the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Jan 2024
Response of protonated, adduct, and fragmented ions in Vocus proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS)
Abstract. Volatile organic compounds (VOCs) affect secondary pollutant formation via active chemistry. Proton-transfer-reaction mass spectrometry (PTR-MS) is one of the most important techniques to study the highly variable spatial and temporal characteristics of VOCs. The response of protonated, adduct, and fragmented ions in PTR-MS in changing instrument settings and varying relative humidity (RH) requires rigorous characterization. Herein, dedicatedly designed laboratory experiments were conducted to investigate the response of these ions for 21 VOCs, including 12 oxygenated VOCs and two nitriles, using the recently developed Vocus PTR-MS. Our results show that the focusing ion-molecule reactor (FIMR) axial voltage increases sensitivity by three to four orders of magnitude but does not significantly change the fractions of protonated ions. Reducing the FIMR pressure, however, substantially increases fragmentation. Applying a high radio frequency (RF) amplitude radially on FIMR can enhance sensitivity by one to two orders of magnitude without affecting the protonated ion fractions. The change in big segmented quadrupole (BSQ) amplitude mainly affects sensitivity and protonated ion fraction by modifying ion transmission. The relationship between sensitivity and proton-transfer reaction rate constant is complicated by the influences from both ion transmission and protonated ion fraction. The protonated ions of most VOCs studied (19 out of 21) show less than 15 % variations in sensitivity as RH increases from ~5 % to ~85 %, except for some long-chain aldehydes which show a positive RH variation of up to 30 %. Our results suggest that the Vocus PTR-MS can reliably quantify the majority of VOCs under ambient conditions with varying RH. However, caution is advised for small oxygenates such as formaldehyde and methanol due to their low sensitivity, as well as for long-chain aldehydes for their slight RH dependence and fragmentation.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2024-16', Anonymous Referee #1, 15 Feb 2024
This study investigated the response of VOC product ions in the Vocus proton transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) by manipulating instrument settings and varying relative humidity (RH). The findings demonstrated the reliable quantification capability of Vocus PTR-MS for a wide range of VOCs under ambient conditions with varying RH, while noting some underestimation for small oxygenates and long-chain aldehydes. The manuscript is well-written and have significance findings for future applications in the environmental atmospheric field. However, there are several issues that require addressing prior to publication.
Major Comments:
- The quantification of VOCs using PTR-MS primarily relies on their proton transfer reactions with H₃O⁺. However, it is also necessary to consider the reaction involving H₃O⁺(H₂O)ₙ (where n = 1, 2, 3...) for species with a higher proton affinity (PA) than that of water clusters. Therefore, it is crucial to conduct all experiments in an environment where the reaction with H₃O⁺ dominates and to identify the key parameters (E/N, FIMR, RF, BSQ…) that primarily minimize the influence of H₃O⁺(H₂O)ₙ.
- The experiment utilized two gas cylinders containing mixed standard gases. So how to make sure that the results of the adduct/fragmented ions in Figure 2 are not interfered by different species in the mixed standard gases as the reactions occurring in VOCUS (R1-R5) are considerably complex.
- The findings regarding methanol present a puzzling scenario. Figure 2 shows that the majority of product ions from methanol are fragmented ions of [MH+H2O]+ (m/Q=51), accounting for 97% of the total. However, Figure 4 suggests that the sensitivity and transmission of methanol remain unchanged when considering all product ions. It is well-established that methanol can be effectively analyzed using traditional PTR and has been widely utilized in atmospheric analysis. Therefore, it is imperative to determine whether the reduced sensitivity of protonated methanol in VOCUS is primarily due to lower transmission or the prevalence of [MH+H2O]+
Minor Comments:
- Please ensure the form of symbols is written consistently in Section 4.2.
- Line 202. The label of the green color in Figure 2 should be consistently expressed as [MH+H2O]+.
- Lines 492-493. “the reverse reaction of R1 might be important for compounds with low PA and low kptr values” needs to be cited with relevant references.
- Lines 414-415. The title and content are reversed in Figures 4 c and d. It seems only the m/Q values of MH+ are used in the x axis of panel c.
- Line 522. What does “RH+ ions” mean?
- There are many expressions of “drift tube”. I suggest replacing the “drift tube”with the FIMR, as the “drift tube” is the reaction chamber in traditional PTR and it has been optimized in VOCUS.
Citation: https://doi.org/10.5194/egusphere-2024-16-RC1 -
AC1: 'Reply on RC1', Yong Jie Li, 26 Feb 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-16/egusphere-2024-16-AC1-supplement.pdf
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RC2: 'Reviewer comment on egusphere-2024-16', Anonymous Referee #2, 15 Feb 2024
This manuscript describes a series of tests performed on the newly developed Vocus PTR-MS to characterize the instrument response to humidity and various internal settings (voltages and pressures) for 21 commonly measured volatile organic compounds. The authors show that sensitivities and ion distributions for most species showed little variation with humidity, and provide recommendations for instrument settings to maximize sensitivities. The manuscript is well written, straightforward, easy to follow, and provides useful benchmarks that instrument users and developers will certainly find helpful. I would recommend publication following clarification of a few minor points:
It should be mentioned somewhere which species are in which standard tank, and some evidence needs to be provided that the various fragments of each compound in an individual tank do not overlap with each other in any way that could obfuscate the results. Were these compounds ever tested outside the tanks to check that?
