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https://doi.org/10.5194/amt-2024-94
© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2024-94
© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
21 Jun 2024
| 21 Jun 2024
Status: this preprint is currently under review for the journal AMT.
Calibration of Hydroxyacetonitrile (HOCH2CN) and Methyl isocyanate (CH3NCO) Isomers using I- Chemical Ionization Mass Spectrometry (CIMS)
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.
How to cite. Finewax, Z., Chattopadhyay, A., Neuman, J. A., Roberts, J., and Burkholder, J.: Calibration of Hydroxyacetonitrile (HOCH2CN) and Methyl isocyanate (CH3NCO) Isomers using I- Chemical Ionization Mass Spectrometry (CIMS), Atmos. Meas. Tech. Discuss. [preprint], https://doi.org/10.5194/amt-2024-94, in review, 2024.
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Status: open (until 27 Jul 2024)
Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor
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RC1: 'Comment on amt-2024-94', Delphine Farmer, 21 Jun 2024
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This is a very useful manuscript providing an update for groups conducting iodide CIMS measurements. The work is carefully done, and shows that iodide CIMS is not sensitive to methyl isocyanate, but is to hydroxyacetonitrile. This is surprising as the I-CIMS system is sensitive to isocyanic acid, but not acetonitrile - and suggests that updates are warranted for several papers that have shown the presence of this ion and assumed it was methyl isocyanate. I will note that the attribution to methyl isocyanate was never central to the key points in any of the papers, but the attribution was certainly there and this work is an important correction. As an author on some of these papers, I have long been curious about the source of methylisocyanate, and this work certainly helps resolves my nagging doubts over the presence of this compound. Overall, this manuscript is timely, clearly written, and constructive. I recommend publication. I suggest some minor wording changes that provide a more diplomatic context to previous work, and raise a question for the authors to ponder (but don't require changes in the text).The lack of sensitivity for iodide reagent ions to methyl isocyanate is intriguing to me because the instrument is well-established by this same research group to have decent sensitivity to isocyanic acid - and well known to not be sensitive to acetonitrile. While adding a methyl group to isocyanic acid would certainly change its binding strength to iodide ions, it is interesting that this is enough to drop the sensitivity that much. Similarly, it is intriguing that adding a hydroxyl group to acetonitrile is enough to make this molecule bind with iodide. This is not to say that I don't believe the results - I do, as they are quite striking - but to say that this work raises additional questions of which types of organic nitrogen compounds the I-CIMS is sensitive to - and what that says about the binding energies with iodide. I encourage the authors to consider this question - what makes I-CIMS so sensitive to hydroxyacetonitrile, and not to methyl isocyanate? I also encourage them to note the reason why previous studies have typically assigned this observed ion to methyl isocyanate and note to hydroxyacetonitrile. It was, in my opinion, a rational line of reasoning - and one that is often used in the CIMS community to suggest attribution of ions. What this work more broadly suggests is that the I-CIMS community needs to question lines of reasoning regarding structural similarity more robustly - not just for these organic nitrogen compounds, but perhaps for other structural moieties as well.A minor wording comment: The Abstract notes "These results contradict several recent field studies that have reported the detection of MIC using I-CIMS instruments." I would argue (as an author on several of these manuscripts) that "contradiction" is a strong term that suggests an argument is necessary in the literature. I do not think the authors of this manuscript suggest that there will be a debate as this work is convincing, though surprising. The papers report their best interpretation at the time, and instead suggest less aggressive wording along the lines of "These results suggest an updated interpretation to results presented in recent field studies that have reported detection of MIC using I-CIMS instruments."Citation: https://doi.org/
10.5194/amt-2024-94-RC1
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Zachary Finewax
Chemical Sciences Laboratory, National Oceanic and Atmospheric Administration (NOAA), Boulder, CO, USA
Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO, USA
present address: Air Pollution Control Division, Colorado Department of Health and Environment, Denver, CO, USA
Aparajeo Chattopadhyay
Chemical Sciences Laboratory, National Oceanic and Atmospheric Administration (NOAA), Boulder, CO, USA
Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO, USA
present address: Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY, USA
J. Andrew Neuman
Chemical Sciences Laboratory, National Oceanic and Atmospheric Administration (NOAA), Boulder, CO, USA
Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO, USA
James Roberts
Chemical Sciences Laboratory, National Oceanic and Atmospheric Administration (NOAA), Boulder, CO, USA
James Burkholder
CORRESPONDING AUTHOR
Chemical Sciences Laboratory, National Oceanic and Atmospheric Administration (NOAA), Boulder, CO, USA
Short summary
This work provides a comprehensive sensitivity calibration of a chemical ionization instrument commonly used in field measurements for the measurement of the toxic isomers methyl isocyanate and hydroxyacetonitrile that are found in the atmosphere. The results from this work has demonstrated that the hydroyacetonitrile isomer was observed in previous field studies rather than the stated identification of methyl isocyanate.
This work provides a comprehensive sensitivity calibration of a chemical ionization instrument...