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Kinetic investigations of proton transfer and complex formation reactions by laser ion mobility spectrometry

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.