Sensitive detection of atmospheric N₂O isotopomers using a quantum cascade laser based spectrometer

Stable isotope analytical techniques have a crucial role to play in the study of atmospheric pollutant and trace gas sources, sinks and transport processes. Quantum cascade laser absorption spectroscopy (QCLAS) allows the site-selective and high-precision analysis of many isotopic species at trace levels, yet not sensitive enough for atmospheric N₂O mixing ratios. In this study, a quantum cascade laser based spectrometer combining with a liquid nitrogen-free preconcentration unit was developed to demonstrate the applicability of high sensitivity and high precision measurement of atmospheric N₂O isotopomers. Primary laboratory tests have been performed for simultaneous determination of the mixing ratios of the most abundant nitrous oxide isotopic species: ¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O and ¹⁴N₂ ¹⁶O. The experiment results showed that site-selective ¹⁵N/¹⁴N can be measured with the precision of 0.59‰ with an optimal averaging time of 258 s at 100 ppm. Details of spectral absorption line pairs selection and N₂O adsorption/desorption processes are also discussed.