Dopant-assisted direct analysis in real time mass spectrometry with argon gas

RATIONALE: Dopants used with Atmospheric Pressure Photoionization (APPI) were examined with the Direct Analysis in Real Time (DART (R)) ion source operated with argon gas. Charge-exchange and proton transfer reactions were observed by adding toluene, anisole, chlorobenzene and acetone to the DART gas stream, complementing the information obtained by helium DART. METHODS: Mass spectra were acquired with a time-of-flight mass spectrometer equipped with a DART ion source operated with argon gas. A syringe pump was used to introduce dopants directly into the DART gas stream through deactivated fused-silica capillary tubing. Samples including polycyclic aromatic hydrocarbons (PAHs), diesel fuel, trinitrotoluene and cannabinoids were deposited onto the sealed end of melting tube, allowed to dry, and the tube was then suspended in the dopant-enhanced DART gas stream. RESULTS: PAHs could be detected as molecular ions at concentrations in the low parts-per-billion range by using a solution of 0.5% anisole in toluene as a dopant. Argon DART analysis of a diesel fuel sample with the same dopant mixture showed a simpler mass spectrum than obtained by using helium DART. The argon DART mass spectrum was dominated by molecular ions for aromatic compounds, whereas the helium DART mass spectrum showed both molecular ions and protonated molecules. In contrast O-2(-) attachment DART showed saturated hydrocarbons and oxygen-containing species. Mass spectra for trinitrotoluene with argon DART in negative-ion mode showed a prominent [M-H](-) peak, whereas conventional helium DART showed both M- and [M-H](-). Lastly, in analogy to a report in the literature using APPI, positive ions produced by argon DART ionization for delta-9-tetrahydrocannabinol (THC) and cannabidiol showed distinctive product-ion mass spectra. CONCLUSIONS: Dopant-assisted argon DART operates by a mechanism that is analogous to those proposed for dopant-assisted atmospheric-pressure photoionization. Copyright (C) 2016 John Wiley & Sons, Ltd.