Earth and Planetary System Science, Faculty of Science, Hokkaido University, N10W8 Kita-ku, Sapporo, Hokkaido 060-0810, Japan.
Rapid Commun Mass Spectrom. 2010 Apr 15;24(7):1085-92. doi: 10.1002/rcm.4483.
We have developed a rapid, sensitive, and automated analytical system to simultaneously determine the concentrations and stable isotopic compositions (delta(15)N, delta(18)O, and delta(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 per thousand and 0.3 per thousand and better than 1.4 per thousand and 2.6 per thousand were obtained for the delta(15)N and delta(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 per thousand and 2.1 per thousand were obtained for the delta(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).
我们开发了一种快速、灵敏且自动化的分析系统，可通过连续流动同位素比值质谱和氦气吹气系统相结合，同时测定水中痕量（纳摩尔级）氧化亚氮（N2O）和甲烷（CH4）的浓度及其稳定同位素组成（δ15N、δ18O 和 δ13C）。该系统由冷阱和毛细管气相色谱仪组成，使用超纯氦气作为载气，可完全提取水样中的 N2O 和 CH4，并实现 N2O、CH4 与其他气体之间的分离。从气相色谱仪出来后的流路可周期性地改变，使气体通过燃烧炉将 CH4 和其他烃类转化为 CO2，或者绕过燃烧炉，将洗脱的 N2O 直接引入质谱仪，通过监测 CO2（+）和 N2O（+）的 m/z 44、45 和 46 的离子，来测定稳定同位素组成。分析系统可通过个人计算机上的顺序软件自动操作。当分别注入超过 6.7 nmol 和 0.2 nmol 的 N2O 时，N2O 的 δ15N 和 δ18O 的分析精度优于 0.2‰和 0.3‰；当分别注入超过 5.5 nmol 和 0.02 nmol 的 CH4 时，CH4 的 δ13C 的分析精度优于 0.07‰和 2.1‰。这样，我们可以同时测定浓度低至 1.7 nmol/kg 的 N2O 和 0.2 nmol/kg 的 CH4 的 120 mL 水样的稳定同位素组成。
