the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Jan 2025
The Coupling of a High-efficiency Aerosol Collector with Electrospray Ionisation/Orbitrap Mass Spectrometry as a novel tool for Real-time Chemical Characterisation of Fine and Ultrafine Particles
Abstract. The chemical properties of aerosols in the atmosphere significantly influence their impact on the global climate forcing and human health. However, a real-time molecular-level characterisation of aerosols remains challenging due to the complex nature of their chemical composition. The current study constructed an instrumental system for the real-time chemical characterisation of aerosol particles. The proposed setup consists of a custom-built high-efficiency aerosol collector (HEAC) used to collect aerosol samples into a working fluid and an electrospray ionisation (ESI) Orbitrap Mass spectrometer (MS) for the subsequent chemical analysis of the liquid sample. The HEAC/ESI-Orbitrap-MS was calibrated against six chemical compounds to investigate the system’s sensitivity and limit of detection (LOD). Results showed that the coupled system has high sensitivities towards the tested chemical compounds and a similar, if not better, LOD than other related instrumental techniques. The capability of the HEAC/ESI-Orbitrap-MS system to identify the chemical composition of organic aerosols (OA) was also examined. Sample OA was generated by α-pinene ozonolysis, and the chemical characterisation results were compared to similar studies. Our data showed that the HEAC/ESI-Orbitrap-MS system can identify most of the α-pinene ozonolysis products reported in the literature, including cis-pinonic acid, pinalic acid and 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA). Monomeric and dimeric reaction products were accurately identified in the mass spectra, even at a total OA mass concentration < 2 µg m-3. The present study showed that the HEAC/ESI-Orbitrap-MS system is a robust technique for the real-time chemical characterisation of OA particles under atmospheric relevant conditions.
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RC1: 'Comment on amt-2024-192', Anonymous Referee #1, 31 Jan 2025
This paper describes a measurement system that couples two sophisticated analytical techniques; a vortex wet chemical aerosol collector, and electrospray Orbitrap high resolution mass spectrometry (ESI-MS). The method was demonstrated for six different organic compounds, analyzed individually. This is an interesting and potentially useful technique. The paper is quite short and lacking in important detail. There are a number of general and specific comments and questions that will need to be dealt with before it is acceptable for publication.
General;
The mass spectrometer apparently has very impressive resolution. Show us. Pick one or two sets of ions at a couple of nominal masses and show us a mass spectrum.
The materials of construction (tubing materials and dimensions) of the analytical system are not adequately described.
There is at least one previous study that coupled and aerosol sampling method with ESI/MS (Stockwell et al., 2018) that showed that SMPS alone can have problems when relied on as the primary calibration method. Also, that methods relies on some assumptions in order derive the actual aerosol mass concentration. For example, are the authors assuming the generated particles are spheres with densities equal to the pure materials? Multiple charging can introduce errors, as noted by Stockwell et al. 2018. Did the authors consider that in their analysis of the SMPS?
Specific;
Abstract – there could be a lot more detail here. Just tell us compounds that were used in the study, there are only six. Tell us the actual mass resolution that was achieved in this study. Tell us what the limits of detection were, and how they were derived (1 sigma?).
Lines 38-48. One aspect missing from this is the fact that many sampling methods (e.g. filters) loose semi-volatile species. The authors seem unaware of the FIGAERO inlet work that couples short-term filters with HR-ToF-MS (Lopez-Hilfiker et al., 2014).
Line 59. The ToF MS systems currently in commercial use can often provide adequate resolution at lower masses (<100 amu), they can struggle with complicated mixtures at higher masses.
Lines 61-77. The authors appear unaware of the work of Stockwell et al., 2018, who coupled a PiLS system with negative ion electrospray (quadrupole) MS.
Line 76 – 89. These sentences and paragraphs fragments are all mixed up and out of order. Lines 82-86 belong at the end of the Line 61-76 paragraph. The sentence on lines 76-77 and the other pieces in lines 78-89 should form the last paragraph of the introduction where you tell us what you will be presenting in this paper.
Methodology: This entire section was described without once telling us what polarity you were using for the ESI inlet.
Lines 118-120. You have skipped a step here. These are solids, so you must have used a solvent in order to nebulize them. What solvent and at what concentrations?
Line 123. This would be a good place to present all the assumptions that go into using the SMPS.
Lines 124-129. Please show us some plots of MS signal versus time, what do those look like, the y-axis error bars in Figure 2 imply there is quite a bit of variability in some of them.
Line 156. How accurate is the assumed density and what factors might affect that?
Line 162. I can’t tell what the phrase ‘prove the functionalities’ means, please explain.
Section 2.3. Please show us a close-up of the mass separation achievable by the Orbitrap at one or two nominal masses.
