Online Calibration of a Chemical Ionization Mass Spectrometer for Multifunctional Biogenic Organic Nitrates

Multifunctional organic nitrates derived from biogenic volatile organic compounds are important for understanding ozone and secondary organic aerosol production from oxidation reactions in the presence of nitrogen oxides. Their measurement is challenging, in part because the quantification of these compounds is difficult and time consuming due to the techniques required to synthesize and purify authentic standards. We describe a novel online synthesis and separation technique and demonstrate its use for calibration of a chemical ionization mass spectrometer using iodide reagent ions (I^– CIMS) to measure four isomers of isoprene hydroxy nitrate (IHN; C₅H₉NO₄), two isomers of methyl vinyl ketone hydroxy nitrate (MVKHN; C₄H₇NO₅), and four isomers of monoterpene hydroxy nitrate (MTHN; C₁₀H₁₇NO₄). We further apply our separation technique to an isoprene + NO₃ + HO₂ online reactor to calibrate for six isomers of isoprene hydroperoxide nitrate (C₅H₉NO₅). We find a large range of detection sensitivities and ion molecule reactor (IMR) temperature dependencies among the reported analytes measured as iodide (I^–) clusters. We report a wide range of normalized sensitivities (normalized Hz pptv^–1; nHz pptv^–1) normalized by the [I·H₂O]⁻ reagent ion signal for this class of analytes (0.2–82 nHz pptv^–1). The (4,3)-MVKHN isomer is exceptional for its high sensitivity with this ion chemistry (82 ± 5 nHz pptv^–1), which can lead to an inaccurate representation of the organic nitrate budget if a moderate sensitivity is assumed. The I^– CIMS demonstrates a much smaller range of sensitivities to IHNs (10–34 nHz pptv^–1), with the two most abundant isomers having similar sensitivities ((1,2): 24 ± 3 nHz pptv^–1; (4,3): 30 ± 4 nHz pptv^–1). These calibrations reveal a significantly different distribution of organic nitrates than would be determined assuming uniform sensitivity for measurements with an I^– CIMS at a ground site in Pasadena, CA, during the summer of 2021. A comparison with another calibrated CIMS (using CF₃O^– reagent ions) for select compounds showed good agreement for IHN and MVKHN.