Computational insights into phthalate ester‐linked VOCs: A density functional theory (DFT)‐based approach for chemical ionization mass spectrometry (CI‐MS) analysis

Abstract

RationaleThe presence of volatile organic compounds (VOCs), notably diethyl phthalate, dimethyl phthalate, di‐n‐butyl phthalate, di(2‐ethylhexyl) phthalate, and similar compounds in soft drinks, raises significant concerns due to their known or potential adverse health effects. Monitoring these compounds is imperative to comprehend their implications on human health and the overall quality of soft drinks. Chemical ionization mass spectrometry (CI‐MS) techniques emerge as powerful tools for VOC quantification in soft drinks, offering fast analysis times, high detection sensitivity, real‐time analysis capabilities, and versatility across various scientific fields.MethodsAchieving absolute quantification of VOCs using proton transfer reaction mass spectrometry (PTR‐MS) presents challenges, with individual VOC calibration proving labor intensive. Theoretical approaches pioneered by Su and colleagues, including density functional theory (DFT), offer avenues for approximating VOC concentrations and understanding ion‐molecule reactions. Specifically, DFT method B3LYP/6–311++G(d, p) computes molecular parameters like dipole moment, polarizability, proton affinity, and ionization energy for large phthalate esters. Rate constants of ion‐molecule reactions are determined using the parametrized trajectory method under varying E/N and temperature conditions.ResultsThe analysis of computed parameters across seven complex molecules reveals notable findings. Bis(2‐methoxyethyl) phthalate, for instance, exhibits a superior dipole moment, suggesting intensified electrostatic interactions with ions and heightened rate constants. The increased proton affinity observed in certain molecules renders them suitable for specific ionization methods. Furthermore, enthalpy change and free energy computations affirm the reactivity of ions with phthalate esters, with distinct variations noted in rate constants based on dipole moment and polarizability.ConclusionsIn conclusion, the parametrized trajectory method, coupled with computational analysis of molecular parameters, offers a means to compute rate constants for ion‐molecule reactions, enabling determination of VOC concentrations in soft drinks without external calibration standards in PTR‐MS analyses. The observed variations in rate constants with temperature and reagent ions align with collision theory principles and existing literature findings, underscoring the utility of these approaches in VOC identification and quantification using PTR‐MS.