the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Jun 2024
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.
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RC1: 'Comment on amt-2024-94', Delphine Farmer, 21 Jun 2024
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 -
AC2: 'Reply to Delphine Farmer review', James B. Burkholder, 04 Oct 2024
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2024-94/amt-2024-94-AC2-supplement.pdf
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AC2: 'Reply to Delphine Farmer review', James B. Burkholder, 04 Oct 2024
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RC2: 'Comment on amt-2024-94', John Crounse, 11 Sep 2024
This note reports calibration experiments conducted at NOAA Chemical Sciences Laboratory for two I- chemical ionization mass spectrometer instruments towards two distinct isomers with molecular formula C2H3ON, namely methyl isocyanate (CH3NCO) and hydroxyacetonitrile (HOCH2CN). The results indicate that these instruments are highly sensitive to HOCH2CN while insensitive to CH3NCO.
It is not at all surprising that I- forms moderately strong complexes with HOCH2CN as compared with CH3NCO. The hydrogen in the polarized (acidic) H-O bond is undoubtedly largely responsible for this difference. Similarly, the polar H-NCO bond is likely the dominant reason for I- forming clusters with HNCO. These functionalities promote stronger binding to reagent ions through hydrogen bonding (J. Phys. Chem. A 2018, 122, 269−279).
For the most part this paper is clear and well-written and the subject matter is pertinent for the AMT journal and I recommend that it be published after the authors have addressed the following points to the editor’s satisfaction:
0). The authors should modify the strong, confrontational language used in abstract, introduction and conclusion where they apply results from this work to reports of previous measurements. Such statements may be hard to prove, and serve no good end. Statements including modifiers such as ‘likely’, ‘may have’, and ‘it may be’ should suffice to convey the authors' point.
1). Are there other stable isomers having formula C2H3NCO besides the two discussed in this paper? If so, authors should discuss the likelihood that these could contribute to ambient CIMS signals. Are there other ‘nearby’ isobars (ie, different atomic composition), considering the resolution of spectrometers in question, which could contribute to signal with nominal mass of C2H3NCO clusters?
2). The IR absorption bands and cross-sections used to quantify CH3NCO should be included. How stable was this compound in AL cylinders? How do Nr calibration of CH3NCO mixtures in AL bottles agree with FTIR determinations?
3). Are there other possible ‘N’ compounds produced in the syringe pump std method for HOCH2CN? How does the mixing ratio calculated from pumping rate and gas flow compare with Nr determination?
Figure 1: y-axis of Panel B is Nr signal or something else? If Nr, you should keep this label, and state in text explicitly the assumption that 100% (or whatever the assumption is) Nr signal is comprised of HAN.
Figure 3: How do the authors interpret and deal with the ToF 30C and 35C and Quad 20C curves that seem to be out of family with the other curves, and the mechanism. Are these curves reproducible?
LN272-281: This PP should be reformulated. Suggest that if the authors wish to put forward the idea that HAN is observed in the atmosphere it would be more appropriate, straightforward, and convincing, if they present their own data, rather than simply re-assigning previously published by other groups. [reviewer notes that the instrumentation calibrated within this work has been deployed I number of previous field campaigns from aircraft and ground-based platforms, with plenty of biomass burning influence]. In addition, the authors should discuss the more general importance of HOCH2CN to the nitrile budget. What fraction of nitriles does HOCH2CN comprise? Is there reason (and if so what are the reasons) to study its chemistry in more detail?
Citation: https://doi.org/10.5194/amt-2024-94-RC2 -
AC1: 'Reply to Crounse review', James B. Burkholder, 04 Oct 2024
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2024-94/amt-2024-94-AC1-supplement.pdf
-
AC1: 'Reply to Crounse review', James B. Burkholder, 04 Oct 2024
Status: closed
-
RC1: 'Comment on amt-2024-94', Delphine Farmer, 21 Jun 2024
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 -
AC2: 'Reply to Delphine Farmer review', James B. Burkholder, 04 Oct 2024
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2024-94/amt-2024-94-AC2-supplement.pdf
-
AC2: 'Reply to Delphine Farmer review', James B. Burkholder, 04 Oct 2024
-
RC2: 'Comment on amt-2024-94', John Crounse, 11 Sep 2024
This note reports calibration experiments conducted at NOAA Chemical Sciences Laboratory for two I- chemical ionization mass spectrometer instruments towards two distinct isomers with molecular formula C2H3ON, namely methyl isocyanate (CH3NCO) and hydroxyacetonitrile (HOCH2CN). The results indicate that these instruments are highly sensitive to HOCH2CN while insensitive to CH3NCO.
It is not at all surprising that I- forms moderately strong complexes with HOCH2CN as compared with CH3NCO. The hydrogen in the polarized (acidic) H-O bond is undoubtedly largely responsible for this difference. Similarly, the polar H-NCO bond is likely the dominant reason for I- forming clusters with HNCO. These functionalities promote stronger binding to reagent ions through hydrogen bonding (J. Phys. Chem. A 2018, 122, 269−279).
For the most part this paper is clear and well-written and the subject matter is pertinent for the AMT journal and I recommend that it be published after the authors have addressed the following points to the editor’s satisfaction:
0). The authors should modify the strong, confrontational language used in abstract, introduction and conclusion where they apply results from this work to reports of previous measurements. Such statements may be hard to prove, and serve no good end. Statements including modifiers such as ‘likely’, ‘may have’, and ‘it may be’ should suffice to convey the authors' point.
1). Are there other stable isomers having formula C2H3NCO besides the two discussed in this paper? If so, authors should discuss the likelihood that these could contribute to ambient CIMS signals. Are there other ‘nearby’ isobars (ie, different atomic composition), considering the resolution of spectrometers in question, which could contribute to signal with nominal mass of C2H3NCO clusters?
2). The IR absorption bands and cross-sections used to quantify CH3NCO should be included. How stable was this compound in AL cylinders? How do Nr calibration of CH3NCO mixtures in AL bottles agree with FTIR determinations?
3). Are there other possible ‘N’ compounds produced in the syringe pump std method for HOCH2CN? How does the mixing ratio calculated from pumping rate and gas flow compare with Nr determination?
Figure 1: y-axis of Panel B is Nr signal or something else? If Nr, you should keep this label, and state in text explicitly the assumption that 100% (or whatever the assumption is) Nr signal is comprised of HAN.
Figure 3: How do the authors interpret and deal with the ToF 30C and 35C and Quad 20C curves that seem to be out of family with the other curves, and the mechanism. Are these curves reproducible?
LN272-281: This PP should be reformulated. Suggest that if the authors wish to put forward the idea that HAN is observed in the atmosphere it would be more appropriate, straightforward, and convincing, if they present their own data, rather than simply re-assigning previously published by other groups. [reviewer notes that the instrumentation calibrated within this work has been deployed I number of previous field campaigns from aircraft and ground-based platforms, with plenty of biomass burning influence]. In addition, the authors should discuss the more general importance of HOCH2CN to the nitrile budget. What fraction of nitriles does HOCH2CN comprise? Is there reason (and if so what are the reasons) to study its chemistry in more detail?
Citation: https://doi.org/10.5194/amt-2024-94-RC2 -
AC1: 'Reply to Crounse review', James B. Burkholder, 04 Oct 2024
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2024-94/amt-2024-94-AC1-supplement.pdf
-
AC1: 'Reply to Crounse review', James B. Burkholder, 04 Oct 2024
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