Evaluation of NO<sup>+</sup> reagent ion chemistry for online measurements of atmospheric volatile organic compounds
2016
A. R. Koss | C. Warneke | B. Yuan | M. M. Coggon | P. R. Veres | J. A. de Gouw
NO<sup>+</sup> chemical ionization mass spectrometry (NO<sup>+</sup> CIMS) can achieve fast (1 Hz and faster) online measurement of trace atmospheric volatile organic compounds (VOCs) that cannot be ionized with H<sub>3</sub>O<sup>+</sup> ions (e.g., in a PTR-MS or H<sub>3</sub>O<sup>+</sup> CIMS instrument). Here we describe the adaptation of a high-resolution time-of-flight H<sub>3</sub>O<sup>+</sup> CIMS instrument to use NO<sup>+</sup> primary ion chemistry. We evaluate the NO<sup>+</sup> technique with respect to compound specificity, sensitivity, and VOC species measured compared to H<sub>3</sub>O<sup>+</sup>. 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<sup>+</sup> is useful for isomerically resolved measurements of carbonyl species; (2) NO<sup>+</sup> can achieve sensitive detection of small (C<sub>4</sub>–C<sub>8</sub>) branched alkanes but is not unambiguous for most; and (3) compound-specific measurement of some alkanes, especially isopentane, methylpentane, and high-mass (C<sub>12</sub>–C<sub>15</sub>) n-alkanes, is possible with NO<sup>+</sup>. We also demonstrate fast in situ chemically specific measurements of C<sub>12</sub> to C<sub>15</sub> alkanes in ambient air.
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