Stable hydrogen isotopic analysis of nanomolar molecular hydrogen by automatic multi-step gas chromatographic separation

摘要

We have developed a new automated analytical system that employs a continuous flow isotope ratio mass spectrometer to determine the stable hydrogen isotopic composition (δD) of nanomolar quantities of molecular hydrogen (H_2) in an air sample. This method improves previous methods to attain simpler and lower-cost analyses, especially by avoiding the use of expensive or special devices, such as a Toepler pump, a cryogenic refrigerator, and a special evacuation system to keep the temperature of a coolant under reduced pressure. Instead, the system allows H_2 purification from the air matrix via automatic multi-step gas chromatographic separation using the coolants of both liquid nitrogen (77 K) and liquid nitrogen + ethanol (158 K) under 1 atm pressure. The analytical precision of the δD determination using the developed method was better than 4° for >5 nmol injections (250 mL STP for 500 ppbv air sample) and better than 15° for 1 nmol injections, regardless of the δD value, within 1 h for one sample analysis. Using the developed system, the δD values of H_2 can be quantified for atmospheric samples as well as samples of representative sources and sinks including those containing small quantities of H_2, such as H _2 in soil pores or aqueous environments, for which there is currently little δD data available. As an example of such trace H_2 analyses, we report here the isotope fractionations during H_2 uptake by soils in a static chamber. The δD values of H_2 in these H_2-depleted environments can be useful in constraining the budgets of atmospheric H_2 by applying an isotope mass balance model.