Measuring the Rate Constants of Reactions Between OH⁻ and Ketone/Acids by Proton Extraction Reaction Mass Spectrometry

Due to its characteristics of short response time (1-10 s), low limit of detection (LOD) (10⁻¹² (V/V)), and self-quantitative measurement, proton transfer reaction mass spectrometry (PTR-MS) has become a powerful technique for real-time trace detection of volatile organic compounds (VOCs). Different from traditional VOCs detection technology, PTR-MS is termed as self-quantitative detection, can provide empirical concentration of the analytes without calibration, which makes it more advantageous in the quantitative detection of multi-component VOCs. Self-quantitative measurement detects the concentration of VOCs by calculating the reaction rate constants of ion-molecule reactions, there is no need for prior calibration unless exceptionally high measurement accuracy is desired. In 2014, our research team developed a new method of proton-extraction-reaction mass spectrometry (PER-MS) with OH⁻ anion as the reagent ions, which can be applied for the detection of VOCs and even inorganic compounds. Due to the lack of reaction rate constant data of OH⁻ and VOCs, the ability for self-quantitative measurements is limited. Therefore, in order to expand the data on the reaction rate constants of OH⁻ and VOCs and promote the self-quantitative measurement ability of PER-MS technique, a new method of proton extraction reaction mass spectrometry (PER-MS) was developed for measuring reaction rate constants of OH⁻ and VOCs. The reaction rate constants were calculated based on the reaction kinetic equations after detecting the signal intensity of product ions and reactive ions by PER-MS and measuring the number density of VOCs and the reaction time. In this work, the pressure in the ion detection system was 7.4×10⁻⁵ Pa, the pressure in the drift tube was 200 Pa, and the temperature of drift tube was kept at 308 K. Acetonitrile was used as a reference to measure the reaction rate constants of acetone with OH⁻. The measured results are agreement with the data reported in the literature. The reaction rate constants of seven VOCs (acetone, 2-butanone, 2-pentanone, 2-hexanone, 2-heptanone, formic acid and acetic acid) reacting with OH⁻ ion were measured by the proposed method. The results enrich the contents of the reaction rate constants between OH⁻ and VOCs, and improve the capability of self-quantitative detection for PER-MS, and also provide a useful reference for the research of ion-molecular reactions.