the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Jan 2026
Characterization of a Portable, Light-Weight, Low-Power Chemical Ionization Time-of-Flight Mass Spectrometer
Abstract. We have developed and characterized the performance of a portable time of flight chemical ionization mass spectrometer (Portable-TOF-CIMS) capable of detecting trace gases at parts per trillion by volume (pptv) mixing ratios in ambient air. The instrument is compact (0.063 m3), weighs less than 30 kg, and operates on 270 W of 24 VDC power. These characteristics allow it to be readily deployed on a range of mobile or stationary platforms with little electrical or structural engineering considerations. The mass spectrometer achieves a mass resolving power of (m/Δm) at mass-to-charge of 381 (m/Q) and a mass accuracy of < 10 ppm. The instrument can operate in both positive or negative polarity and therefore can detect a suite of different analytes depending upon the reagent ion chemistry. We demonstrate the instrument response to inorganic and organic trace gases using Iodide anion adduct and Benzene cation reagent ion chemistries and illustrate its performance sampling ambient air during a multi-week stationary deployment and mobile deployments from two different personal automobiles and from a cargo e-bike using only a battery to power the instrument during operation.
Competing interests: All authors declare that they have no conflict of interest. Felipe Lopez-Hilfiker and Urs Rohner are employees of Tofwerk AG who is a manufacturer & supplier of chemical ionization time of flight mass spectrometers.
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: final response (author comments only)
- RC1: 'Comment on egusphere-2025-5685', Anonymous Referee #1, 19 Jan 2026
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RC2: 'Comment on egusphere-2025-5685', Anonymous Referee #2, 15 Feb 2026
This paper developed and characterized the performance of a Portable-ToF-CIMS. This new lightweight and low-power instrument has high sensitivities and acceptable mass resolving power, allowing its great use in a range of mobile or stationary platforms for characterizing ppt level of trace gases. The authors demonstrate the performance of this instrument through short-period field measurements. This paper has important implications in atmospheric chemistry and air quality studies and therefore well within the scope of AMT. It can be recommended for publication after addressing the following comments.
Specific comments:
Lines 112-115: Can the authors provide more details on how calibration was performed, e.g., the number of data points? It looks to me that six different levels of standard gases were measured (Fig. 2a) while only four data points were used to generate the calibration curve (Fig. 2b). How would the calibration curve change if all data were included?
Line 133-135: How long would it take to power on the instrument and get a stable reagent ion signal?
Line 166-167: The correlation coefficient of the fit should be given.
Line 201-202: The authors stated that “The dynamic range (<1 pptv to > 1 ppbv), ppt-level precision and low detection limits of the instrument are evident.”. However, only normalized signals were shown in Fig. 5. The mixing ratios of these key pollutants should be provided.
Line 202-205: The morning peak of HONO started increasing at around 3 am. Is this due to automobile traffic or ground elements? Is the signal in Fig. 5b in units of ncps or pptv? If it is pptv, the HONO mixing ratio at noon time is high. Is this consistent with previous studies?
Line 211-213: The maximum mixing ratio of HONO can reach 10 ppbv. It seems like a high value. What is the typical concentration of NOx in this area?
Line 224-227: Did C6H10O5 correlate with acetonitrile, another typical tracer of biomass burning? Mixing ratios should be given in Fig. 6, just like what the authors did for Fig. S1.
Line 236-238: Are there any other VOCs emitted from grilling detected, e.g., alkanals (Klein et al., 2016)?
Technical comments:
Line 16: Iodide and Benzene should be in lowercase.
Line 41: Spell out the full term of VOC and VCP. Check out the whole manuscript for similar issues.
References:
Klein, F., Platt, S.M., Farren, N.J., Detournay, A., Bruns, E.A., Bozzetti, C., Daellenbach, K.R., Kilic, D., Kumar, N.K., Pieber, S.M., Slowik, J.G., Temime-Roussel, B., Marchand, N., Hamilton, J.F., Baltensperger, U., Prévôt, A.S.H., El Haddad, I., 2016. Characterization of Gas-Phase Organics Using Proton Transfer Reaction Time-of-Flight Mass Spectrometry: Cooking Emissions. Environ. Sci. Technol. 50, 1243-1250.
Citation: https://doi.org/10.5194/egusphere-2025-5685-RC2 -
CC1: 'Comment on egusphere-2025-5685', Uma Puttu, 22 Feb 2026
This manuscript describes an impressive portable CIMS. Because benzene cation is used as the reagent ion in this study, I would like to draw the authors’ attention to our recent work1, which focuses on benzene ion chemistry and has recently been accepted. It may be relevant to the interpretation of your measurements.
In our study, we observed several behaviors that were somewhat puzzling and I wonder whether you have observed similar phenomena. For example, during isoprene calibrations in zero air, we observed multiple fragment and oxygenated ions in addition to the parent ion (C11H14+). I am curious whether a similar fragmentation pattern was observed in your experiments.
We also found a strong water-dependent sensitivity, which was challenging to correct using the measured water signal alone. I would be interested in your perspective on how this issue might be addressed when converting ncps to more meaningful mixing ratios. I fully understand that this question may be beyond the scope of the present study, but I raise it in the spirit of interactive discussion.
Lastly, I noticed a potential inconsistency between Figures 2A and 3A. In Figure 2A, the ratio of xylene to isoprene appears to be approximately 8, whereas in Figure 3A the same ratio appears to be closer to 2, assuming C8H10+ and C11H14+ are used to quantify xylene and isoprene, respectively.
References:
- Puttu, U., Kamp, J. R., Chen, X., Chen, J.-H., Li, J., Gonzalez-Meler, M. A., Wang, J., and Xu, L.: Chemical ionization mass spectrometry utilizing benzene cations for measurements of volatile organic compounds and nitric oxide, EGUsphere, 1–29, https://doi.org/10.5194/egusphere-2025-4103, 2025.
Citation: https://doi.org/10.5194/egusphere-2025-5685-CC1
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Please find the referee comment in the attached pdf.