The potential of NO+ and O2 +• in switchable reagent ion proton transfer reaction time-of-flight mass spectrometry

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

Rationale

The 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.

Methods

Achieving 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.

Results

The 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.

Conclusions

In 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.

Reliable Detection of Chemical Warfare Agents Using High Kinetic Energy Ion Mobility Spectrometry

High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) ionize and separate ions at reduced pressures of 10-40 mbar and over a wide range of reduced electric field strengths E/N of up to 120 Td. Their reduced operating pressure is distinct from that of conventional drift tube ion mobility spectrometers that operate at ambient pressure for trace compound detection. High E/N can lead to a field-induced fragmentation pattern that provides more specific structural information about the analytes. In addition, operation at high E/N values adds the field dependence of ion mobility as an additional separation dimension to low-field ion mobility, making interfering compounds less likely to cause a false positive alarm. In this work, we study the chemical warfare agents tabun (GA), sarin (GB), soman (GD), cyclosarin (GF) and sulfur mustard (HD) in a HiKE-IMS at variable E/N in both the reaction and the drift region. The results show that varying E/N can lead to specific fragmentation patterns at high E/N values combined with molecular signals at low E/N. Compared to the operation at a single E/N value in the drift region, the variation of E/N in the drift region also provides the analyte-specific field dependence of ion mobility as additional information. The accumulated data establish a unique fingerprint for each analyte that allows for reliable detection of chemical warfare agents even in the presence of interfering compounds with similar low-field ion mobilities, thus reducing false positives.

Pitfalls in the Detection of Volatiles Associated with Heated Tobacco and e-Vapor Products When Using PTR-TOF-MS

We investigated the applicability of proton transfer reaction-time-of-flight mass spectrometry (PTR-TOF-MS) for quantitative analysis of mixtures comprising glycerin, acetol, glycidol, acetaldehyde, acetone, and propylene glycol. While PTR-TOF-MS offers real-time simultaneous determination, the method selectivity is limited when analyzing compounds with identical elemental compositions or when labile compounds present in the mixture produce fragments that generate overlapping ions with other matrix components. In this study, we observed significant fragmentation of glycerin, acetol, glycidol, and propylene glycol during protonation via hydronium ions (H3O+). Nevertheless, specific ions generated by glycerin (m/z 93.055) and propylene glycol (m/z 77.060) enabled their selective detection. To thoroughly investigate the selectivity of the method, various mixtures containing both isotope-labeled and unlabeled compounds were utilized. The experimental findings demonstrated that when samples contained high levels of glycerin, it was not feasible to perform time-resolved analysis in H3O+ mode for acetaldehyde, acetol, and glycidol. To overcome the observed selectivity limitations associated with the H3O+ reagent ions, alternative ionization modes were investigated. The ammonium ion mode proved appropriate for analyzing propylene glycol (m/z 94.086) and acetone (m/z 76.076) mixtures. Concerning the nitric oxide mode, specific m/z were identified for acetaldehyde (m/z 43.018), acetone (m/z 88.039), glycidol (m/z 73.028), and propylene glycol (m/z 75.044). It was concluded that considering the presence of multiple product ions and the potential influence of other compounds, it is crucial to conduct a thorough selectivity assessment when employing PTR-TOF-MS as the sole method for analyzing compounds in complex matrices of unknown composition.

Detecting Hexafluoroisopropanol Using Soft Chemical Ionization Mass Spectrometry and Analytical Applications to Exhaled Breath

Here we explore the potential use of proton transfer reaction/selective reagent ion-time-of-flight-mass spectrometry (PTR/SRI-ToF-MS) to monitor hexafluoroisopropanol (HFIP) in breath. Investigations of the reagent ions H₃O⁺, NO⁺, and O₂^+• are reported using dry (relative humidity (rH) ≈ 0%) and humid (rH ≈ 100%)) nitrogen gas containing traces of HFIP, i.e., divorced from the complex chemical environment of exhaled breath. HFIP shows no observable reaction with H₃O⁺ and NO⁺, but it does react efficiently with O₂^+• via dissociative charge transfer resulting in CHF₂⁺, CF₃⁺, C₂HF₂O⁺, and C₂H₂F₃O⁺. A minor competing hydride abstraction channel results in C₃HF₆O⁺ + HO₂^• and, following an elimination of HF, C₃F₅O⁺. There are two issues associated with the use of the three dominant product ions of HFIP, CHF₂⁺, CF₃⁺, and C₂H₂F₃O⁺, to monitor it in breath. One is that CHF₂⁺ and CF₃⁺ also result from the reaction of O₂^+• with the more abundant sevoflurane. The second is the facile reaction of these product ions with water, which reduces analytical sensitivity to detect HFIP in humid breath. To overcome the first issue, C₂H₂F₃O⁺ is the ion marker for HFIP. The second issue is surmounted by using a Nafion tube to reduce the breath sample's humidity prior to its introduction into drift tube. The success of this approach is illustrated by comparing the product ion signals either in dry or humid nitrogen gas flows and with or without the use of the Nafion tube, and practically from the analysis of a postoperative exhaled breath sample from a patient volunteer.

Recent developments and applications of selected ion flow tube mass spectrometry (SIFT-MS)

Selected ion flow tube mass spectrometry (SIFT-MS) is now recognized as the most versatile analytical technique for the identification and quantification of trace gases down to the parts-per-trillion by volume, pptv, range. This statement is supported by the wide reach of its applications, from real-time analysis, obviating sample collection of very humid exhaled breath, to its adoption in industrial scenarios for air quality monitoring. This review touches on the recent extensions to the underpinning ion chemistry kinetics library and the alternative challenge of using nitrogen carrier gas instead of helium. The addition of reagent anions in the Voice200 series of SIFT-MS instruments has enhanced the analytical capability, thus allowing analyses of volatile trace compounds in humid air that cannot be analyzed using reagent cations alone, as clarified by outlining the anion chemistry involved. Case studies are reviewed of breath analysis and bacterial culture volatile organic compound (VOC), emissions, environmental applications such as air, water, and soil analysis, workplace safety such as transport container fumigants, airborne contamination in semiconductor fabrication, food flavor and spoilage, drugs contamination and VOC emissions from packaging to demonstrate the stated qualities and uniqueness of the new generation SIFT-MS instrumentation. Finally, some advancements that can be made to improve the analytical capability and reach of SIFT-MS are mentioned.