Kinetic investigations of proton transfer and complex formation reactions by laser ion mobility spectrometry
2006
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.
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