Novel proton transfer reaction-mass spectrometry by using protonated molecules as a primary ion

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A two-stage ion source for proton transfer reaction (PTR) ionization was investigated in order to achieve more selective mass spectrometric (MS) detection of selected volatile organic compounds (VOCs) than that achieved with commonly used PTR-MS instruments, which are based on single-step PTR ionization with H_3O^+. The two-stage PTR ion source generated reagent ions other than H_3O^+ by an initial PTR between H_3O^+ and a selected VOC, and then a second PTR ionization occurred only for VOCs with proton affinities larger than the affinity of the reagent VOC. Acetone Using two-stage PTR-MS, we differentiated isomeric VOCs by means of differences in their proton affinities. This technique was applied to chamber experiment of isoprene-oxidation and to analysis of secondary organic aerosol.