Ultra-sensitive detection of nitric oxide isotopologues

This paper reviews recent technical advances of resonance enhanced multiphoton ionization (REMPI) spectroscopy and laser-induced fluorescence (LIF) spectroscopy, both extremely sensitive techniques for the determination of low nitric oxide (NO) concentrations. Because NO is involved in a multitude of biological functions, labeling and isotope-specific detection are important tools for elucidating chemical pathways. Mass selective REMPI discovers spectroscopic windows for monitoring the most abundant isotopologues such as 14N16O, 15N16O and 14N18O without these species interfering spectroscopically with each other. This has been realized by using the A2Σ+(v'=0)<-X2ΠΩ(v''=0) transition (NO γ-band) in the ultraviolet region around 226 nm. Selected rovibrational transitions are employed for the ultra-sensitive and isotope-specific detection of exhaled NO by LIF. The detection limit for 14N18O has been improved to 0.8 parts per trillion. Furthermore, preliminary results from a study of the A2Σ+(v' =1)<-X2ΠΩ(v''=0) transition interfering with the B2Π(v'=1)<-X2ΠΩ(v''=0) transition are presented as an alternative approach for isotopologue selective measurements of 14N18O. From experimental data the rotational constant B=1.10310 cm-1 and the spin-orbit coupling constant A=33.170 cm-1 of 14N16O (B2Π(v'=1)) have been calculated.