Edited by: Paolo Tosi, University of Trento, Italy
Reviewed by: Oh-Hoon Kwon, Ulsan National Institute of Science and Technology, South Korea; Fuminori Misaizu, Tohoku University, Japan; Pawel K. Misztal, University of California, Berkeley, United States
This article was submitted to Physical Chemistry and Chemical Physics, a section of the journal Frontiers in Chemistry
†Present Address: Noora Hyttinen, Nano and Molecular Systems Research Unit, University of Oulu, Oulu, Finland
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In this study, we present reactions of
In the 1990's proton transfer reaction mass spectrometry (Hansel et al.,
is exothermic and will occur on every collisions, which means that the reaction rate is close to the collisional limit value (Lindinger et al.,
Here we present detailed results on the mechanism of chemical ionization of eight monoterpenes (C10H16) by
The experimental setup is illustrated in
Illustration of the experimental setup. Calibration compounds were either taken from the calibration gas standards or from the diffusion source, which was flushed with a constant synthetic air flow controlled by a mass flow controller (MFC). Different humidity steps were adjusted with the Liquid Calibration Unit (LCU) and monitored by an infrared gas analyzer (IRGA). The humidified synthetic air containing the respective calibration compound(s) was introduced to a selective reagent ion time of flight mass spectrometer (SRI-ToF-MS).
The Liquid Calibration Unit (LCU, Ionicon Analytik, Austria) was used to quantitatively evaporate certain amounts of water into the synthetic air stream resulting in absolute humidities in the range of 3–30 ppth. The calibration of the ketones was performed by dynamic dilution of calibration gas standards (Apel Riemer Environmental Inc., Broomfield (CO), USA) in a humidified carrier gas generated by the LCU. Highly water-soluble compounds can be calibrated precisely with the LCU. As monoterpenes are non-polar compounds they are not quite soluble in water. Therefore, we decided to build a temperature-controlled diffusion device, which was combined with the humidified synthetic air stream from the LCU to generate known amounts of monoterpenes in the parts per billion range.
Each diffusion tube containing several milliliters of the respective liquid calibration compound consisted of a 1/8 inch PFA tubing plug (Parker-Hannifin Corporation, Tucson, USA) connected to a PEEK capillary (Vici Valco, Switzerland) of defined length and inner diameter. The capillary was connected through a gas tight 1/8–1/4 inch tee reducer with the carrier gas stream from the LCU. As described in Fuller et al. (
where D [cm2 s−1] is the diffusion coefficient, M [g mol−1] the molecular weight of the analyte, P [Pa] the total pressure, A [cm2] the cross-sectional area of the capillary, L [cm] the length of the capillary,
The Selective Reagent Ion Time-of-Flight Mass Spectrometer (SRI-ToF-MS) used in this study is based on the PTR-ToF-MS described by Graus et al. (
In the drift tube, the ions travel as a result of the applied electric field strength E with an increased drift velocity vd through the buffer gas (Lindinger et al.,
We used ion mobility values μ0 of
The reduction of reagent ions [
Where [A] is the density of analyte A, k the reaction rate coefficient and t the drift time. The ratio
We compare the calculated sensitivity with the experimentally observed sensitivity ε
The center-of-mass kinetic energy
With
where
Collisional limiting rate coefficients (kc) of ion molecule reactions are calculated according to Su and Chesnavich (
Overview of collisional rate coefficients kc, collision energies KEcm, calculated sensitivities εcalc, measured sensitivities εmeas and reaction efficiencies (eff) at dry and humid conditions.
