Note: a novel vacuum ultraviolet light source assembly with aluminum-coated electrodes for enhancing the ionization efficiency of photoionization mass spectrometry

Development and Application of a Chemical Ionization Focusing Integrated Ionization Source TOFMS for Online Detection of OVOCs in the Atmosphere

Single photon ionization (SPI) based on vacuum ultraviolet (VUV) lamps has been extensively investigated and applied due to its clean mass spectra as a soft ionization method. However, the photon energy of 10.6 eV and photon flux of 1011 photons s-1 of a commercial VUV lamp limits its range of ionizable analytes as well as its sensitivity. This work designs a chemical ionization focusing integrated (CIFI) ionization source time-of-flight mass spectrometry (TOFMS) based on a VUV lamp for the detection of volatile organic compounds (VOCs) and oxygenated volatile organic compounds (OVOCs). The photoelectrons obtained from the VUV lamp via the photoelectric effect ionized the oxygen and water in the air to obtain the reagent ions. The ion-molecule-reaction region (IMR) is constituted by a segmented quadrupole that radially focuses the ions using a radio-frequency electric field. This significantly enhances the yield and transport efficiency of the product ions leading to a great improvement in sensitivity. As a result, a 44-fold and 1154-fold increase in the signal response for benzene and pentanal were achieved, respectively. To verify the reliability of the ionization source, the linear correspondence and repeatability of benzene and pentanal were investigated. Satisfactory dynamic linearity was obtained in the mixing ratio range of 5-50 ppbv, and the relative standard deviation (RSD) of inter-day reached 3.91% and 6.26%, respectively. Finally, the CIFI-TOFMS was applied to the determination of OVOCs, and the LOD of 12 types of OVOCs reached the pptv level, indicating that the ionization source has the potential for accurate and sensitive online monitoring of atmospheric OVOCs.

A Simple High-Flux Switchable VUV Lamp Based on an Electrodeless Fluorescent Lamp for SPI/PAI Mass Spectrometry

Single-photon ionization (SPI) is a unique soft ionization technique for organic analysis. A convenient high-flux vacuum ultraviolet (VUV) light source is a key precondition for wide application of SPI techniques. In this study, we present a novel VUV lamp by simply modifying an ordinary electrodeless fluorescent lamp. By replacing the glass bulb with a stainless steel bulb and introducing 5% Kr/He (v/v) as the excitation gas, an excellent VUV photon flux over 4.0 × 1014 photons s-1 was obtained. Due to its rapid glow characteristics, the VUV lamp can be switched on and off instantly as required by detection, ensuring the stability and service life of the lamp. To demonstrate the performance of the new lamp, the switchable VUV lamp was coupled with an SPI-mass spectrometer, which could be changed to photoinduced associative ionization (PAI) mode by doping gaseous CH2Cl2 to initiate an associative ionization reaction. Two types of volatile organic compounds sensitive to SPI and PAI, typically benzene series and oxygenated organics, respectively, were selected as samples. The instrument exhibited a high detection sensitivity for the tested compounds. With a measurement time of 11 s, the 3σ limits of detection ranged from 0.33 to 0.75 pptv in SPI mode and from 0.03 to 0.12 pptv in PAI mode. This study provides an extremely simple method to assemble a VUV lamp with many merits, e.g., portability, robustness, durability, low cost, and high flux. The VUV lamp may contribute to the development of SPI-related highly sensitive detection technologies.

Combustion, Chemistry, and Carbon Neutrality

Combustion is a reactive oxidation process that releases energy bound in chemical compounds used as fuels─energy that is needed for power generation, transportation, heating, and industrial purposes. Because of greenhouse gas and local pollutant emissions associated with fossil fuels, combustion science and applications are challenged to abandon conventional pathways and to adapt toward the demand of future carbon neutrality. For the design of efficient, low-emission processes, understanding the details of the relevant chemical transformations is essential. Comprehensive knowledge gained from decades of fossil-fuel combustion research includes general principles for establishing and validating reaction mechanisms and process models, relying on both theory and experiments with a suite of analytic monitoring and sensing techniques. Such knowledge can be advantageously applied and extended to configure, analyze, and control new systems using different, nonfossil, potentially zero-carbon fuels. Understanding the impact of combustion and its links with chemistry needs some background. The introduction therefore combines information on exemplary cultural and technological achievements using combustion and on nature and effects of combustion emissions. Subsequently, the methodology of combustion chemistry research is described. A major part is devoted to fuels, followed by a discussion of selected combustion applications, illustrating the chemical information needed for the future.

Study of the thermal decomposition mechanism of FOX-7 by molecular dynamics simulation and online photoionization mass spectrometry

