An automated, high through-put method for accurate and precise measurements of dissolved nitrous-oxide and methane concentrations in natural waters

We describe a technique for measuring dissolved CH4 and N2O concentrations from discrete water samples using an automated purge and trap gas extraction system, coupled with a gas chromatograph-mass spectrometer (PT-GCMS). The automated system measures blanks, standards, and 25 samples in less than five hours with only similar to 30 min of operator involvement. Rigorous testing of the PT-GCMS demonstrates sensitivity, accuracy and precision that is comparable or better than conventional methods for CH4 and N2O analysis. Measured concentrations of CH4 and N2O in air-equilibrated water samples showed good agreement with expected values derived from solubility calculations, and results of a multilaboratory intercalibration exercise showed that our measurements agree with those made using conventional methods. Precision of replicate water samples is 3.3% for CH4 and 3.0% for N2O. Detection limits are well below the expected concentrations in most natural waters with a five milliliters sample, and can be lowered substantially by analyzing a larger sample volume. To demonstrate the utility of the method, we present depth profiles of CH4 and N2O from Saanich Inlet, a coastal anoxic fjord in British Columbia. The Saanich Inlet water column exhibits rapid changes in CH4 and N2O across depth-dependent and seasonally variable redox conditions. Our high through-put, automated method facilitates the measurement of aqueous N2O and CH4 concentrations, and will thus help to improve our understanding of the natural cycling of these climate-active trace gases.