Abstract. NO+ chemical ionization mass spectrometry (NO+ CIMS) can achieve fast (sub 1-Hz) on-line 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 (C4–C8) branched alkanes, but is not unambiguous for most; and (3) compound-specific measurement of some alkanes, especially iso-pentane, methylpentanes, and high mass (C12–C15) n-alkanes, is possible with NO+. We also demonstrate fast in-situ chemically specific measurements of C12 to C15 alkanes in ambient air.
Evaluation of NO<sup>+</sup> reagent ion chemistry for on-line measurements of atmospheric volatile organic compounds