Acetone | 4.19 | 0.055 | 78.0 | 3.3 | 4.2 | 7.8 | 9.9 | 4.0 | 0.078 | 46.4 | n.d. | – | n.d. | – |
MVK | 4.22 | 0.056 | 78.6 | 4.6 | 5.9 | 8.6 | 11.0 | 4.0 | 0.080 | 47.0 | n.d. | – | n.d. | – |
MEK | 4.05 | 0.056 | 75.5 | 7.1 | 9.4 | 11.5 | 15.2 | 3.9 | 0.080 | 45.3 | n.d. | – | n.d. | – |
α-Pinene | 2.44 | 0.057 | 45.5 | 14.5 | 31.9 | 8.6 | 19.0 | 2.4 | 0.084 | 28.6 | 10.7 | 37.5 | 10.0 | 34.8 |
β-Pinene | 2.61 | 0.057 | 48.7 | 13.3 | 27.3 | 9.1 | 18.8 | 2.6 | 0.084 | 30.2 | 8.7 | 28.8 | 9.5 | 31.3 |
Camphene | 2.48 | 0.057 | 46.2 | 15.6 | 33.6 | 11.0 | 23.8 | 2.5 | 0.084 | 29.0 | 10.0 | 34.5 | 9.7 | 33.6 |
3-Carene | 2.52 | 0.057 | 46.9 | 15.3 | 32.6 | 7.8 | 16.6 | 2.5 | 0.084 | 29.5 | 10.6 | 35.9 | 9.0 | 30.5 |
Limonene | 2.56 | 0.057 | 47.6 | 9.6 | 20.2 | 5.9 | 12.3 | 2.5 | 0.084 | 29.8 | 5.1 | 17.1 | 5.3 | 17.9 |
Myrcene | 2.68 | 0.057 | 49.8 | 10.6 | 21.3 | 5.8 | 11.6 | 2.7 | 0.084 | 31.2 | 8.9 | 28.5 | 7.6 | 24.4 |
Ocimene | 3.70 | 0.057 | 68.8 | 12.6 | 18.4 | 4.8 | 7.0 | 3.5 | 0.084 | 41.4 | 12.4 | 29.8 | 6.0 | 14.5 |
Sabinene | 2.76 | 0.057 | 51.5 | 16.3 | 31.7 | 13.2 | 25.6 | 2.6 | 0.084 | 30.7 | 12.5 | 40.5 | 13.0 | 41.2 |
Proton affinities are not available for most monoterpenes, thus quantum chemical calculations were performed. Known proton affinities for ammonia, acetone, methyl vinyl ketone and methyl ethyl ketone were compared with our calculations. Additionally, the change of standard enthalpies for the ion-molecule reactions, possible protonation sites and probable
(R)-(-)-limonene (analytical standard), ocimene (mixture of isomers, >90%), camphene (>95%), sabinene (75%), (+)-α-pinene (>99%), and myrcene (analytical standard) were obtained from Sigma Aldrich (Vienna, Austria). (+)-3-carene (>98.5%) and (-)-β-pinene (>99.0%) were purchased from Fluka. Pressured synthetic air grade 5.8 was obtained from Messer (Gumpoldskirchen, Austria), the calibration standard gases were fabricated by Apel Riemer Environmental Inc. (Broomfield, United States). Bottled NH3 grade 3.8 was purchased from Linde AG (Pullach, Germany).
The monoterpenes β-pinene and limonene are also present in our calibration gas standards from Apel Riemer Environmental Inc., Broomfield (CO), USA. They certify an accuracy of typically ±10%. We compared the gas calibration results with our home build diffusion source. The agreement between the estimated sensitivities of the diffusion source and the gas standard differed not more than 25% for these two compounds. As many physical and chemical properties of the investigated monoterpenes are not experimentally determined, we had to rely on calculated values for saturation vapor pressures Ps and diffusion coefficients D. Thus, the error of the diffusion source seems very reasonable. To understand the principles of the investigated ion-molecule reactions it is of greater importance that the diffusion rate of the analyte remains constant over the entire measurement period. In our experiment we detected volume mixing ratio drifts of the diffusion source in the range of ± 3% only. Overall we estimate a calibration error of less than ± 30% taking into account also dilution errors from calibrated flow controllers.
Typical reagent ion distributions are shown in
Typical reagent ion distributions are shown at dry (left) and humid (right) conditions as a function of extraction voltage settings at an E/N value of 51 (top) and 81 Td (bottom), respectively.
