Thermal decomposition of polypropylene by tunable synchrotron vacuum ultraviolet photoionization mass spectrometry

The application of synchrotron vacuum ultraviolet (VUV) photoionization combined with molecular-beam mass spectrometry, also called synchrotron VUV photoionization mass spectrometry (SVUV-PIMS), in the research of the thermal decomposition of polypropylene (PP) was studied, and some main pyrolysis products formed at different photon energies have been identified. Using SVUV-PIMS, some isomers can be distinguished, which are much helpful for further understanding of the thermal decomposition of PP.     

Recent developments in synchrotron vacuum ultraviolet photoionization coupled to mass spectrometry

In recent years, increasing attention has been paid to “soft” photoionization (PI), which will potentially become a standard, universal ionization method. Tunable synchrotron vacuum ultraviolet (SVUV) light, a quasi-continuous light with good energy resolution and high photon flux, has proved an ideal source for “soft” PI in various research fields (e.g., combustion chemistry and molecular imaging).This review focuses on combinations of SVUV light with commonly used techniques (e.g., molecular-beam sampling, laser desorption, ion desorption, and thermal vaporization). These couplings have successful applications in flame chemistry, organic analysis, chemical imaging and aerosol mass spectrometry.     

The characterization of selected drugs with infrared laser desorption/tunable synchrotron vacuum ultraviolet photoionization mass spectrometry

Some selected drugs including captopril, fudosteine and racecadotril have been analyzed by infrared (IR) laser desorption/tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry (PIMS). The molecular ions of captopril and racecadotril are exclusively observed without any fragments at near threshold single-photon ionization (SPI). However, fudosteine easily forms fragments even at a photon energy near the ionization threshold, indicating the instability of its molecular ion. For these drugs, a number of fragments are yielded with the increase of photon energy. The structures of such fragments proposed by IR LD/VUV PIMS are supported by electron ionization time-of-flight mass spectrometry (EI-TOFMS) results. Fragmentation pathways are discussed in detail.     

On-line product analysis of pine wood pyrolysis using synchrotron vacuum ultraviolet photoionization mass spectrometry

The pyrolysis process of pine wood, a promising biofuel feedstock, has been studied with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry. The mass spectra at different photon energies and temperatures as well as time-dependent profiles of several selected species during pine wood pyrolysis process were measured. Based on the relative contents of three lignin subunits, the data indicate that pine wood is typical of softwood. As pyrolysis temperature increased from 300 to 700 °C, some more details of pyrolysis chemistry were observed, including the decrease of oxygen content in high molecular weight species, the observation of high molecular weight products from cellulose chain and lignin polymer, and potential pyrolysis mechanisms for some key species. The formation of polycyclic aromatic hydrocarbons (PAHs) was also observed, as well as three series of pyrolysis products derived from PAHs with mass difference of 14 amu. The time-dependent profiles show that the earliest products are formed from lignin, followed by hemicellulose products, and then species from cellulose.

Study of combustion intermediates in fuel‐rich methyl methacrylate flame with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry

A fuel‐rich premixed laminar methyl methacrylate (MMA)/O₂/Ar flame at low pressure (30 Torr) with the equivalence ratio (?) of 1.60 is studied in this work. Synchrotron vacuum ultraviolet photoionization combined with molecular beam mass spectrometry is employed to identify the combustion intermediates including isomeric intermediates. The observed combustion intermediates can be classified as four types: radicals, non‐cyclic hydrocarbons, cyclic hydrocarbons and oxygenates. Benzene is the unique aromatic hydrocarbon detected in this work, and several oxygenates with two oxygen atoms are identified. Mole fraction profiles of most intermediates are evaluated, which can help understand the MMA combustion mechanism under fuel‐rich conditions. The similarities among rich flames of MMA and other oxygenated fuels, as well as the characteristics of rich MMA flame, are also discussed. The results show that combustion of MMA not only reduces soot emissions, but also has low concentrations of some potential toxic by‐products. Copyright © 2008 John Wiley & Sons, Ltd.     

