the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Apr 2026
Isomer-resolved online analysis of organic aerosols using ion mobility mass spectrometry
Abstract. Secondary organic aerosol (SOA) makes up much of the particulate matter in the troposphere and impacts global climate and human health, though uncertainties regarding the sources and properties of SOA limit our understanding of these effects. New analytical techniques are required to better characterize the molecular composition of SOA, including methods that can identify isomeric compounds that may have different contributions to SOA properties such as hygroscopicity or volatility. We present a method for isomer-resolved analysis of SOA using a commercially available chemical ionization ion-mobility time-of-flight mass spectrometer (CI-IMS-TOF) and a Vaporization Inlet for Aerosols (VIA). The compatibility of the VIA and the CI-IMS-TOF was assessed through the analysis of 10 carboxylic acid standards across a large temperature range (30 - 170 °C). Ion drift times were found to be stable to within 0.075% of their initial values after drift time calibration. The VIA-CI-IMS-TOF was also used to collect real-time ion mobility and mass spectra of SOA constituents during an α-pinene ozonolysis chamber experiment. Several reaction products were identified in the SOA using synthetic standards, including structural isomers of C8H12O4 and C9H14O4. Temporal evolution of reaction products was used to assess formation timescales and determine the generation of oxidation for individual isomers. Both iodide and bromide reagent ions were used in the VIA-CI-IMS-TOF to achieve a more comprehensive analysis of SOA. This study demonstrates the performance of the VIA-CI-IMS-TOF for online, isomer-resolved analysis of organic aerosol and its potential for improving the current understanding of SOA composition.
Competing interests: MRC and HS are employees of Aerodyne Research Inc., which sells the VIA and CI-IMS-TOF
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 22 May 2026)
- RC1: 'Comment on ar-2026-13', Kangwei Li, 19 May 2026 reply
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Figure Files Schaum et al. https://cires1.colorado.edu/jimenez/group_pubs.html
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- 1
This manuscript develops a new technique called VIA-CI-IMS-TOF for online and isomer-resolved aerosol chemical analysis. The authors evaluated the performance of this instrumentation using 10 different carboxylic acids over a wide range of temperature. They further applied this technique into a chamber experiment with ozonolysis of a-pinene, which is able to prove its ability for identifying some isomers at real time. SOA are complicated mixture with diverse composition and structures. IMS is a promising technique coupling with MS for real-time isomer-resolved measurement. This technique is expected to deepen our molecular-level characterization and understanding of SOA. I strongly recommend its publication after addressing the below comments
Major comments
1. I never work with VIA and CI-IMS-TOF. Coupling both is a smart idea and the key aspect of this study. Since VIA and CI-IMS-TOF are both commercially available, I wonder whether this combination (or so-called coupling) could be readily used/achieved straightforward if both are available? What is the key challenge of achieving such combination?
2. I also wonder whether it is possible to achieve real-time and isomer-resolved gas-phase and particle-phase analysis simultaneously during one single chamber experiment, e.g. by switching two lines of charcoal denuder and HEPA particle filter in front of VIA?
3. Line 145-150 The authors mentioned that standards are vaporized into the chamber and then condensing onto seed particle, as their first sets of chamber expts. What type of seed particles is used during these expts? Is there any specific reason that standards are prepared in methanol not water? Also, I am a bit confused with the later text that some standards were pipetted onto a PTFE filter that placed between VIA and AIM (CI-IMS-TOF), but using heated chamber air to evaporate analytes from PTFE filter. It seems these are two methods of bringing standards into the instrument. Why two methods are required? Are they targeting for different standards? Can the authors explain why not directly injecting these standards by nebulizing the standard solution into organic particles following a dryer before entering into VIA-CI-IMS-TOF?
4. Fig. 1 shows that these standards are nicely separated at different drift time. Can drift time information be used to infer to some chemical properties? e.g. HPLC retention time is an indicator of polarity. Also, why only using carboxylic acids as standards in the entire study? Could this technique apply for other non-carboxylic acid compounds such as esters, peroxides, carbonyls, etc?
5. For this VIA-CI-IMS-TOF technique, I wonder whether it is possible to demonstrate its quantification ability by varying the concentration of standards, and providing quantification capability information such as detection limit (e.g. LOD, LOQ) and linear range like traditional chemical analysis?
6. For chamber expt as shown in Fig. 3 and Fig. 5, many isomer peaks are not baseline separated. Is there any hope that this issue could be further improved in IMS settings?
7. I also wonder how did the authors fit those not-well-resolved peaks, e.g. how the software treating the data automatically and identifying those isomer peaks in IMS for each m/z? Currently only a few formulas with standards are shown for chamber SOA, but I assume there should be many isomer peaks for many other compounds. I wonder how many isomers are found for other formulas, e.g. dimers and other compounds? The most simple way is to show a typical molecular fingerprint like HPLC-HRMS using rt vs. m/z, therefore I am curious if the authors could provide a scatter plot and/or excel sheet for chamber a-pinene SOA, by showing drift time vs. m/z while dots were coloured by peak area/intensity? This plot could be used in the maintext to strengthen the current method for a broader identification of isomer-resolved peaks
8. It seems there are two identical chamber SOA expts but switching between Br- and I- reagent, am I right? For Fig. 5, I guess Br- and I- give two different drift tube time for each standard. Can the authors label the name of the corresponding isomer peak for each standard? Indeed, Br- and I- reagent ions could provide more detailed chemical information, but it might also make the data analysis and interpretation even more complicated, and I think the current result is still very preliminary.
9. I could not find the detailed information that how the drift time calibration was performed in sec 3.1. If I understand correctly, this drift time calibration is only necessary for VIA temperature ramping experiment. It seems VIA was operated at a fixed temperature of 200 C for chamber SOA expts. Is there any specific reason of choosing this fixed temperature that is different than ramping 30-170 C used for standards? By the way, would VIA operating at multiple temperature give more information?
Minor comments
Line133 “peak s” → “peaks”
For Fig. 2, terpenylic acid formula should be C8H12O4
For the data showing in Fig. 3 and Fig. 5, what is the IMS collected time during the chamber SOA expt? Are they referring to the averaged ~1 minute near the end of the experiment?