Lines 190-192. Would inorganic salts create matrix effects that would alter sensitivities.?
Figure 2. It seems inappropriate to extrapolate the fit line for VA way past the data points as it is done in this figure.
Lines 233. It would be interesting to see the background signals that are being referred to.
Figure 4. The colors are hard to discern and are not color-blind compatible.
Figure 6. The designations ±H and ±O mean nothing to those not intimately familiar with mass defects. Please show us which direction is which and explain what they mean as far as H and O content go.
Line 281. What do you mean by ‘the agreement between’ are you talking about the agreement in the time profile?
Line 299. Isn’t there a limit in how small the inner diameter of tubing can be? It would help if you told us in the Methods Section what it is.
Figure 7. It would help in understanding this figure if you put shaded areas behind the time traces for each sampling mode.
Line 318. The ‘apparent increase’ is not very convincing. Maybe if you showed time averaged data.?
Lines 344-345. What is responsible for the 2-minute time constant? What is the residence time of the liquid in the vortex collector?
Lines 350-352. We have no context with which to evaluate these statements on improving the time response since we haven’t been given the information of materials and dimensions of the tubing and we don’t know what the time constant of the vortex collector is. It seems like the vortex would be something of a well-mixed element that would tend to smear out the signals in time.
Lines 385. What was the mass resolution (M/DM) for actual mass spectra?
The reference list needlessly hard to read. Please put line breaks between citations or indent the first line of a citation
SI – please put page numbers on.
SI Eq1. It looks like you are quoting 1 sigma LODs? If true, that needs to be specified when you put those numbers in the abstract and elsewhere.
Table S1: Adonitol vapor pressure is 1.51 Pa and Erythritol vapor pressure is 6.36 ´10-4 Pa? I don’t believe it, something is wrong. I checked my version of EPI suite and it says the same thing as the table, but it doesn’t make sense if you look the two structures.
Table S4. This way of sorting the data, C#, is not very useful. I’d rather see it by exact mass. It would show where the clusters are (which would give you a good guide as to what nominal mass could be used to show the MS resolution). It also makes the mass defect O# count dependence clear.
References:
Lopez-Hilfiker, F. D., Mohr, C., Ehn, M., Rubach, F., Kleist, E., Wildt, J., Mentel, T. F., Lutz, A., Hallquist, M., Worsnop, D., and Thornton, J. A.: A novel method for online analysis of gas and particle composition: description and evaluation of a Filter Inlet for Gases and AEROsols (FIGAERO), Atmos. Meas. Tech., 7, 983-1001, 2014.
Stockwell, C. E., Kupc, A., Witkowski, B., Talukdar, R. K., Liu, Y., Selimovic, V., Zarzana, K. J., Sekimoto, K., Warneke, C., Washenfelder, R. A., Yokelson, R. J., Middlebrook, A. M., and Roberts, J. M.: Characterization of a catalyst-based conversion technique to measure total particle nitrogen and organic carbon and comparison to a particle mass measurement instrument, Atmos. Meas. Tech., 11, 2749-2768, 2018.
Citation: https://doi.org/10.5194/amt-2024-192-RC1 -
RC2: 'Comment on amt-2024-192', Anonymous Referee #2, 10 Mar 2025
This manuscript presents an instrumental system combining a high-efficiency aerosol collector (HEAC) with ESI Orbitrap MS. This would be a potentially useful online instrument. While some descriptions are too much simplified, and some important details on the instrument are missing. The following comments need to be addressed:
1. The ionization efficiency of ESI is very sensitive to the sample matrix and ambient conditions. The authors also indicate these aspects in section 4.3. I may suggest to consider these effects more carefully.
(1) How to correct the variation of detected intensity caused by different sample matrix? If there are inorganic ions or organic molecules with high ionization efficiency, the ionization efficiency of one molecule would vary a lot. I agree that we cannot exclude the problems of ion suppression during the ionization of ESI, while the correction procedures need to be detailed here.
(2) How about the stability of the instrument? I just wonder how much the detected intensity will change when ambient conditions, such as temperature, humidity, vary. For the measurement of ambient aerosol samples, ambient conditions are always changing.
2. Six chemical compounds were used to Mass spectrometer equipped with ESI is quite good at analyzing organic molecules with -NO2, -OSO3 function groups, which has been widely used to quantify compounds such as nitro-aromatics, organosulfates, etc. I may suggest to test the sensitivity and LOD of these compound groups, at least test some commercial nitroaromatic compounds.
3. The author mentioned the possibility of thermal decomposition in ESI. Even if this cannot be concluded in the current study, some basis or evidence should be mentioned.
Citation: https://doi.org/10.5194/amt-2024-192-RC2
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