An Iodide-Adduct High-Resolution Time-of-Flight Chemical-Ionization Mass Spectrometer: Application to Atmospheric Inorganic and Organic Compounds

Abstract

A high-resolution time-of-flight chemical-ionization mass spectrometer (HR-ToF-CIMS) using Iodide-adducts has been characterized and deployed in several laboratory and field studies to measure a suite of organic and inorganic atmospheric species. The large negative mass defect of Iodide, combined with soft ionization and the high mass-accuracy (<20 ppm) and mass-resolving power (R>5500) of the time-of-flight mass spectrometer, provides an additional degree of separation and allows for the determination of elemental compositions for the vast majority of detected ions. Laboratory characterization reveals Iodide-adduct ionization generally exhibits increasing sensitivity toward more polar or acidic volatile organic compounds. Simultaneous retrieval of a wide range of mass-to-charge ratios (m/Q from 25 to 625 Th) at a high frequency (>1 Hz) provides a comprehensive view of atmospheric oxidative chemistry, particularly when sampling rapidly evolving plumes from fast moving platforms like an aircraft. We present the sampling protocol, detection limits and observations from the first aircraft deployment for an instrument of this type, which took place aboard the NOAA WP-3D aircraft during the Southeast Nexus (SENEX) 2013 field campaign.

Highly functionalized organic nitrates in the southeast United States: Contribution to secondary organic aerosol and reactive nitrogen budgets

“…The two other isomers (cis and trans δ-1,4) of isoprene hydroxynitrate were detected less sensitively by this technique likely because of the location of the hydroxy functional group relative to that of the nitrate, which can increase the polarity of the hydroxy group and therefore, the stability of the iodide-hydroxynitrate adduct (SI Appendix). Sensitivity generally increases with increasing functionality and increasing number of atoms on the molecule but not indefinitely (14). Our reported mass concentrations would be higher by a factor of ∼2.2 if we applied the mean sensitivity of the cis and trans δ-1,4 and lower by a factor of ∼2.5 if we applied the highest sensitivity to any compound tested thus far using iodide-adduct HRToF-CIMS.…”

“…The iodide-adduct ionization is most sensitive to polar organics with hydroxy groups (14), and the dominant ON signal in the particle phase observed during the SOAS was of highly oxygenated compounds. As such, we apply the highest sensitivity value obtained (8.5 counts second −1 per parts per trillion per million total reagent ion) from these calibrations-that of the β-4,3 isomer of C 5 H 9 NO 4 , with structure that presumably allows the iodide ion to interact most stably with the hydroxy functional group-to all observed ONs to calculate the mass concentration (micrograms meter −3 ) of each.…”

Gas-phase pesticide measurement using iodide ionization time-of-flight mass spectrometry

“…The elemental composition of analyte ions can be determined for a broad range of m/z ratios (typically 0-1000) (Aljawhary et al, 2013;Ehn et al, 2014;Lee et al, 2014;Brophy, 2015;de Gouw and Warneke, 2007). Iodide is an obvious target reagent ion for pesticide CIMS, as iodide has been used to measure oxidized nitrogen and halogenated speciesincluding N 2 O 5 , ClNO 2 , and ClNO 3 -and more recently semi-volatile organic compounds, particularly organic acids (Huey et al, 1995;Kercher et al, 2009;Lee et al, 2014). Pesticides often contain one or multiple of these previously detected functional groups, suggesting that iodide is an appropriate reagent ion for their detection.…”

“…Due to the ligand-switch mechanism described above, analyte detection by iodide ToF-CIMS is expected to vary with ambient relative humidity (RH) (Kercher et al, 2009;Lee et al, 2014). As field measurements are a desired outcome in the development of a real-time pesticide detector, we investigated the sensitivity of trifluralin and metolachlor with replicate injections of trifluralin (2.4 µL of 98 ng µL −1 solution) and metolachlor (3 µL of 103 ng µL −1 solution) over an RH range of 0-80 % (Fig.…”

“…The fitting procedures used in the Tofware software package allow for confirmation of peak identity not only by the exact mass of the peak fit but also by the fit of isotope-containing peaks. Such fitting has allowed for measurement of trace compounds in complex environmental and laboratory samples (e.g., LopezHilfiker et al, 2014;Lee et al, 2014;Aljawhary et al, 2013;Ehn et al, 2014). For example, Fig.…”