Synonyms: 
CH3O2H
Methyl hydroperoxide
Methylhydroperoxide

Peroxide Chemical Ionization Mass Spectrometer

The measurement of gas phase peroxide species, H2O2 and CH3OOH, contribute to our scientific understanding of the photochemistry of trace gases and particles prior to and after their transport and processing through deep convective clouds. The PCIMS instrument used to make these measurements in the DC3/SEAC4RS mission is new and this will be its first use in an airborne science campaign.

The PCIMS instrument is a slightly modified CIMS instrument manufactured by THS Instruments LLC. Mechanically it consists of a differentially pumped quadrupole mass spectrometer. The instrument operates in negative ion mode and currently I- and O2- reagent ions are used to measure hydrogen peroxide and methylhydroperoxide, respectively, by the formation of cluster ions at masses 80 and 161. The reagent ions are produced by flowing a N2/CH3I/O2 mixture past a 210Po foil.

On the G-V, the PCIMS inlet system starts with a PFA Teflon lined heated G-V HIMIL inlet. From the HIMIL the inlet line is comprised of PFA Teflon and is also heated (Hot-Tube, Clayborn Lab). Analytical blanks are performed by diverting the ambient sample flow through a trap filled with Carulite 200 catalyst. Gas phase calibrations are performed through standard additions to ambient air. H2O2 is added from a urea hydrogen peroxide solid decomposition source or by the evaporation of a nano-fluidic flow of a dilute aqueous solution. CH3OOH is added by the evaporation of a nano-fluidic flow of a dilute aqueous solution. The ambient, calibration and reagent gases are vented overboard through the G-V common exhaust.

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NSF G-V
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URI Peroxides and Formaldehyde Instrument

POPS measures CH2O, H2O2, and CH3OOH.

CH2O is measured by aqueous collection followed by enzyme fluorescence detection.

H2O2 and CH3OOH is measured by aqueous collection followed by HPLC separation and enzyme fluorescence detection.

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Chemical Ionization Mass Spectrometer

The single mass analyzer CIMS (S-CIMS) was developed for use on NASA’s ER-2 aircraft. Its first measurements were made in 2000 (SOLVE). Subsequently, it has flown on the NASA DC-8 aircraft for INTEX-NA, DICE, TC4, and ARCTAS, as well as on the NCAR C-130 during MILAGRO/INTEX-B. HNO3 is measured by selective ion chemical ionization via the fluoride transfer reaction: CF3O- + HNO3 → HF • NO3- + CF2O In addition to its fast reaction rate with HNO3, CF3O- can be used to measure additional acids and nitrates as well as SO2 [Amelynck et al., 2000; Crounse et al., 2006; Huey et al., 1996]. We have further identified CF3O- chemistry as useful for the measurement of less acidic species via clustering reactions [Crounse et al., 2006; Paulot et al., 2009a; Paulot et al., 2009b; St. Clair et al., 2010]: CF3O- + HX → CF3O- • HX where, e.g., HX = HCN, H2O2, CH3OOH, CH3C(O)OOH (PAA) The mass analyzer of the S-CIMS instrument has recently been upgraded from a quadrupole to a time-of-flight (ToF) analyzer. The ToF admits the sample ion beam to the ion extractor, where a pulse of high voltage orthogonally deflects and accelerates the ions into the reflectron, which in turn redirects the ions toward the multichannel plate detector. Ions in the ToF follow a V-shaped, 43 cm path from extractor to detector, separating by mass as the smaller ions are accelerated to greater velocities by the high voltage pulse. The detector collects the ions as a function of time following each extractor pulse. The rapid-scan collection of the ToF guarantees a high temporal resolution (1 Hz or faster) and simultaneous data products from the S-CIMS instrument for all mass channels [Drewnick et al., 2005]. We have flown a tandem CIMS (TCIMS) instrument in addition to the SCIMS since INTEX-B (2006). The T-CIMS provides parent-daughter mass analysis, enabling measurement of compounds precluded from quantification by the S-CIMS due to mass interferences (e.g. MHP) or the presence of isobaric compounds (e.g. isoprene oxidation products) [Paulot et al., 2009b; St. Clair et al., 2010]. Calibrations of both CIMS instruments for HNO3 and organic acids are performed in flight using isotopically-labeled reagents evolved from a thermally-stabilized permeation tube oven [Washenfelder et al., 2003]. By using an isotopically labeled standard, the product ion signals are distinct from the natural analyte and calibration can be performed at any time without adversely affecting the ambient measurement. We also fly calibration standards for H2O2 (evolved from urea-hydrogen peroxide) and MHP (from a diffusion vial).

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