AMT12 dissolved methane and nitrous oxide concentration and saturation measurements from CTD bottle samples
Originator's Protocol for Data Acquisition and Analysis
Water samples were collected using a Sea-Bird 911plus CTD system fitted with 24 x 20 litre "Ocean Test Equipment" (Niskin type) external spring water bottles. Casts were completed predawn to minimise diurnal effects. Subsamples from the CTD bottles were collected in 1 litre volumetric flasks (Fisher), allowing overflow of the sample, to preclude contamination by air bubbles. Each sample was poisoned using 200ml of 0.25M aqueous HgCl 2 and sealed with a ground glass stopper. The sample flasks were stored at room temperature for 0-8 hours, which treatment should result in no significant change in N 2 O (Elkins, 1980).
N 2 O and CH 4 partial pressures and concentrations were measured by single phase equilibration gas chromatography, using the analytical apparatus described by Upstill-Goddard et al. (1996). The system was flushed with equilibrator gas (compressed air) and sealed. The sample to be analysed was warmed, in its sealed flask, to a constant 25°C. The glass stopper was then replaced with the equilibrator manifold, and the joint sealed with PTFE tape and a joint clip.
A headspace was created in the sample flask by introduction of compressed air with ambient N 2 O and CH 4 mixing ratios of 318 ppbv and 1.79 ppmv respectively. The headspace was circulated through a closed circuit to return to the sample flask through a solvent filter (Jones Chromatography) with a period of 12 minutes. Two sample loops (one each for N 2 O and CH 4 ) were simultaneously flushed with ultra high purity nitrogen carrier gas. Each sample run was followed by a standard (made from secondary standards), and then by ambient air analysis. Shimadzu Class VP software was used for the analysis of N 2 O and CH 4 hromatograms. Run time of the system was typically 35 minutes. The water phase of the sample was measured gravimetrically.
N 2 O analysis
CO 2 and water vapour were removed from the gas stream using stainless steel tubing (internal diameter 3.8 cm) packed with 1.22 cm 3 Carbosorb and 1.2 cm 3 magnesium perchlorate (Mg(ClO 4 )), respectively. The N 2 O in the headspace was detected with an Electron Capture Detector (ECD), utilizing a 15 m Ci 63 Ni source, and operated at 320°C and 1.0 nA. The flow rate of 24 ml min -1 resulted in a retention time of 7.43 minutes.
CH 4 analysis
Water vapour was removed from the gas stream using stainless steel tubing (internal diameter 3.8 cm) packed with 1.2 cm 3 magnesium perchlorate (Mg(ClO 4 )). The CH 4 in the headspace was determined by Flame Ionisation Detection using CP grade hydrogen (BOC) and compressed air (BOC) to support combustion at 120°C. The flow rate of 20 ml min -1 resulted in a retention time of 0.97 minutes.
References Cited
Elkins J.W., 1980. Determination of dissolved nitrous oxide in aquatic systems by gas chromatography using electron-capture detection and multiple phase equilibration. Analytical Chemistry, 52, 262-267.
Upstill-Goddard R.C., Rees A.P. and Owens N.J.P., 1996. Simultaneous high-precision measurements of methane and nitrous oxide in water and seawater by single-phase equilibration gas chromatography. Deep-Sea Research Part I, 43(10), 1669-1682.
BODC Data Processing Procedures
Data were submitted via email in an Excel spreadsheet archived under BODC's accession number UEA050040. Sample metadata (Latitude, longitude, ctd cast, station number and depth) were checked against information held in the database. There were no discrepancies.
The concentration data were provided in nanomoles per litre and percent. These units for the concentration data were consistent with the BODC parameter code units and no conversions were applied.
The data were reformatted and loaded in BODC's samples database under Oracle Relational Database Management System. Data were marked up with BODC parameter codes and loaded into the database. Individual samples were matched through CTD cast and depth.
A parameter mapping table is provided below:
| Originator's Parameter | Units | Description | BODC Parameter Code | Units | Comments |
|---|---|---|---|---|---|
| Methane concentration | nmol l -1 | Concentration of methane {CH4} per unit volume of the water body [dissolved plus reactive particulate phase] by gas chromatography | CH4CGCXX | nmol l -1 | n/a |
| Methane saturation | Percent | Saturation of methane {CH4} in the water body [dissolved plus reactive particulate phase] by gas chromatography and computation from concentration | CH4SGCX1 | Percent | n/a |
| Nitrous oxide concentration | nmol l -1 | Concentration of nitrous oxide {N2O} per unit volume of the water body [dissolved plus reactive particulate phase] by gas chromatography | DN2OGCTX | nmol l -1 | n/a |
| Nitrous oxide saturation | Percent | Saturation of nitrous oxide {N2O} in the water body [dissolved plus reactive particulate phase] by gas chromatography and computation from concentration | SN2OGCX1 | Percent | n/a |
Data Quality Report
BODC was not advised by the data originator of any quality issues with the data.
