constantly draws gas in through an inlet and expels it through an outlet.

Until recently, they were not configured for syringe-sample analysis.

—Measuring methane emissions from peatlands

2 ACC U R ATE S Y R I N G E-S A M PL E CH₄ M E A SU R EM ENTS I N PE ATL A N DS W P/A N A LY TI C A L /0 2 0 - EN R E V. A

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Solution

Researchers in the UK (ECOHYDROLOGY, www.interscience.wiley.com DOI: 10.1002/eco.109) have attempted to overcome this limitation by making a simple modification to an ABB analyzer. This analyzer uses a cavity enhanced laser absorption technology called Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) to measure concentrations of the greenhouse gases methane, carbon dioxide and water vapor. In it, a wavelength-tuned laser beam is reflected back and forth thousands of times within a cavity containing the gas to be measured.

The reflections create an extremely long path length for increased measurement sensitivity. But being in flow-through mode via a vacuum pump, the analyzer cannot readily measure discrete gas samples by direct injection (at time of publication).

The modification consists of a 50 cm3 stainless steel, double-ended cylindrical chamber connected by tubing to the inlet and outlet ports of the analyzer.

To facilitate injection of gas samples, researchers drilled a 9.5 mm hole at the center of the sample cylinder. They then fitted a threaded 31.75 mm section of copper tubing sealed with a gas-tight 14 mm rubber turnover septum.

Next, the researchers determined the moles of gas in the newly established analyzer sample loop. To measure the methane concentration in samples of gas taken from field flux chambers, they first open the loop to allow it to purge before every syringe sample. This prevents the loop from becoming over-pressurized and creating leaks.

To check for accuracy and drift of the new measurement technique, researchers compared results with laboratory measurements via gas chromatography coupled with a flame ionization detector. They developed an initial calibration curve for the modified analyzer in syringe injection mode using four standard gases of different methane concentrations. If necessary, the samples were diluted by ambient air to bring the sample into the optimum operating range of the analyzer.

—01 Famous black grouse in the peatlands

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Results

Using the new measurement technique at a near site location, researchers reported methane flux rates for a peatland in West Wales. The area consisted of typical patterns of peatland microforms such as hummocks, hollows, lawns and pools. The study investigated variability in methane emissions for different microforms and estimated total methane emission rates from the area that included winter fluxes. They installed 20 permanent collars across replicate examples of each microform type over an area of 10,000 square meters.

To measure methane flux at each collar site, a 27 liter chamber was fitted to the collar every one to two weeks and left in place for 25 to 40 minutes.

At 5 minute intervals, 10 ml samples of chamber gas were extracted in disposable syringes and taken to a nearby location for measurement by the modified LGR greenhouse gas analyzer within 48 hours.

The flexibility of the new technique permits long-term, high frequency sampling conducted without time-consuming trips to a laboratory. It eliminates the high cost and spatial constraints of autochambers.

The short sample storage times affords the flexibility of using inexpensive, reusable disposable syringes.

Los Gatos Research now offers an option for manual injection of gas samples via syringe in most of its gas and isotopic analyzers, including the greenhouse gas analyzer.

—02 Syringe-injection in an LGR analyzer

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