Trace Analysis of NO 2 in the Presence of NO by Laser Photofragmentation/Fragment Photoionization Spectrometry at Visible Wavelengths

Abstract

Trace concentrations of NO and NO₂ molecules are differentiated spectrally by using a visible dye laser and a simple flow cell with a pair of miniature electrodes for ion detection. NO is detected near 452 nm by (2+2) resonance-enhanced multiphoton ionization via its A²∑⁺-X²II (0,0) transitions, while NO₂ is detected by laser photofragmentation with subsequent fragment NO ionization via the A²∑⁺-X²II (0,0) and (1,1) transitions. Spectral differentiation is possible since the internal energy of the NO photofragment differs from that of “ambient” NO. Measurement of vibrationally excited NO via its A²∑⁺-X²II (0,3) band is also demonstrated at 517 nm. Rotationally resolved spectra of NO and fragment NO are analyzed by using a multiparameter computer program based on two-photon energy level expressions and line strengths for A²∑⁺-X²II transitions. Boltzmann analysis of the P₁₂ + O₂₂ branch of the (0,0) band reveals that the rotational temperature of fragment NO is approximately 500 K compared to room-temperature NO. Limits of detection [signal-to-noise (S/N) = 3] of NO and NO₂ are in the 20–40-ppbv range at 449.2, 450.7, and 452.6 nm for a 10-s integration time. The limit of detection of NO₂ at 517.5 nm is 75 ppbv. The analytical utility of the technique for ambient air analysis is evaluated and discussed.