A vacuum ultraviolet photoionization mass spectrometric study of acetone

The photoionization and dissociative photoionization of acetone have been studied at the photon energy range of 8-20 eV. Photoionization efficiency spectra for ions CH3COCH3+, CH3+, C2H3+, C3H3+, C3H5+, CH(2-)CO+, CH3CO+, C3H4O+, and CH3COCH2+ have been measured. In addition, the energetics of the dissociative photoionization has been examined by ab initio Gaussian-3 (G3) calculations. The computational results are useful in establishing the dissociation channels near the ionization thresholds. With the help of G3 results, the dissociation channels for the formation of the fragment ions CH3CO+, CH2CO+, CH3+, C3H3+, and CH3COCH2+ have been established. The G3 results are in fair to excellent agreement with the experimental data.     

Thermal decomposition of glycidyl azide polymer studied by synchrotron photoionization mass spectrometry

In this work, the thermal decomposition reactions and products of glycidyl azide polymer (GAP) at low pressure have been investigated by tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam sampling mass spectrometry. It has been observed that thermal decomposition of GAP began at a lower temperature (about 70 degrees C), compared to about 170 degrees C in the air. Most observed species in the thermal decomposition process have been clearly identified by measurements of the photoionization mass spectrum and photoionization efficiency (PIE) spectra. Many species have been detected at the initiation of the degradation. Compared with previous studies on thermal decomposition of GAP, some free radicals, such as C2H3O, C3H5O, C6H6N, C3H5ON3, and so forth, have been identified in the present work. The formation mechanisms of some important radicals have been discussed, and the most probable reaction routines have also been proposed, which should be of importance in understanding the energy-releasing mechanism of GAP thermal decomposition.     

Study on thermal decomposition of polymers by evolved gas analysis using photoionization mass spectrometry (EGA-PIMS)

Photoionization mass spectrometry (PIMS) with vacuum ultraviolet (VUV) light source provides an efficient and fragmentation-free method for the soft ionization of gaseous compounds, in order to facilitate an understanding of thermal decomposition behavior and chemical composition of polymeric materials. The PIMS was applied to the evolved gas analysis (EGA) system equipped with a skimmer interface which is constituted based upon a jet separator principle between a vacuum MS chamber and an atmospheric sample chamber in a furnace. A photoionization source with a deuterium (D₂) lamp was closely installed to the vacuum ionization chamber of a mass spectrometer to improve the ionization efficiency. The thermal decomposition of typical polymers in inert gas atmosphere was investigated by the EGA-PIMS and the resulting PI mass spectrum was characterized satisfactorily by only the parent ions with no contribution as a result of fragmentation during the ionization. The results suggested that the EGA-PIMS was an especially powerful and desirable in situ thermal analysis method for polymeric materials which evolve organic gases simultaneously and concurrently. The combination of EGA equipped with skimmer interface with no change of evolved gaseous species and PIMS with fragmentation-free during the ionization is described briefly, and the effective results are presented by comparing with EGA using conventional electron impact ionization mass spectrometry.     

Direct identification of propargyl radical in combustion flames by vacuum ultraviolet photoionization mass spectrometry

We have developed an effusive laser photodissociation radical source, aiming for the production of vibrationally relaxed radicals. Employing this radical source, we have measured the vacuum ultraviolet (VUV) photoionization efficiency (PIE) spectrum of the propargyl radical (C(3)H(3)) formed by the 193 nm excimer laser photodissociation of propargyl chloride in the energy range of 8.5-9.9 eV using high-resolution (energy bandwidth = 1 meV) multibunch synchrotron radiation. The VUV-PIE spectrum of C(3)H(3) thus obtained is found to exhibit pronounced autoionization features, which are tentatively assigned as members of two vibrational progressions of C(3)H(3) in excited autoionizing Rydberg states. The ionization energy (IE = 8.674 +/- 0.001 eV) of C(3)H(3) determined by a small steplike feature resolved at the photoionization onset of the VUV-PIE spectrum is in excellent agreement with the IE value reported in a previous pulsed field ionization-photoelectron study. We have also calculated the Franck-Condon factors (FCFs) for the photoionization transitions C(3)H(3) (+)(X;nu(i),i = 1-12)<--C(3)H(3)(X). The comparison between the pattern of FCFs and the autoionization peaks resolved in the VUV-PIE spectrum of C(3)H(3) points to the conclusion that the resonance-enhanced autoionization mechanism is most likely responsible for the observation of pronounced autoionization features. We also present here the VUV-PIE spectra for the mass 39 ions observed in the VUV synchrotron-based photoionization mass spectrometric sampling of several premixed flames. The excellent agreement of the IE value and the pattern of autoionizing features of the VUV-PIE spectra observed in the photodissociation and flames studies has provided an unambiguous identification of the propargyl radical as an important intermediate in the premixed combustion flames. The discrepancy found between the PIE spectra obtained in flames and photodissociation at energies above the IE(C(3)H(3)) suggests that the PIE spectra obtained in flames might have contributions from the photoionization of vibrationally excited C(3)H(3) and/or the dissociative photoionization processes involving larger hydrocarbon species formed in flames.     

