Attomole detection of nitroaromatic vapours using resonance enhanced multiphoton ionization mass spectrometry

Abstract

A very sensitive and selective procedure has been developed for the detection of nitrobenzene (C₆H₅NO₂) and o-nitrotoluene (C₆H₄CH₃NO₂) vapours using resonance enhanced multiphoton ionization mass spectrometry. The time-of-flight mass spectra of these two nitroaromatic molecules are characterized by a prominent NO⁺ ion signal (m/z 30) together with a characteristic pattern of hydrocarbon fragment ions. The intense NO⁺ ion signal arises via efficient two-photon resonant ionization of neutral nitrogen monoxide (NO) molecules produced by dissociation of the nitroaromatic species. In the wavelength range studied to date, 224–260 nm, NO⁺ ion generation is observed to be strongly dependent on laser wavelength, with an intensity maximum occurring at 226.3 nm. At this particular wavelength, NO⁺ ion signals have been detected with less than 1 amol (<10^–18 mol) of nitrobenzene vapour in the laser beam. The two aromatics can be distinguished by observing differences in the laser induced mass spectra and in the wavelength dependence of fragment ion production. Furthermore, it is possible to distinguish NO⁺ ion formation from NO and NO₂ gases and NO⁺ ion formation from nitroaromatic molecules in vacuum by studying the wavelength dependence of the NO⁺ ion signal in the range 245–250 nm and it is hoped that this procedure can be used to make similar distinctions with atmospheric samples.