Quantification of gas phase methyl iodide using H3O+ as the reagent ion in the PTR-MS technique

Abstract

In this work, the proton transfer reaction between the hydronium ion (H3O+) and methyl iodide (CH3I) is studied to investigate if consistent quantification of the gas phase CH3I is possible in humid air. The neutral CH3I molecule was chosen because this compound is of environmental importance in the field of nuclear power plant safety and nuclear energy. Water was used as a reagent ion source in a conventional Insbruck PTR-MS to produce H3O+ reagent ions. The use of H3O+ ions allows for fast, sensitive and specific detection of gas phase CH3I via a proton-transfer reaction H3O++CH3I→[CH3I-H]++H2O. The instrument response was linear in the tested 5–96ppbV range and the PTR-MS sensitivity was observed to be humidity dependent. The observed sensitivity was in found to range between 1.6 and 3.3cps/ppb at relative humidity between 63 and 15% at T=23°C. A typical H3O+ primary ion signal was 107cps and the normalized sensitivity was in the range between 0.16 and 0.33ncps/ppb. The instrument CH3IH+ ion background rate was 6.8±1.4cps and the dwell time was 1s. The detection limit was calculated as 3 times the standard deviation of the background level and ranged between 1.3 and 3.8ppb. The theoretical collision rate based on the dipole moment and molecular polarizability is calculated. The theoretical collision rate is compared with the experimentally obtained values. The results indicate that the PTR-MS technique is a good analytical method to detect and quantify gas phase CH3I concentrations.