the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Characterisation of the multi-scheme chemical ionisation inlet-2 and the detection of gaseous iodine species
Jiali Shen
Siddharth Iyer
Paxton Juuti
Jiangyi Zhang
Mrisha Koirala
Mikko M. Kytökari
Douglas R. Worsnop
Matti Rissanen
Markku Kulmala
Norbert M. Maier
Jyri Mikkilä
Mikko Sipilä
Juha Kangasluoma
Abstract. The multi-scheme chemical ionisation inlet 1 (MION1) allows fast switching between measuring atmospheric ions without chemical ionisation and neutral molecules by multiple chemical ionisation methods. In this study, the upgraded multi-scheme chemical ionisation inlet 2 (MION2) is presented. The new design features improved ion optics that increase the reagent ion concentration, a generally more robust operation and the possibility to run multiple chemical ionisation methods with the same ionisation time.
To simplify the regular calibration of MION2, we developed an open-source flow reactor chemistry model (MARFORCE) to quantify the chemical production of sulfuric acid (H2SO4), hypoiodous acid (HOI) and hydroperoxyl radical (HO2). MARFORCE simulates convection-diffusion-reaction processes inside typical cylindrical flow reactors with uniform inner diameters. The model also provides options to simulate the chemical processes 1) when two flow reactors with different inner diameters are connected together and 2) when two flows are merged into one (connected by a Y-shape tee), but with reduced accuracy. Additionally, the chemical mechanism files in the model are compatible with the widely-used Master Chemical Mechanism, thus allowing future adaptation to simulate other chemical processes in flow reactors.
We further carried out detailed characterisation of the bromide (Br−) and nitrate (NO3−) chemical ionisation methods with different ionisation times. We calibrated H2SO4, HOI and HO2 by combining gas kinetic experiments with the MARFORCE model. Sulfur dioxide (SO2), water (H2O) and molecular iodine (I2) were evaluated using dilution experiments from a gas cylinder (SO2), dew point mirror measurements (H2O), and a derivatization approach in combination with high-performance liquid chromatography quantification (I2), respectively. We find that the detection limit is negatively correlated with the fragmentation enthalpy of the analyte-reagent ion (Br−) cluster, i.e., a stronger binding (larger fragmentation enthalpy) leads to a lower detection limit. Additionally, a moderately longer reaction time enhances the detection sensitivity thus decreasing the detection limit. For example, the detection limit for H2SO4 is estimated to be 2.9 × 104 molec. cm−3 with a 300 ms ionisation time. A direct comparison suggests that this is even better than the widely-used Eisele-type chemical ionisation inlet.
While the NO3− chemical ionisation method is generally more robust, we find that the Br− chemical ionisation method (Br−-MION2) is significantly affected by air water content. Higher air water content results in lower sensitivity for HO2 and SO2 within the examined conditions. On the other hand, a steep sensitivity drop of H2SO4, HOI and I2 is only observed when the dew point is greater than 0.5–10.5 ℃ (equivalent to 20–40 % RH; calculated at 25 ℃ hereafter). Future studies utilising atmospheric pressure Br− chemical ionisation method, including Br−-MION2, should carefully address the humidity effect on a molecular basis. By combining methods such as water-insensitive NO3−-MION2 with Br−-MION2, MION2 should be able to provide greater details of air composition than either of these methods alone.
Combining instrument voltage-scanning, chemical kinetic experiments and quantum chemical calculations, we find that the HIO3 detection is not interfered with by iodine oxides under atmospherically relevant conditions. The IO3−, HIO3NO3− and HIO3Br− ions measured using the Br− and NO3− chemical ionisation methods are primarily, if not exclusively, produced from gaseous HIO3 molecules.
Xu-Cheng He et al.
Status: open (until 28 Mar 2023)
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RC1: 'Comment on amt-2023-30', Anonymous Referee #1, 24 Mar 2023
reply
In "Characterisation of the multi-scheme chemical ionisation inlet-2 and the detection of gaseous iodine species", authors Xu-Cheng He and co-workers describe a new version of an inlet (MION) for chemical ionization mass spectrometry (CIMS) that allows for switching between multiple schemes and reaction times. Overall, they are presenting careful work that employed scientifically sound and appropriate methodology. Its findings will be very interesting for the CIMS community, especially of course if using a MION inlet or similar. Unfortunately, the manuscript itself was not prepared as carefully.
General positive points are the abstract, which I believe provides a good summary (except for some ambiguities noted below), as well as the figures, which are of mostly easy to read and of high quality and well-chosen by their relevance to the presented research.
However, for an AMT paper introducing a new CIMS inlet, its description is confusing and substantially lacking important details, as I try to elaborate in my detailed comments. This deficiency is most apparent in Section 2 (Methods), but found also in some parts of Section 3 ("Results") where additional experimental and analytical methodology are described.
In addition, the text will need some proofreading/copy-editing to deal with numerous grammatical errors. Some semantic errors disrupt the reading as well. Nonetheless, the text is in principle understandable.For these reason, I suggest to reconsider the manuscript only after major revisions.
[Disclaimer: I do not feel qualified to judge the various methods used for the DFT calculations (Section 2.4).]
Specific comments:Title: Much of this study deals with the Br- ionization scheme, and indeed provides useful insights into that specific scheme in particular, applicable also beyond the MION inlet systems. NO3-, for the most part, is rather used as a reference. Anyway, I would point that out already in the title. E.g., "... using Br- as reagent", or "... using Br- and NO3- as reagents".
Abstract: I suggest disclosing somewhere near the beginning that MION inlets operate (or are at least designed to operate) at atmospheric pressure.
L4 (abstract): "generally more robust operation" ... please be more specific.
L20 (abstract): Should specify if the detection limit for H2SO4 is achieved via Br- or NO3- reagents (or both).
