Laser-based ion mobility spectrometer for the direct analysis of aromatic compounds in liquids

A laser-based ionization source for the direct analysis of liquid samples in ion mobility (IM) spectrometry is presented and characterized. Ionization of aromatic substances in liquids is achieved, analogous to atmospheric pressure laser ionization (APLI) in mass spectrometry, by vaporizing the liquid and subsequently ionizing the aromatic substances by resonance-enhanced multiphoton ionization (REMPI). The effects of parameters, such as composition and flow rate of the solvent as well as laser wavelength and pulse energy, are systematically investigated. The characterization of the IM spectrometer is carried out by means of selected substances from diverse fields of applications, e.g., polycyclic aromatic hydrocarbons (PAH), pesticides, wood preservatives and drug compounds. Limits of detection (LOD) down to 10 fmol and linear ranges up to three orders of magnitude are established. In addition to direct laser ionization, indirect laser ionization via dopants (toluene) for substances with low ionization efficiencies is investigated. Ionization occurs as a result of proton transfer from toluene radical cations to substances of sufficiently high proton affinities. As a result of indirect laser ionization, LOD could be decreased by up to two orders of magnitude. Ionization products are investigated by means of a combination of IM and mass spectrometer. Depending on the substance investigated primary ions (radical cations) and secondary ions (protonated molecules) resulting from ion molecule reactions are formed.