Presentation
Since 2017, the OCEANIS team has been using an Agilent microGC 490, distributed by SRA, to measure dissolved methane in the pore waters of marine sediments.
There are several versions of the Agilent MicroGC 490, but the one used by the OCEANIS team is the two-channel version. Our microGC, a pre-integrated package, whitch has an Electronic Gas Control (EGC) injector, a column and a micro TCD detector. A computer with Soprane control software completes the system.
The microGC 490 is a miniaturised device (15 cm x 30 cm x 27 cm) that can be easily carried on board. It has a single carrier gas, helium (5.5 bar). The instrument is calibrated linearly (between 0.01% and 90%) using a single point of 1% standard methane gas in a 20 ml flask, flushed for 5 minutes.

The headspace method is used to measure interstitial fluids. Samples are taken by depositing 6 ml of sediment in a 60 ml glass vial (pre-weighed) containing 35 ml of a 1 mol/l KOH solution, leaving a headspace of around 20 ml and preserving the sample over time. The sample bottle was immediately stoppered with a septum, sealed with an aluminium cap and then shaken vigorously. The samples are then stored upside down, in a cool, dark place. They will keep for several weeks.

Once in the laboratory, the measurement is carried out by pumping 3 successive pumps of 2 ml of Head-Space at atmospheric pressure. Analysis of a sample takes around 5 minutes.
The OCEANIS team mainly uses this microGC to measure methane in the sedimentary column of interface cores taken from the Rhône delta. We have also measured interstitial methane in mudflats in the Loire river. In addition, in order to properly qualify the measurements, an intercomparison was carried out with Laura Lapham from the University of Maryland, a specialist in dissolved methane, and the results were very consistent. Finally, given the range of concentrations that can be measured, methane levels in the water column are generally not detectable. However, we were able to measure methane levels in the water of Lac Pavin, at the transition between the oxic and anoxic layers where methane accumulates.