The use of dopants in atmospheric pressure ionization sources of mass spectrometers
Creators: name: Dousty, Faezeh
Publisher: name: University of British Columbia
Year: 2015

description: Dopants, as “ionization assisting” chemicals, have been used in different ionization techniques. However, they have been used most frequently in the field of Atmospheric Pressure Photoionization (APPI). These chemicals that have high photoabsorption and photoionization cross sections easily get photoionized and “preserve” to a certain extent the energy of the photon flux, which otherwise is significantly lost due to the absorption by matrix components. Subsequently, these photo-dopant ions ionize analyte molecules through ion-molecule chemical reactions. Obtaining a profound understanding of ion formation mechanisms is a crucial step in successful applications of mass spectrometry. The first part of this thesis focuses on comprehensively describing ionization mechanisms and specifically the role of dopants. With the understanding obtained from in depth research on ion formation mechanisms, two novel dopants for APPI are proposed, i.e., carbon disulfide and isoprene. The potential of these chemicals as dopants, alongside their underlying ionization mechanisms, were investigated with a commercial APPI source coupled to a liquid chromatograph and a custom-built APPI source coupled to a gas chromatograph. Carbon disulfide was proven to be an effective charge transfer reagent for the positive ion APPI (PI-APPI) and promoted the ionization of non-polar compounds, for which the proton transfer route was not possible. The advantage of carbon disulfide over commonly used dopants is its high ionization energy (10 eV), which enables the ionization of analytes with high IEs through a charge transfer route, when other commonly used dopants with less IEs suppressed their ionization. Isoprene, which is considered a green chemical, gave effective results in the negative ion APPI (NI-APPI). In contrast to LC, in which mobile phases can interfere with the ionization of analytes and complicate the ionization matrix, GC provides a very simple matrix for photoionization. Thus, underlying ion formation mechanisms can be more reliably studied and interpreted. The investigation of ionization mechanisms in both LC- and GC-based/MS analyses in parallel contributed to better formulating the role of all chemicals present in the source. Last but not least, the role of dopants in enhancing ionization responses in Atmospheric Pressure Laser Ionization (APLI) for GC/MS applications was investigated. In APLI, instead of a one-step VUV ionization event used in APPI, a two-step UV ionization event is employed. APLI has only been recently introduced; thus, the utilization of dopants in this technique has not been explored as extensively as for APPI. The potential of carbon disulfide and isoprene as dopants were also examined for this technique. The short lifetime of the excited transition states of carbon disulfide suppressed its laser ionization. Therefore, laser ionization of carbon disulfide did not increase the total ion production in order to assist the ionization in the PI mode. Positive electron affinity of carbon disulfide disqualifies its applications for negative ionization. Therefore, dopant-assisted studies in PI-APLI were limited to using toluene as dopant, which produced an abundant intensity of toluene radical cations. Laser ionization of isoprene produced a range of radical cations. Thus, the elevated chemical noise and the presence of many ions at the low m/z range of the isoprene spectrum can interfere with ion products of small molecules, which render its application as a dopant for PI-APLI problematic, similar as for PI-APPI. In NI APLI, toluene offered more effective results than isoprene. Therefore, toluene was chosen to investigate the role of dopants in enhancing the ionization responses and to study the corresponding ionization mechanisms in PI/NI APLI.