Evaluation of the optimization space for atmospheric pressure photoionization (APPI) in comparison with APCI

Abstract

The usefulness of atmospheric pressure photoionization (APPI) is difficult to evaluate for unknowns due to the fragmented literature. Specifically, the variation of dopants with a wide set of compounds or the use of APPI in the negative mode have rarely been explored. Thirty compounds were selected that were not suitable for ESI with a wide variety of functional groups and investigated with atmospheric pressure chemical ionization (APCI) and APPI in the positive and negative ion modes. The influence of the mobile phase (eluents containing acetonitrile or methanol) and--for APPI--four different dopants (acetone, chlorobenzene, toluene, and toluene/anisole) were explored. Stepwise variation of the organic mobile phase allowed to elucidate the ionization mechanism. Atmospheric pressure photoionization was especially useful for compounds, where the M(●+) and not the [M + H](+) was formed. The dopants chlorobenzene and anisole promoted the formation of molecular ions M(●+) for about half of the compounds, and its formation was also positively influenced by the use of mobile phases containing methanol. In the negative ion mode, APPI offered no advantage toward APCI. Best results were generally achieved with the dopant chlorobenzene, establishing that this dopant is suitable for a wide set of compounds. For one quarter of the compounds, significantly better results were achieved with mobile phases containing methanol for both APPI and APCI than those with acetonitrile, but only in the positive mode. With either of the methods--APPI or APCI--about 10% of the compounds were not detected. Strategies to get results quickly with difficult unknowns will be discussed.

Mechanism behind the paper spray chemical ionization phenomenon and the choice of solvent

“…Based on the data presented in Figure 4, it was concluded that the sensitivity of PSCI was improved with the usage of solvents in the order of hexane < toluene < chlorobenzene. These data are consistent with a previous study, which suggested chlorobenzene as a sensitive dopant for molecular ion generation during atmospheric pressure photoionization (APPI) 44 . Previous studies showed that chemical reactions such as charge transfer and/or proton transfer between solvent and analyte are important for APPI mechanism 45–47 .…”

“…These data are consistent with a previous study, which suggested chlorobenzene as a sensitive dopant for molecular ion generation during atmospheric pressure photoionization (APPI). 44 Previous studies showed that chemical reactions such as charge transfer and/or proton transfer between solvent and analyte are important for APPI mechanism. [45][46][47] Therefore, there is a common element shared by PSCI, atmospheric pressure chemical ionization, and APPI mechanisms, and that can explain the improved ionization efficiency with chlorobenzene for both APPI and PSCI.…”

Ionization efficiency ladders as tools for choosing ionization mode and solvent in liquid chromatography/mass spectrometry

“…Additionally, influence of mobile phases on ionization in an APCI source was studied by Fredenhagen and Kühnöl and it was concluded that for APCI+ mobile phases containing MeOH were advantageous compared with MeCN. For some compounds with MeCN signal‐to‐noise ratios were higher; however, this was due to an extensive in‐source fragmentation while utilizing MeOH as organic mobile phase.…”

The use of isoprene as a novel dopant in negative ion atmospheric pressure photoionization mass spectrometry coupled to high‐performance liquid chromatography

“…The ionization mechanisms in the DA‐NI‐APPI, as well as the role of commonly used eluents and buffers in LC and the effect of operational parameters of MS on the observed ions in the mass spectra, have been studied thoroughly. A summary of ionization pathways in the NI‐APPI mode, which were adopted from the literature, is presented in Table .…”