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

Abstract

AbstractWe 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 ∼ 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.