Using dopants in the atmospheric pressure chemical ionization ion source to determine the site of protonation by ion mobility spectrometry

Rationale

Compounds like caffeine metabolites with more than one proton acceptor site can produce a mixture of isomeric protonated ions (protomers) in electrospray ionization and atmospheric pressure chemical ionization (APCI) ion sources. Discrimination between the protomers is of interest as the charge location influences ion structure and chemical and physical properties.

Methods

Protonation of caffeine in an APCI ion source was studied using ion mobility spectrometry. The hydronium ions, H₃O⁺(H₂O)_n, are the main reactant ions in the APCI ion source. Different dopant gases including NO₂, NH₃, and CH₃NH₂ were used to produce new reactant ions NO⁺, NH₄⁺, and CH₃NH₃⁺, respectively. Density functional theory was employed to explain the experimental results and calculate the energies of the ionization reactions.

Results

The ion mobility spectrum of caffeine showed three peaks. In the presence of NO₂ dopant and NO⁺ reactant ion, caffeine was ionized via charge transfer and formation of M⁺ ion. As NH₃ and CH₃NH₂ are stronger bases than H₂O, the reactant ions NH₄⁺ and CH₃NH₃⁺ selectively protonated the more basic site of caffeine, that is, the imidazole nitrogen. Using these dopants, we could attribute the first ion mobility peak to M⁺ ion, the second peak to the protonation of caffeine at the carbonyl oxygen atom, and the third peak to the protonation of the imidazole nitrogen atom. The calculated collisional cross-sections of M⁺ and the protomers of caffeine confirmed the peaks' assignment.

Conclusions

The criterion for the selection of an appropriate dopant is that its proton affinity (PA) should be between those of the proton acceptor sites of the molecule studied.