Optimization of H3O+/O2+ Dual-mode Ionization in PTR-MS for Simultaneous Detection of Alkanes, Olefins and Aromatic Compounds

Abstract:

Measurements of VOC composition from fossil fuels are analytically challenging because of the complex mixture of hydrocarbons (saturated, unsaturated, aromatics, etc). Speciated chemical measurements typically rely on relatively slow GC separation. Proton transfer reaction mass spectrometry (PTR-MS) is advantageous due to its fast response and high sensitivity. The most common ionization mechanism applied to VOC detection by PTR-MS is proton transfer from hydronium ion (H₃O⁺). However, alkanes cannot be detected using H₃O⁺ ionization chemistry because their proton affinities are too low. Ionization of alkanes is possible via electron transfer and/or hydride abstraction using O₂⁺ or NO⁺.

We used PTR-MS to analyze aromatic, alkene and alkane (linear, branched and cyclic) compounds simultaneously not by switching the ionization agents, but by adjusting the drift tube voltage and optimizing the ratio of H₃O⁺/O₂⁺ produced in the instrument’s ion source. The highest detection sensitivity for aromatic and alkene compounds was produced by proton transfer from H₃O⁺, while hydride abstraction by O₂⁺ allowed detection of alkanes. For alkanes, sensitivities ranged from 1.1±0.01 cps/ppbv for n-decane to 74.7±0.25 cps/ppbv for decalin. Sensitivities in O₂⁺ mode were from 6 (Adamantane) to 146 (4-Methyl nonane) times higher than those obtained in H₃O⁺ mode under the same ion source and drift tube voltage conditions. Sensitivities for butyl benzene and 1-decene were 157±0.57 and 66.8±0.21 cps/ppbv, respectively. Sensitivity differences among C₁₀ hydrocarbons are related to their structure, which affects their ionization energies (IE) and hence ease of hydride abstraction. Sensitivities at the parent ion mass were inversely correlated with IE (142 cps/ppbv/eV). This suggests higher electronic stability for cyclic non substituted compounds, followed by cyclic substituted, branch linear and linear C₁₀ hydrocarbons. Although selectivity is a known shortcoming of quadrupole-based PTR-MS, we demonstrated that, when optimized, it can be a useful tool to study alkanes, olefins, and aromatics simultaneously in high concentration plumes or in a laboratory.

AMO acknowledges financial support of UCMEXUS-CONACyT Program for the Postdoctoral Fellowship