Coupling a Gas Chromatograph Simultaneously to a Flame Ionization Detector and Chemical Ionization Mass Spectrometer for Isomer-Resolved Quantification of Particle-Phase Organic Compounds

Abstract:

Atmospheric oxidation of volatile organic compounds generates thousands of unique chemicals that have distinctly different physical and chemical properties depending on their structure and chemical functionality. Measurement techniques that can achieve characterization with detail down to molecular structure (i.e. isomer-resolved resolution) are consequently necessary to understanding differences in fate and transport between isomers produced in the oxidation process. In this study, a field‐deployable thermal desorption aerosol gas chromatograph (TAG) was simultaneously coupled to a time-of-flight chemical ionization mass spectrometer (“TAG‐CIMS”) using iodide as a reagent, and a flame ionization detector (FID), providing near-universal response to all analytes. This instrument measures molecular formulas of unknowns alongside identification of known compounds and precise quantification of all analytes. We present here detailed characterization of the particle-phase oxidation products of common indoor emissions (e.g. limonene) over hours to days of atmospheric oxidation (by OH and O₃). We found that chemical formulas identified in CIMS has an average of about five isomers and the sensitivities of those isomers can vary by up to two orders of magnitudes. Furthermore, we compare the directly measured sensitivity to currently adopted approaches to calibration (e.g. “voltage scanning”), yielding new insight into the capabilities and limitations of this reagent ion chemistry. Finally, a multi-reagent ionization mode is investigated in which both zero air and iodide are introduced as reagent ions, to examine the feasibility of extending the use of an individual CIMS to a broader range of analytes. While this approach reduces iodide-adduct ions by a factor of two, other product ions such as [M-H]^- and [M+O₂]^- increase by a factor of five to ten, potentially providing additional structural information and measurements of compounds too non-polar to form an iodide adduct.