Site selective real-time measurements of atmospheric N 2O isotopomers by laser spectroscopy
We describe the first high precision real-time analysis of the N 2O site-specific isotopic composition at ambient mixing ratios. Our technique is based on mid-infrared quantum cascade laser absorption spectroscopy (QCLAS) combined with an automated preconcentration unit. The QCLAS allows for simultaneous and specific analysis of the three main stable N 2O isotopic species, 14N 15N 16O, 15N 14N 16O, 14N 14N 16O, and the respective site-specific relative isotope ratio differences δ 15N α and δ 15N β. Continuous, stand-alone operation is achieved by using liquid nitrogen free N 2O preconcentration, a quasi-room-temperature quantum cascade laser (QCL), quantitative sample transfer to the QCLAS and an optimized calibration algorithm. The N 2O site-specific isotopic composition (δ 15N α and δ 15N β) can be analysed with a long-term precision of 0.2‰. The potential of this analytical tool is illustrated by continuous N 2O isotopomer measurements above a grassland plot over a three week period, which allowed identification of microbial source and sink processes.