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Evaluation of NO+ reagent ion chemistry for online measurements of atmospheric volatile organic compounds
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    Atmospheric Measurement Techniques, 9(7), 2909-2925.
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  • Description:
    NO+ chemical ionization mass spectrometry (NO+ CIMS) can achieve fast (1aEuro-Hz and faster) online measurement of trace atmospheric volatile organic compounds (VOCs) that cannot be ionized with H3O+ ions (e.g., in a PTR-MS or H3O+ CIMS instrument). Here we describe the adaptation of a high-resolution time-of-flight H3O+ CIMS instrument to use NO+ primary ion chemistry. We evaluate the NO+ technique with respect to compound specificity, sensitivity, and VOC species measured compared to H3O+. The evaluation is established by a series of experiments including laboratory investigation using a gas-chromatography (GC) interface, in situ measurement of urban air using a GC interface, and direct in situ measurement of urban air. The main findings are that (1) NO+ is useful for isomerically resolved measurements of carbonyl species; (2) NO+ can achieve sensitive detection of small (C-4-C-8) branched alkanes but is not unambiguous for most; and (3) compound-specific measurement of some alkanes, especially isopentane, methylpentane, and high-mass (C-12-C-15) n-alkanes, is possible with NO+. We also demonstrate fast in situ chemically specific measurements of C-12 to C-15 alkanes in ambient air.
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