Evaluation of NO⁺ reagent ion chemistry for online measurements of atmospheric volatile organic compounds

NO⁺ chemical ionization mass spectrometry (NO⁺ CIMS) can achieve fast (1Hz 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 (C4-C8) branched alkanes but is not unambiguous for most; and (3) compound-specific measurement of some alkanes, especially isopentane, methylpentane, 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.