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

Abstract

Abstract. 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.