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(EN) The first part of the thesis describes the ionization reactions in the APCI source in the presence (enclosed configuration) and absence (open configuration) of the ion source housing. Commercial APCI source was modified and used in enclosed, as well as open configuration. All other parameters were kept the same (including the source geometry). In the positive mode, the biggest differences were observed for non-polar analytes dissolved in aprotic solvents like toluene, chloroform and carbon disulfide. Protonated species were dominant in the open configuration, while radical cations were mostly present in the enclosed configuration. The excessive protonation in open ion source was caused by the diffusion of water vapor molecules into the ionization region from the atmosphere. Water vapor molecules were also responsible for the formation of an ion [M + 19]+ from alkynes in the open configuration. The fragmentation study confirmed that the ion [M + 19]+ was a 2-methylketone formed by the addition reaction. The formation of such artifacts can pose problems in qualitative analysis. On the other hand, the ion [M + 55]+ was observed in the enclosed ion source as a reaction product of unsaturated compounds with acetonitrile solvent molecules. Its fragmentation can be used for the double bond...