Calibration of Hydroxyacetonitrile (HOCH2CN) and Methyl isocyanate (CH3NCO) Isomers using I- Chemical Ionization Mass Spectrometry (CIMS).

The toxic reduced nitrogen compound methyl isocyanate (CH₃NCO, MIC) has been reported present in wildfire and bio-mass burning emissions, agricultural fumigation plumes, and indoor air. Its isomer, hydroxyacetonitrile (HOCH₂CN, glycolonitrile, HAN) has not been observed in the Earth's atmosphere to date. In this study, absolute sensitivity calibrations for these isomers using two I^- chemical ionization mass spectrometry (I-CIMS) instruments, time-of-flight (ToF) and quadrupole (Quad) instruments, commonly used in laboratory and field measurements, were performed, for the first time, over a range of ion-molecule reactor temperature (10–40 °C) and I(H₂O)^-/I^- ratio (0.01–1). This study demonstrates that I-CIMS, under typical operating conditions, is not sensitive to MIC with limits of detection (LOD) of >860 and >570 ppb for the ToF and Quad I-CIMS instruments, respectively. Both I-CIMS instruments are, however, highly sensitive to the HAN isomer with 0.3 and 3 ppt LODs for the ToF-CIMS and Quad-CIMS instruments, respectively. These results contradict several recent field studies that have reported the detection of MIC using I-CIMS instruments. This study demonstrates that HAN, rather than MIC, was most likely the C₂H₃NO isomer observed in those field studies, although the source chemistry for HAN remains uncharacterized. [ABSTRACT FROM AUTHOR]