How long were the tanks allowed to equilibrate on the instrument before ramping concentrations, humidity, and other variables? And was it ever demonstrated that the wait times of 30 minutes for one tank and 120 minutes for the other (L 171-173) were sufficient for stabilisation? From Figure S5 it appears that some species' signals are still creeping up by the end of the step. How much error could this introduce in the calibrations and the determinations of their sensitivities to instrument parameters?
Similarly, was it ever demonstrated that 15 minute steps of humidity (L 181-182) were sufficient to allow for signal stabilisation at each step?
L 484: "the rest one" should be "the other one"
Citation: https://doi.org/10.5194/egusphere-2024-16-RC2 - AC2: 'Reply on RC2', Yong Jie Li, 26 Feb 2024
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-16', Anonymous Referee #1, 15 Feb 2024
This study investigated the response of VOC product ions in the Vocus proton transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) by manipulating instrument settings and varying relative humidity (RH). The findings demonstrated the reliable quantification capability of Vocus PTR-MS for a wide range of VOCs under ambient conditions with varying RH, while noting some underestimation for small oxygenates and long-chain aldehydes. The manuscript is well-written and have significance findings for future applications in the environmental atmospheric field. However, there are several issues that require addressing prior to publication.
Major Comments:
- The quantification of VOCs using PTR-MS primarily relies on their proton transfer reactions with H₃O⁺. However, it is also necessary to consider the reaction involving H₃O⁺(H₂O)ₙ (where n = 1, 2, 3...) for species with a higher proton affinity (PA) than that of water clusters. Therefore, it is crucial to conduct all experiments in an environment where the reaction with H₃O⁺ dominates and to identify the key parameters (E/N, FIMR, RF, BSQ…) that primarily minimize the influence of H₃O⁺(H₂O)ₙ.
- The experiment utilized two gas cylinders containing mixed standard gases. So how to make sure that the results of the adduct/fragmented ions in Figure 2 are not interfered by different species in the mixed standard gases as the reactions occurring in VOCUS (R1-R5) are considerably complex.
- The findings regarding methanol present a puzzling scenario. Figure 2 shows that the majority of product ions from methanol are fragmented ions of [MH+H2O]+ (m/Q=51), accounting for 97% of the total. However, Figure 4 suggests that the sensitivity and transmission of methanol remain unchanged when considering all product ions. It is well-established that methanol can be effectively analyzed using traditional PTR and has been widely utilized in atmospheric analysis. Therefore, it is imperative to determine whether the reduced sensitivity of protonated methanol in VOCUS is primarily due to lower transmission or the prevalence of [MH+H2O]+
Minor Comments:
- Please ensure the form of symbols is written consistently in Section 4.2.
- Line 202. The label of the green color in Figure 2 should be consistently expressed as [MH+H2O]+.
- Lines 492-493. “the reverse reaction of R1 might be important for compounds with low PA and low kptr values” needs to be cited with relevant references.
- Lines 414-415. The title and content are reversed in Figures 4 c and d. It seems only the m/Q values of MH+ are used in the x axis of panel c.
- Line 522. What does “RH+ ions” mean?
- There are many expressions of “drift tube”. I suggest replacing the “drift tube”with the FIMR, as the “drift tube” is the reaction chamber in traditional PTR and it has been optimized in VOCUS.
Citation: https://doi.org/10.5194/egusphere-2024-16-RC1 -
AC1: 'Reply on RC1', Yong Jie Li, 26 Feb 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-16/egusphere-2024-16-AC1-supplement.pdf
-
RC2: 'Reviewer comment on egusphere-2024-16', Anonymous Referee #2, 15 Feb 2024
This manuscript describes a series of tests performed on the newly developed Vocus PTR-MS to characterize the instrument response to humidity and various internal settings (voltages and pressures) for 21 commonly measured volatile organic compounds. The authors show that sensitivities and ion distributions for most species showed little variation with humidity, and provide recommendations for instrument settings to maximize sensitivities. The manuscript is well written, straightforward, easy to follow, and provides useful benchmarks that instrument users and developers will certainly find helpful. I would recommend publication following clarification of a few minor points:
It should be mentioned somewhere which species are in which standard tank, and some evidence needs to be provided that the various fragments of each compound in an individual tank do not overlap with each other in any way that could obfuscate the results. Were these compounds ever tested outside the tanks to check that?
How long were the tanks allowed to equilibrate on the instrument before ramping concentrations, humidity, and other variables? And was it ever demonstrated that the wait times of 30 minutes for one tank and 120 minutes for the other (L 171-173) were sufficient for stabilisation? From Figure S5 it appears that some species' signals are still creeping up by the end of the step. How much error could this introduce in the calibrations and the determinations of their sensitivities to instrument parameters?
Similarly, was it ever demonstrated that 15 minute steps of humidity (L 181-182) were sufficient to allow for signal stabilisation at each step?
L 484: "the rest one" should be "the other one"
Citation: https://doi.org/10.5194/egusphere-2024-16-RC2 - AC2: 'Reply on RC2', Yong Jie Li, 26 Feb 2024
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Cited
Fangbing Li
Dan Dan Huang
Linhui Tian
Wen Tan
Liang Zhu
Penglin Ye
Douglas Worsnop
Ka In Hoi
Kai Meng Mok
Yong Jie Li
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(1806 KB) - Metadata XML
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Supplement
(3020 KB) - BibTeX
- EndNote
- Final revised paper