Trace Analysis of NO2 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.