In the first part of this thesis, a different design for the use of a photoionization source (UV Lamp) in Ion Mobility spectrometry (IMS) has been proposed. Ion mobility spectra of several chemical compounds were obtained while the effect of dopant and solvent on the spectra were studied. In this design a curtain electrode was mounted in front of the UV lamp to divide the ionization zone into two distinct regions. Dopant was directly photoionized in the first region and then solvent molecules were ionized via proton transfer with the dopant photoions, in the second region. Finally, the analyte were ionized by protonated solvent molecules. This design consumes much less dopant than the conventional sources and leads to increased photoionization yield. The second part of the thesis deals with employing two ion sources, namely photoionization (UV) and corona discharge (CD) source in an ion mobility spectrometer. The design is capable of using each source individually or the two sources simultaneously. Ion mobility spectra of some chemical compounds obtained by each ionization sources were reported. The limits of detection for acridine were obtained to be 0.11 and 0.30 ng for CD and UV, respectively. Finally, simultaneous operation of the two ionization sources was investigated by recording ion mobility spectra of selected samples. In the third part of the thesis, attempts were made to assign the peaks in ion mobility spectra of some compounds with different protonation sites, such as caffeine, obtained by the two ionization sources. Experimental and theoretical evidences were collected to link the observed peaks to related ionic species. In the experimental part, the effects of sample concentration as well as the nature of the reactant ions in the existence and intensity of each peak were evaluated. In the theoretical part, some parameters such as thermodynamic stability, proton affinity, dipole moment, charge distribution, electron density and chemical bond length as well as volume and surface area of protonated isomers of sample molecules were calculated. Using the collected data and evidences, some predictions about the origin of each peak were proposed. Furthermore, the new concept of “internal proton affinity” (IPA) was introduced which expresses the tendency of holding an added proton for different sites in a molecule.