The thermal decomposition mechanism of energetic materials is important for analyzing the combustion mechanisms of propellants and evaluating the safety of propellants during transport and storage. 1,1-Diamino-2,2-dinitroethylene (FOX-7) is an important insensitive energetic material that can be used as an oxidizer in propellants. However, the initial decomposition mechanism of FOX-7 is not clear to date. The ReaxFF molecular dynamics method is widely used in the investigation of the thermal decomposition mechanisms of energetic materials. Meanwhile, the combination of thermogravimetry with online photoionization time-of-flight mass spectrometry (TG-PI-TOF-MS) and online single-photon ionization time-of-flight mass spectrometry (SPI-TOF-MS) can reveal the decomposition products, which may be integrated with the results of the simulation. In this study, the primary thermal decomposition mechanism of 1,1-diamino-2,2-dinitroethylene (FOX-7) was studied by the ReaxFF molecular dynamics simulations and online photoionization mass spectrometry. The results of the molecular dynamics simulations showed that the primary decomposition step of FOX-7 is C–NO₂ cleavage; after this, C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O formation occurs via a three-membered ring transition state, followed by NO elimination. The remaining structure loses NH₂ and H, resulting in the formation of the NHCCO structure, which finally breaks down into HNC and CO. NH₂ reacts with an H atom to produce NH₃. A reversible intramolecular hydrogen transfer was also observed at 2500 K; however, it failed to dominate the decomposition reaction. During the decomposition of FOX-7, the major products are N₂, NH₃, CO₂, and H₂N₂ and the minor products are H₂O, HN₂, and H₂. The TG-PI-TOF-MS spectrum shows three signals, i.e., m/z = 18, 28, and 30, which can be assigned to H₂O, CO, and NO, respectively. Moreover, four signals at m/z = 72.72, 55.81, 45.79, and 29.88 corresponding to the products (NH₂)₂CCO, (NH₂)CCO, NO₂, and NO have been obtained in the SPI-TOF-MS spectrum. The experimental data obtained via online photoionization mass spectrometry further validated the results of the molecular dynamics simulations.

In this work, the primary thermal decomposition mechanism of 1,1-diamino-2,2-dinitroethylene (FOX-7) was studied by ReaxFF molecular dynamics simulations and online photoionization mass spectrometry.

Experimental study of polyethylene pyrolysis and combustion over HZSM-5, HUSY, and MCM-41

The effects of temperatures, catalysts, and catalyst contents on polyethylene (PE) pyrolysis were investigated by using single-photon ionization time-of-flight mass spectrometry (SPI-TOFMS). The mass spectra of pyrolyzed PE and PE/catalysts from 300°C to 800°C illustrate that the pyrolysis reactions were apparently promoted and varied by introducing HZSM-5, HUSY, and MCM-41. As microporous catalysts, HZSM-5 and HUSY were found to accelerate the BTX formation at 400°C, which could not be observed for pure PE until 800°C. With the existence of MCM-41, only alkenes were produced below 600°C. The pyrolysis processes could to be accelerated by adding catalysts. Principal components analysis (PCA) was finally employed to identify the main factors with influence on the products distribution. Analytical results showed that the yield of the majority of products could be affected by different experimental conditions, that the type of catalysts makes the most significant influence. The impact of different types of catalysts on fire hazard of PE was studied by using the cone calorimeter. The results indicated that the time to ignition (TTI) and the peak heat release rate (pHRR) were changed remarkably. It is worth noting that with the addition of MCM-41, the pHRR is the minimum.

All-solid-state deep ultraviolet laser for single-photon ionization mass spectrometry

We report here the development of a reflectron time-of-flight mass spectrometer utilizing single-photon ionization based on an all-solid-state deep ultraviolet (DUV) laser system. The DUV laser was achieved from the second harmonic generation using a novel nonlinear optical crystal KBe2BO3F2 under the condition of high-purity N2 purging. The unique property of this laser system (177.3-nm wavelength, 15.5-ps pulse duration, and small pulse energy at ∼15 μJ) bears a transient low power density but a high single-photon energy up to 7 eV, allowing for ionization of chemicals, especially organic compounds free of fragmentation. Taking this advantage, we have designed both pulsed nanospray and thermal evaporation sources to form supersonic expansion molecular beams for DUV single-photon ionization mass spectrometry (DUV-SPI-MS). Several aromatic amine compounds have been tested revealing the fragmentation-free performance of the DUV-SPI-MS instrument, enabling applications to identify chemicals from an unknown mixture.

Ultrasonic nebulization extraction/low pressure photoionization mass spectrometry for direct analysis of chemicals in matrices

A novel ultrasonic nebulization extraction/low-pressure photoionization (UNE-LPPI) system has been designed and employed for the rapid mass spectrometric analysis of chemicals in matrices. An ultrasonic nebulizer was used to extract the chemicals in solid sample and nebulize the solvent in the nebulization cell. Aerosols formed by ultrasonic were evaporated by passing through a transferring tube, and desolvated chemicals were ionized by the emitted light (10.6 eV) from a Krypton discharge lamp at low pressure (∼68 Pa). First, a series of semi/non-volatile compounds with different polarities, such as polycyclic aromatic hydrocarbons (PAHs), amino acids, dipeptides, drugs, nucleic acids, alkaloids, and steroids were used to test the system. Then, the quantification capability of UNE-LPPI was checked with: 1) pure chemicals, such as 9,10-phenanthrenequinone and 1,4-naphthoquinone dissolved in solvent; 2) soil powder spiked with different amounts of phenanthrene and pyrene. For pure chemicals, the correlation coefficient (R(2)) for the standard curve of 9,10-phenanthrenequinone in the range of 3 ng-20 μg mL(-1) was 0.9922, and the measured limits of detection (LOD) was 1 ng ml(-1). In the case of soil powder, linear relationships for phenanthrene and pyrene from 10 to 400 ng mg(-1) were obtained with correlation coefficients of 0.9889 and 0.9893, respectively. At last, the feasibility of UNE-LPPI for the detection of chemicals in real matrices such as tablets and biological tissues (tea, Citrus aurantium peel and sage (Salvia officinalis) leaf) were successfully demonstrated.