Simultaneous determination of δ 15 N and δ 18 O of N 2 O and δ 13 C of CH 4 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 ( δ 15 N, δ 18 O, and δ 13 C) of nanomolar quantities of nitrous oxide (N 2 O) and methane (CH 4 ) 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 N 2 O and CH 4 in a water sample and separation among N 2 O, CH 4 , 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 CH 4 and the other hydrocarbons into CO 2 , or to bypass the combustion furnace for the direct introduction of eluted N 2 O into the mass spectrometer, for determining the stable isotopic compositions through monitoring the ions of m/z 44, 45, and 46 of CO   2 +and N 2 O + . 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 δ 15 N and δ 18 O of N 2 O, respectively, when more than 6.7 nmol and 0.2 nmol of N 2 O, respectively, were injected. Simultaneously, analytical precisions better than 0.07‰ and 2.1‰ were obtained for the δ 13 C of CH 4 when more than 5.5 nmol and 0.02 nmol of CH 4 , 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 N 2 O and 0.2 nmol/kg for CH 4 . Copyright © 2010 John Wiley & Sons, Ltd.