Thermal decomposition study of 4-methyloxybenzyl-glycoside by TG/DTA and on-line pyrolysis-photoionization mass spectrometry

A flavor precursor of 4-methyloxybenzyl-2, 3, 4, 6-tetra-O-acetyl-β-D-glucopyranoside (MBGL) was synthesized via a modified Koenigs-Knorr reaction. The thermal decomposition behaviour and pyrolysis intermediate products of the glycoside were studied by simultaneous thermogravimetric/differential thermal analysis (TG/DTA) and synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry (PIMS). TG/DTA results showed that the largest mass loss rate appeared at a Tp of 246.7 °C. PIMS was used to identify the pyrolysis products of MBGL at 300 °C, 500 °C and 700 °C, respectively. The experimental apparatus had some advantages in real-time analysis and fewer secondary reactions. Some important pyrolysis intermediates, such as the ions of the 4-methyloxybenzyl group at m/z 121 and the glycone moiety at m/z 347, were detected by PIMS. The results indicate that the MBGL was probably showed a different pyrolysis way compared with the other glycosides. This work reports a useful application of synchrotron VUV PIMS in a thermal decomposition study of glycoside flavor precursors.

Determination of the two-compartment model parameters of exhaled HCN by fast negative photoionization mass spectrometry

Breath exhaled hydrogen cyanide (HCN) has been identified to be associated with several respiratory diseases. Accurately distinguishing the concentration and release rate of different HCN sources is of great value in clinical research. However, there are still significant challenges due to the high adsorption and low concentration characteristics of exhaled HCN. In this study, a two-compartment kinetic model method based on negative photoionization mass spectrometry was developed to simultaneously determine the kinetic parameters including concentrations and release rates in the airways and alveoli. The influences of the sampling line diameter, length, and temperature on the response time of the sampling system were studied and optimized, achieving a response time of 0.2 s. The negative influence of oral cavity-released HCN was reduced by employing a strategy based on anatomical lung volume calculation. The calibration for HCN in the dynamic range of 0.5-100 ppbv and limit of detection (LOD) at 0.3 ppbv were achieved. Subsequently, the experiments of smoking, short-term passive smoking, and intake of bitter almonds were performed to examine the influences of endogenous and exogenous factors on the dynamic parameters of the model method. The results indicate that compared with steady-state concentration measurements, the kinetic parameters obtained using this model method can accurately and significantly reflect the changes in different HCN sources, highlighting its potential for HCN-related disease research.

Vacuum ultraviolet photoionization and dissociative photoionization of toluene: Experimental and theoretical insights

The photoionization and dissociative photoionization of toluene have been studied using synchrotron radiation vacuum ultraviolet light with photon energy in the range of 8.50-25.50 eV. The ionization energies (8.82 eV) and double ionization energies (23.80 eV) of toluene as well as the appearance energies for its major fragments C₇H₇⁺ (11.17/10.71 eV), C₆H₅⁺ (13.73 eV), C₅H₆⁺ (13.58/12.50 eV), C₅H₅⁺ (16.23 eV), C₄H₅⁺ (15.64 eV), C₄H₄⁺ (16.10 eV) and C₄H₃⁺ (17.11 eV) are determined, respectively by using photoionization efficiency spectrometry. With the help of experimental and theoretical results, seven dissociative photoionization channels have been proposed: C₇H₇⁺ + H, C₆H₅⁺ + CH₃, C₅H₆⁺ + C₂H₂, C₅H₅⁺ + C₂H₂ + H, C₄H₅⁺ + C₃H₃, C₄H₄⁺ + C₃H₄ and C₄H₃⁺ + C₃H₄ + H. In addition, the geometries of the intermediates, transition states and products involved in these photoionization and dissociative photoionization processes have been performed at the B3LYP/6-311++G(d, p) level. The mechanisms of dissociative photoionization of toluene and the intermediates and transition states involved are discussed in detail. Generally speaking, the experimental results are in agreement with theoretical calculations in this work and published literature results. Especially the mechanisms of dissociative photoionization to C₄H₅⁺, C₄H₄⁺ and C₄H₃⁺ were discussed for the first time in this work. This investigation may provide useful information on understanding the photoionization and dissociative photoionization of toluene.

Impact of photochemical ageing on Polycyclic Aromatic Hydrocarbons (PAH) and oxygenated PAH (Oxy-PAH/OH-PAH) in logwood stove emissions

The combustion of spruce logwood in a modern residential stove was found to emit polycyclic aromatic hydrocarbons (PAH) and oxygenated polycyclic aromatic hydrocarbons (OPAH) with emission factors of 404 μg MJ^-1 of 35 analysed PAH, 317 μg MJ^-1 of 11 analysed Oxy-PAH and 12.5 μg MJ^-1 of 5 analysed OH-PAH, most of which are known as potential mutagens and carcinogens. Photochemical ageing in an oxidation flow reactor (OFR) degraded particle-bound PAH, which was also reflected in declining PAH toxicity equivalent (PAH-TEQ) values by 45 to 80% per equivalent day of photochemical ageing in the atmosphere. OPAH concentrations decreased less than PAH concentrations during photochemical ageing, supposedly due to their secondary formation, while 1-hydroxynaphthalene, 1,5-dihydroxynaphthalene and 1,8-naphthalaldehydic acid were significantly increased after ageing. Furthermore, secondary organic aerosol (SOA) formation and aromatic compounds not included in targeted analysis were investigated by thermal-optical carbon analysis (TOCA) hyphenate to resonance-enhanced multi-photon ionisation time-of-flight mass spectrometry (REMPI-TOFMS). The commonly used PAH-source indicators phenanthrene/anthracene, fluoranthene/pyrene, retene/chrysene, and indeno[cd]pyrene/benzo[ghi]perylene remained stable during photochemical ageing, enabling identification of wood combustion emissions in ambient air. On the other hand, benz[a]pyrene/benz[e]pyrene and benz[a]anthracene/chrysene were found to decrease with increasing photochemical age. Retene/chrysene was not a proper classifier for the wood combustion emissions of this study, possibly due to more efficient combustion than in open wood burning, from which this diagnostic ratio was initially derived. This study motivates in-depth investigation of degradation kinetics of particle-bound species on different combustion aerosol as well as the consequences of photochemical ageing on toxicity and identification of wood combustion emissions in ambient air.