Simultaneous determination of δ15N and δ18O of N2O and δ13C of CH4 in nanomolar quantities from a single water sample

We have developed a rapid, sensitive, and automated analytical system to simultaneously determine the concentrations and stable isotopic compositions (δ¹⁵N, δ¹⁸O, and δ¹³C) of nanomolar quantities of nitrous oxide (N₂O) and methane (CH₄) in water, by combining continuous‐flow isotope‐ratio mass spectrometry and a helium‐sparging system to extract and purify the dissolved gases. Our system, which is composed of cold traps and a capillary gas chromatograph that use ultra‐pure helium as the carrier gas, achieves complete extraction of N₂O and CH₄ in a water sample and separation among N₂O, CH₄, and the other component gases. The flow path following exit from the gas chromatograph was periodically changed to pass the gases through the combustion furnace to convert CH₄ and the other hydrocarbons into CO₂, or to bypass the combustion furnace for the direct introduction of eluted N₂O into the mass spectrometer, for determining the stable isotopic compositions through monitoring the ions of m/z 44, 45, and 46 of CO and N₂O⁺. The analytical system can be operated automatically with sequential software programmed on a personal computer. Analytical precisions better than 0.2‰ and 0.3‰ and better than 1.4‰ and 2.6‰ were obtained for the δ¹⁵N and δ¹⁸O of N₂O, respectively, when more than 6.7 nmol and 0.2 nmol of N₂O, respectively, were injected. Simultaneously, analytical precisions better than 0.07‰ and 2.1‰ were obtained for the δ¹³C of CH₄ when more than 5.5 nmol and 0.02 nmol of CH₄, respectively, were injected. In this manner, we can simultaneously determine stable isotopic compositions of a 120 mL water sample with concentrations as low as 1.7 nmol/kg for N₂O and 0.2 nmol/kg for CH₄. Copyright © 2010 John Wiley & Sons, Ltd.