A liquid nitrogen-free preconcentration unit for measurements of ambient N2O isotopomers by QCLAS
Abstract. Important information about the biogeochemical cycle of nitrous oxide (N2O) can be obtained by measuring its three main isotopic species, 14N15N16O, 15N14N16O, and 14N14N16O, and the respective site-specific relative isotope ratio differences δ15Nα and δ15Nβ. Absorption laser spectroscopy in the mid-infrared is a direct method for the analysis of the 15N isotopic composition of N2O, yet not sensitive enough for atmospheric N2O mixing ratios (320 ppb). To enable a fully-automated high precision analysis of N2O isotopic species at ambient mixing ratios, we built and optimized a liquid nitrogen-free preconcentration unit to be coupled to a quantum cascade laser (QCL) based spectrometer. During standard operation 10 l of ambient air are preconcentrated on a HayeSep D trap and desorbed in 50 ml of synthetic air. Rigorous tests were conducted, using FTIR, quantum cascade laser absorption spectroscopy (QCLAS), GC-FID and component-specific ozone and oxygen analysers to investigate recovery rates, conservation of isotopic signatures and spectral interferences after preconcentration. We achieve quantitative N2O recovery of >99% with only minor, statistically not significant isotopic fractionation and no relevant spectral interferences from other atmospheric constituents. The developed preconcentration unit also has the potential to be applied to other trace gases and their isotopic composition.