L22 (abstract): again, "generally more robust" is too vague.
L95-102: Somewhat confusing description of the MION inlet. Not the only issue, but also: is "reaction time" the same as "ionisation time"? Most critically maybe: how does MION2 allow for two CI methods with same reaction time, and why was that not possible with the MION1? Please clarify.
After also checking Fig. 1, I think I get it. But now I wonder why only two CI at the same time (and same reaction time) (L101), and not three? (Or even six, if the polarity can be switched quickly as well, which remains unclear. And does "two or more (up to six)" simply mean "up to six" or something more elaborate, possibly including limitations regarding reaction time choice as in MION1?)Fig. 1: I suggest to indicate the directions of the various flows, to make the drawing easier to comprehend.
L108: How was the sample flow provided?
L110-122: What are typical/required/desired reagent flows, reagent concentrations, and purge flow(s!)?
Also, how are the reagent concentrations facilitated?Fig. A1 is missing the "purge flow" (and L119-122).
L124-125: How is that "operational stability" manifest or determined? If elaborated on later, please state so. If not, be more specific.
L125-127: How have the ion optics been upgraded? I am not expecting much details, but at least some indication of what type of effort was undertaken and required to increase "reagent ion transmission" (by which the authors might actually mean the amount of reagent ions ending up being detected?)?
Section 2.2.1: What exactly is the "calibration source"? It is referred to multiple times, but never actually specified what it is, except that OH radicals are generated "in it", or how it connects to other parts of the setup. (For example, are the OH radicals actively mixed into that SO2- and I2- containing gas mixture, or is the gas mixture going through a region where OH radicals is generated, and the "source" is the sum of something like that?)
Similarly, Fig. A2 simply refers to a mysterious "calibration box".Section 2.2.2: 1st paragraph is a general introduction to the problem that may fit better to the Introduction section.
Section 2.2.2: Unclear what was done why, and how the I2 was ultimately supplied during the MION calibration experiments. For example, was the permeation tube output used directly, and the process from dissolution in hexane to quantifying a concentrated solution of the derivative was only to gain knowledge of the I2 permeation tube output rate? (Which is presented as the conclusion of the main paragraph.)
Eq. 1: Q is not defined.
Table 1, Section 3.1: Detection limits are given for MION2/T1, MION2/T2 and Eisele inlets. But for H2SO4, either NO3- or Br- were used on MION2/T1, so, which reagent ion do the reported detection limits correspond to? And the Eisele inlet presumably used NO3-? If so, is there a reason that the NO3- scheme with MION2 was not tested for HIO3, as the Eisele inlet was?
Also, it is unclear at this stage what is meant by "APi1" and "APi2".~L344: Is fragmentation at atmospheric pressure (as opposed to only in the ion optics) responsible for the HO2 cal factors for T1 vs T2 being only a factor of 2.3 apart?
L360: I am not following the final sentence. As the authors just pointed out, they found (experimentally) that more strongly fragmenting instrument settings reduced sensitivity to HOI, agreeing with somewhat weaker binding between reagent and analyte, compared to the H2SO4 case, expected theoretically. So, why would one anyway blame "iodine chemistry schemes" or "differences in experimental conditions"? As those terms are rather vague, I may just misunderstand what is being pointed at. (Oh, is it differences in chemistries between the cited studies and this study?)
L383: Please provide a reference to that "earlier study".
L420: I disagree with the implication of the first half of this sentence. I agree that, for instance, within a typical day, ambient absolute humidity often does not vary by very much. But within, say, a week, one would expect substantial variations. And more so the longer of a time period is being considered...
Section 3.4: Figure A9 needs some more explanation, maybe via annotations in the figure (photo). Unclear what is what.
Section 3.5: I appreciate that experiments were carried out using two independent detectors and sample sources. But the discussion of the respective differences is awfully short. (And merely from a statistics point of view, a sample size of two is not that much better than a sample size of one.) Hence, is there anything useful to say about differences between APi1 and APi2 (or APi3 for that matter), beyond time since service? E.g., details on tunings, or purity of gas or calibrant supplies, etc.?
~L553: How was DeltaV50 determined? The shapes of the signal-remaining curves (Fig. 6) indicate that for several species the maximum is not obtained at the lowest tested DeltaV. The clearest case is H2O, for which the signal-remaining drops to 50% at ~3-4V, but the curve is steep and a higher reference value (signal-remaining = 1) would likely be obtained at yet "softer" settings (e.g., DeltaV < 2V). Correspondingly, if dV50 is simply the 50% point from Fig. 6, I expect several points in Fig. 7 being "too high".
Fig. 7: It would be very useful if the same color coding was used in Figs. 6 and 7, i.e., same color for same species.
Section 3.7: L568 vs L580 appear to contradict each other, even though using same reference. Is Reaction 4 exo- or endothermic?
Section 4 ("Conclusions"): This Section is really a summary of the results and the major discussion points presented in Section 3. With the exception of one or two sentences, it does not provide any new discussion nor actual conclusions. Consequently, it should be named accordingly. (Whereas Section 3 would be more aptly named "Results and Discussion".)
Technical comments:L14: More correct, I believe, to write "We calibrated for [...]"
L34: "spectrometer" -> "spectrometry" (for grammar)
L38: missing article ("forming a relatively")
[I will stop commenting on grammatical errors (or semantical errors or typos). Some proofreading/copy-editing service will be more suitable.]
L81: I assume the authors mean the LOD is higher (hence worse), not lower
Fig. A2: stain -> stainless
L150: "calibrator" or "calibration"?
L238-239: I am counting three ways, not two.
Citation: https://doi.org/10.5194/amt-2023-30-RC1
Xu-Cheng He et al.
Xu-Cheng He et al.
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