Simultaneous high-precision measurements of methane and nitrous oxide in water and seawater by single phase equilibration gas chromatography

A novel semi-automated gas chromatographic technique for the simultaneous high-precision measurement of the partial pressures and concentrations of nitrous oxide (N₂O) and methane (CH₄) in discrete samples of water and sir has been developed. For seawater analysis, samples collected in glass 11 volumetric flasks with the exclusion of all air, are thermally equilibrated to 25.0 ± 0.05°C in a water bath and then rapidly equilibrated with a headspace of known N₂O and CH₄ compositions in a closed system at ambient pressure. The equilibrated headspace gases are separated on Porapak Q columns, followed by electron capture detection for N₂O and flame ionisation detection for CH₄. Complex sample purging or cold trapping procedures are avoided, eliminating the sources of major error associated with previous methods. System hardware requirements are comparatively modest, simplifying operation and maintenance, and minimal sample handling restricts the potential for sample outgassing and air contamination during analysis. Seawater samples can be routinely processed at a rate of 5-6 h^-1, and air or calibration standards at a rate of ~20 h^-1. For high-quality oceanographic work, individual seawater samples are each compared with two calibration standards and a sample of ambient air pumped from the ship's bow. In this mode, a 12-bottle hydrocast can be processed in 3-4 h. Routine analytical precisions for atmospheric air and for seawaters with saturations in the range 0-300% N₂O and 0-160% CH₄ relative to atmospheric air, are better than ±0.5% (100σ/x̄) for N₂0 and ±0.9% (100σ/x̄) for CH₄. Detection limits are equivalent to ~40 pmol l^-1 N₂O and ~50-150 pmol l^-1 CH₄ for salinity 35 seawater at 20°C, although in practice the detectors always operate far above these ranges. Some N₂O and CH₄ data for seawater and air from the Bellingshausen Sea off Antarctica (62°-68°S,65°-86°W) and the northwestern Indian Ocean (03°-26°N, 56°-59°E) are presented.