Ion decomposition versus molecular size probed by vacuum ultraviolet photoionization

The molecular size dependence of primary fragmentation is studied for a series of β -naphthyl esters having alkyl chain lengths from C2 to C18. The esters are vaporized at a known ftemperature and ionized by coherent vacuum ultraviolet radiation at 10.5 eV. The photoionization wavelength is energetic enough to cause both metastable and nonmetastable primary fragmentation to m / z 144, but not energetic enough to cause secondary fragmentation to m / z 115 or 116. Under these conditions, the ratio of the nonmetastable-to-metastable daughter ion current, D/m _D ^′ is expected to give a rough indication of the average parent ion dissociation rate. The D / m _D ratio decreases with increasing molecular size, but not as quickly as expected by simple RRK theory. This behavior along with temperature dependence studies suggests that the internal energy required for dissociation is provided in substantial part by both the initial thermal internal energy and the energy imparted by the photoionization step. The role of thermal energy in the dissociation of large ions is discussed.     

Pyrolysis of methyl tert-butyl ether (MTBE). 1. Experimental study with molecular-beam mass spectrometry and tunable synchrotron VUV photoionization

An experimental study of methyl tert-butyl ether (MTBE) pyrolysis (3.72% MTBE in argon) has been performed at low pressure (267 Pa) within the temperature range from 700 to 1420 K. The pyrolysis process was detected with the tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry (MBMS). About thirty intermediates are identified from near-threshold measurements of photoionization mass spectrum and photoionization efficiency spectrum. Among them, H2, CO, CH4, CH3OH and C4H8 are the major pyrolysis products. The radicals such as methyl, methoxy, propargyl, allyl, C4H5 and C4H7 are detected. The isomers of pyrolysis products are identified as well, i.e., propyne and allene, 1,2,3-butatriene and vinylacetylene, isobutene and 1-butene, propanal and acetone. Furthermore, the mole fractions of the pyrolysis products have been evaluated under various temperatures. Meanwhile, the initial formation temperatures of different pyrolysis products can be obtained. This work is anticipated to present a new experimental method for pyrolysis study and help understand the pyrolysis and combustion chemistry of MTBE and other oxygenated fuels.     

Direct detection of isoprene photooxidation products by using synchrotron radiation photoionization mass spectrometry

We report the combination of a vacuum ultraviolet photoionization mass spectrometer, operating on the basis of synchrotron radiation, with an environmental reaction smog chamber for the first time. The gas- and pseudo-particle-phase products of OH-initiated isoprene photooxidation reactions were measured on-line and off-line, respectively, by mass spectrometry. It was observed that aldehydes, methacrolein, methyl vinyl ketone, methelglyoxal, formic acid, and similar compounds are the predominant gas-phase photooxidation products, whereas some multifunctional carbonyls and acids mainly exist in the particle phase. This finding is reasonably consistent with results of studies conducted in other laboratories using different methods. The results indicate that synchrotron radiation photoionization mass spectrometry coupled with a smog chamber is a potentially powerful tool for the study of the mechanism of atmospheric oxidations and the formation of secondary organic aerosols.     