First, we investigated the reaction of
Sensitivities (εmeas) of three ketones (acetone:
where M is a third body, and k+ and k− the reactions rates in the respective direction (Ikezoe et al.,
Overview of the bond energies (BE), reaction enthalpies (ΔHr), proton affinities (PA), and protonated structures, calculated at the CCSD(T)-F12/VDZ-F12//ωB97X-D/aug-cc-pVTZ level of theory at 298 K.
Ammonia | 203.8 | 204a | |||
Acetone | 26.4 | 35.7 | 194.5 | 194a | |
Methyl vinyl ketone (MVK) | 27.3 | 32.9 | 198.2 | 199.5a | |
Methyl ethyl ketone (MEK) | 25.9 | 34.0 | 195.7 | 197.7a | |
α-Pinene | 17.9 | 15.4 | 206.3 | 204–209b, c | |
β-Pinene | 18.2 | 13.4 | 208.7 | ||
Camphene | 18.5 | 15.2 | 207.2 | 205.7c | |
3-Carene | 20.6 | 19.0 | 205.4 | ||
27.8 | 196.7 | ||||
Limonene | 22.3 | 25.0 | 201.2 | ||
26.0 | 200.2 | ||||
Myrcene | 20.9 | 20.5 | 204.2 | ||
Ocimene | 26.0 | 19.1 | 210.7 | ||
Sabinene | 20.6 | 8.5 | 215.9 |
Geometries of the
We investigated the reaction of
Camphene product ion distributions are shown at dry (3 ± 1 ppth; left) and humid (18 ± 1 ppth; right) conditions as a function of extraction voltage settings at an E/N value of 51 Td (top) and 81 Td (bottom), respectively.
First, we will discuss the results for dry conditions at an E/N of 51 Td and an extraction voltage of 20 V only. Camphene (
α-pinene product ion distributions are shown at dry (7 ± 1 ppth; left) and humid (26 ± 1 ppth; right) conditions as a function of extraction voltage settings at an E/N value of 51 Td (top) and 81 Td (bottom), respectively.
Myrcene product ion distributions are shown at dry (7 ± 1 ppth; left) and humid (26 ± 1 ppth; right) conditions as a function of extraction voltage settings at an E/N value of 51 Td (top) and 81 Td (bottom), respectively.
3-Carene product ion distributions are shown at dry (5 ± 1 ppth; left) and humid (26 ± 1 ppth; right) conditions as a function of extraction voltage settings at an E/N value of 51 Td (top) and 81 Td (bottom), respectively.
Limonene product ion distributions are shown at dry (7 ± 1 ppth; left) and humid (26 ± 1 ppth; right) conditions as a function of extraction voltage settings at an E/N value of 51 Td (top) and 81 Td (bottom), respectively.
We performed detailed quantum chemical calculations to better understand the
The structures of protonated compounds are also shown in
Limonene (PA = 201.2 kcal/mol) has the smallest PA, which is even smaller than NH3, and should form exclusively adduct ions only. We observe a cluster ion yield of 85%, which is the highest one of all monoterpenes investigated. But still, there exists a 15% channel at 0.057 eV collision energy producing protonated limonene. At 0.084 eV and dry conditions the yield of the proton transfer channel (including fragment ions) increases to 30% at the lowest extraction voltage (20 V). Increasing the extraction voltage from 20 to 30 V increases this channel even further reaching 50% (
The reaction enthalpy ΔHr of the reaction
The product ion distribution was slightly shifted when changed from dry to humid conditions. More cluster ions (~ 3%) were found under humid conditions (
Since more than a decade different methods have been tested to differentiate the monoterpene isomers using H3O+-CIMS technology. Müller et al. (
In this laboratory study, we investigated the reactions of
The raw data supporting the conclusions of this manuscript will be made available by the authors, without undue reservation, to any qualified researcher.
BS and EC ran the experiments and analyzed the data. NH performed the quantum chemical calculations. EC, NH, BS, LF, and AH took part in the data discussion. LF implemented the raw data analysis software. AH and EC wrote the manuscript. All authors contributed to improvements of the manuscript.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The Supplementary Material for this article can be found online at: