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. Atmospheric oxidation products of volatile organic compounds consist ofthousands of unique chemicals that have distinctly different physical andchemical properties depending on their detailed structures and functionalgroups. Measurement techniques that can achieve molecular characterizationswith details down to functional groups (i.e., isomer-resolved resolution)are consequently necessary to provide understandings of differences of fateand transport within isomers produced in the oxidation process. Wedemonstrate a new instrument coupling the thermal desorption aerosol gaschromatograph (TAG), which enables the separation of isomers, with thehigh-resolution time-of-flight chemical ionization mass spectrometer(HR-ToF-CIMS), which has the capability of classifying unknown compounds bytheir molecular formulas, and the flame ionization detector (FID), which provides anear-universal response to organic compounds. The TAG-CIMS/FID is used toprovide isomer-resolved measurements of samples from liquid standardinjections and particle-phase organics generated in oxidation flow reactors.By coupling a TAG to a CIMS, the CIMS is enhanced with an additionaldimension of information (resolution of individual molecules) at the cost oftime resolution (i.e., one sample per hour instead of per minute). We foundthat isomers are prevalent in sample matrix with an average number of threeto five isomers per formula depending on the precursors in the oxidationexperiments. Additionally, a multi-reagent ionization mode is investigatedin which both zero air and iodide are introduced as reagent ions, to examinethe feasibility of extending the use of an individual CIMS to a broaderrange of analytes with still selective reagent ions. While this approachreduces iodide-adduct ions by a factor of 2, [M − H]− and[M + O2]− ions produced from lower-polarity compounds increase bya factor of 5 to 10, improving their detection by CIMS. The methodexpands the range of detected chemical species by using two chemicalionization reagents simultaneously, which is enabled by the pre-separation of analytemolecules before ionization.

atmospheric measurement techniques

Coupling a gas chromatograph simultaneously to a flame ionization detector and chemical ionization mass spectrometer for isomer-resolved measurements of particle-phase organic compounds