Quantification of the gas phase methyl iodide using O2+ as the reagent ion in the PTR-ToF-MS technique.
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| Title: | Quantification of the gas phase methyl iodide using O2+ as the reagent ion in the PTR-ToF-MS technique. |
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| Authors: | R’Mili, Badr1, Temime-Roussel, Brice1, Monod, Anne1, Wortham, Henri1, Strekowski, Rafal S.1 rafal.strekowski@univ-amu.fr |
| Source: | International Journal of Mass Spectrometry. Aug2018, Vol. 431, p43-49. 7p. |
| Subjects: | Gas phase reactions, Methyl iodide, Oxygen, Molecules, Nuclear energy |
| Abstract: | The charge-transfer-reaction between molecular oxygen ions (O 2 + ) and methyl iodide (CH 3 I) is studied to investigate if consistent environmental quantification of the gas phase CH 3 I is possible without prior calibration. The neutral CH 3 I molecule was chosen because this compound is of atmospheric chemistry and environmental importance in the field of nuclear power plant safety and nuclear energy. Molecular oxygen was used as a reagent ion source in a commercial Proton-Transfer-Reaction Time-of-Flight Mass Spectrometer (PTR-ToF-MS) to produce molecular oxygen ions (O 2 + ). The use of O 2 + ions allows for fast, sensitive and specific detection of gas phase CH 3 I via an electron exchange reaction O 2 + + CH 3 I → CH 3 I + + O 2 . The instrument response was linear in the 0.23–150 ppbv range and its sensitivity was humidity independent. The detection sensitivity of CH 3 I normalized by the O 2 + count rate of 10 6 cps was found to be S = 22.6 ± 0.3 ncps/ppbv, independent of relative humidity. A typical O 2 + primary ion signal was ( 2.0 ± 0.2 ) × 10 6 cps. The lowest measured CH 3 I concentration was 0.23 ± 0.10 ppb. Error is ±σ. The theoretical collision rate based on the dipole moment and molecular polarizability values is calculated using the Langevin collision rate ( k L ) approximation, the average-dipole-orientation (ADO) theory and the capture rate coefficient ( k CAP ) based on trajectory calculations. The experimental rate constant, k exp , for the electron transfer reaction between O 2 + ions and CH 3 I is calculated to be 1.72 ± 0.22 × 10 - 9 cm 3 s −1 . Listed errors are ±σ and represent precision only. The experimentally determined value agrees very well with the theoretical collision rate values, k L = 1.24 × 10 - 9 cm 3 s -1 , k ADO = 1.73 × 10 - 9 cm 3 s −1 and k CAP = 1.48 × 10 - 9 cm 3 s -1 . The obtained results indicate that the PTR-MS technique is an excellent analytical method to quantify gas phase CH 3 I. [ABSTRACT FROM AUTHOR] |
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| Database: | Engineering Source |
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