In this study two issues are addressed, namely laser ionisation of selected nitroaromatic compounds (NAC) and the characterisation of their anions by photodetachment (PD) spectroscopy. Laser ionisation of the NAC at λ = 226.75 nm is investigated by ion mobility (IM) spectrometry at atmospheric pressure. The main product after laser ionisation is the reactive NO+ ion formed in a sequence of photofragmentation and multiphoton ionisation processes. NO+ is trapped by specific ion molecule reactions (IMR). Alternatively, NO, added as laser dopant, can directly be ionised. The formed NO+ reacts with the NAC under complex formation. This allows fragmentless NAC detection. The combination of IM spectrometry and PD spectroscopy provides real-time characterisation of the anions in the IM spectrum. This is useful to differentiate between NAC and interfering substances and, thus, to reduce false-positive detections of NAC. The electrons detached by the PD laser at λ = 532 nm are detected in the same spectrum as the anions. The potential of PD-IM spectrometry in terms of cross section determination, analytical improvements, tomographic mapping, spatial hole burning etc., is outlined.
The detection of polar molecules, like ketones and ethers, in a laser ion mobility spectrometer was investigated. Because
the direct multiphoton ionization (MPI) for these compounds shows too high limits of detection (LOD) and intensive
fragmentation of the molecular ions, alternative ionization methods based on ion-molecule-reactions (IMR) were
investigated. These ionization methods should retain the advantages of the laser ionization. As examples for IMR two
reaction classes, proton transfer reactions (PTR) and complex formation reactions (CFR), were studied. The PTR are
based in a first step on the proton transfer from toluene radical cations to polar molecules. In a second step protonated
dimers are formed. The CFR are characterized by the complex formation between aniline or phenol radical cations and
polar molecules. All products are formed at atmospheric pressure and are characterized by transfer into a time-of-flight
mass spectrometer. In both IMR the ionic reactants are formed selectively by 1+1 REMPI. The rates of the following
IMR are near to the collision limit. Therefore the reactions are very efficient. The LOD for the analysis of selected
ketones and ethers by IMR are in the low ppb-range, much lower than the corresponding LOD for direct MPI. The
required laser intensities for the IMR are up to a factor of 1000 lower than the laser intensities for direct MPI. The
fragmentation of the product ions is much lower for both IMR in comparison to direct MPI. The IMR allow the
quantitative analysis of substance mixtures.
The drift time spectra of polycyclic aromatic hydrocarbons (PAH), alkylbenzenes and alkylphenylethers were recorded with a laser-based ion mobility (IM) spectrometer. The ion mobilities of all compounds were determined in helium as drift gas. This allows the calculation of the diffusion cross sections (Ωcalc) on the basis of the exact hard sphere scattering model (EHSSM) and their comparison with the experimentally determined diffusion cross sections (Ωexp). These Ωexp/Ωcalc-correlations are presented for molecules with a rigid structure like PAH and prove the reliability of the theoretical model and experimental method. The increase of the selectivity of IM spectrometry is demonstrated using resonance enhanced multiphoton ionisation (REMPI) at atmospheric pressure, realized by tuneable lasers. The REMPI spectra of nine alkylbenzenes and alkylphenylethers are investigated. On the basis of these spectra, the complete qualitative distinction of eight compounds in a mixture is shown. These experiments are extended to alkylbenzene isomer mixtures.