Direct gas injection method: a simple modification to an elemental analyzer/isotope ratio mass spectrometer for stable isotope analysis of N and C from N2O and CO2 gases in nanomolar concentrations.

Rationale: Stable isotope analyses of trace amounts of nitrous oxide gas require special instrumentation and laborious sample preparation methods that have hindered many laboratories from measuring this potent greenhouse gas. A simple modification to an Elemental Analyzer (EA) coupled to an Isotope Ratio Mass Spectrometry (IRMS) setup that allows users to measure the N and C isotopic ratios of nitrous oxide (N(2)O) and carbon dioxide (CO(2)) by injecting the gases directly into the EA is described.

Methods: The standard EA was fitted with a gas injection port and a home-made packed column filled with Hayesep Q polymer. A gas mixture of 3.1% N(2)O in helium (He) was injected directly into the EA. This method allowed large volumes of sample to be injected without saturating the column.

Results: The use of the home-made column resulted in better resolution of sample peaks and allowed smaller concentrations of the analyte to be injected. This study showed that this method produced accurate and reproducible stable isotope measurements with sample injection volumes ranging from 100 to 5000 μL, containing between 20 and 1000 nmol of analyte.

Conclusions: This simple, inexpensive method can be useful for the laboratories that do not have access to more advanced and expensive interfaces to analyze nanomolar quantities of N(2)O and CO(2) from microbiological and ecological studies and offers a simple alternative for in-house measurements of these trace gases.