Negative ion formation in laser ion mobility increment spectrometer

A new method is proposed and implemented, according to which small concentrations of organic compounds in the atmosphere are measured by combining the ion mobility increment spectrometer and laser ionization. The experimental setup makes use of the pulsed fourth-harmonic radiation of a YAG:Nd³⁺ laser (λ = 266 nm). At high intensities (q ∼ 10⁷ W/cm²) of laser radiation, ion spectra with reactant peaks are observed, which are identical to the spectra obtained with traditional sources of ionization by corona discharge and β radiation. The ion spectra obtained at low laser radiation intensities (q ∼ 10⁵ W/cm²) are free of background and reactant peaks, which ensures the high selectivity of the analysis. It is concluded that the mechanism of negative ion formation from the molecules of nitro compounds can change depending on the intensity of ionizing laser radiation.