Vacuum ultraviolet photoionization mass spectrometric study of cyclohexene

In this work, photoionization and dissociation of cyclohexene have been studied by means of coupling a reflectron time‐of‐flight mass spectrometer with the tunable vacuum ultraviolet (VUV) synchrotron radiation. The adiabatic ionization energy of cyclohexene as well as the appearance energies of its fragment ions C₆H₉⁺, C₆H₇⁺, C₅H₇⁺, C₅H₅⁺, C₄H₆⁺, C₄H₅⁺, C₃H₅⁺ and C₃H₃⁺ were derived from the onset of the photoionization efficiency (PIE) curves. The optimized structures for the transition states and intermediates on the ground state potential energy surfaces related to photodissociation of cyclohexene were characterized at the ωB97X‐D/6‐31+g(d,p) level. The coupled cluster method, CCSD(T)/cc‐pVTZ, was employed to calculate the corresponding energies with the zero‐point energy corrections by the ωB97X‐D/6‐31+g(d,p) approach. Combining experimental and theoretical results, possible formation pathways of the fragment ions were proposed and discussed in detail. The retro‐Cope rearrangement was found to play a crucial role in the formation of C₄H₆⁺, C₄H₅⁺ and C₃H₅⁺. Intramolecular hydrogen migrations were observed as dominant processes in most of the fragmentation pathways of cyclohexene. The present research provides a clear picture of the photoionization and dissociation processes of cyclohexene in the 8‐ to 15.5‐eV photon energy region. Copyright © 2016 John Wiley & Sons, Ltd.     

Vacuum ultraviolet photoionization mass spectrometric study of ethylenediamine

The photoionization and dissociative photoionizations of ethylenediamine have been studied both experimentally and theoretically. In experiments, photoionization efficiency spectra for ions NH(2)CHCH(3)(+), NH(2)CH=CH(2)(+), CH(2)NH(2)(+), NH(3)(+), NH(2)CH(2)CHNH(2)(+) and NH(2)CH(2)CH(2)NH(2)(+) have been obtained. In addition, the energetics of the dissociative photoionization is investigated with ab initio Gaussian-3 (G3) calculations. The computational results are useful in analyzing the dissociation channels near the ionization thresholds. With the help of the G3 results, the dissociation channels for the formation of the aforementioned fragment ions have been established.     

In situ diagnostics of the decomposition of silacyclobutane on a hot filament by vacuum ultraviolet laser ionization mass spectrometry

The gas-phase reaction products of silacyclobutane (SCB) and 1, 1-dideuterio-silacyclobutane (SCB-d(2)) from a hot-wire chemical vapor deposition (HWCVD) chamber were diagnosed in situ using vacuum ultraviolet (VUV) laser single-photon ionization (SPI) coupled with time-of-flight (TOF) mass spectrometry. The SCB molecule was found to decompose at a filament temperature as low as 900 degrees C. Both Si- (silylene, methylsilylene, and silene) and C-containing (ethene and propene) species were produced from the SCB decomposition on the filament. Ethene and propene were detected by the mass spectrometer. It is demonstrated that the formation of ethene is favored over that of propene. The experimental study of hot-wire decomposition of SCB-d(2) shows that propene is most likely produced by a process that is initiated by a 1,2-H(D) migration to form n-propylsilylene, followed by an equilibration with silacyclopropane, which then decomposes to propene. The detection of ethene in our experiment indicates that a competitive route of fragmentation exists for SCB decomposition on the filament. It has been shown that this competitive route occurs without H/D scrambling. The highly reactive silylene, silene, and methylsilylene species produced from SCB decomposition underwent either insertion reactions into the Si-H bonds of the parent molecule or pi-type addition reaction across the double and triple CC bonds. The dimerization product of silene, 1,3-disilacyclobutane, at m/z = 88 was also observed.     

Photoionization study of L-valine in the gas phase by vacuum ultraviolet synchrotron radiation

The photoionization and photodissociation of L-valine are studied by tunable synchrotron vacuum ultraviolet photoionization mass spectrometry at the photon energy of 13 eV. The ionization energy of L-valine and the appearance energies of major fragments are measured by the photoionization efficiency spectrum in the photon energy range of 8-11 eV. Possible formation pathways of the major fragments, NH(2)CHC(OH)(2)(+) (m/z=75), NH(2)(CH(3))(2)(CH)(2)(+) (m/z=72) and NH(2)CHCO(+) (m/z=57), are discussed in detail with the theoretical calculations at the B3LYP/6-31++G (d, p) level. Hydrogen migration is considered as the key way for the formation of NH(2)CHC(OH)(2)(+) (m/z=75) and NH(2)CHCO(+) (m/z=57). Furthermore, other fragments, NH(2)CHCOOH(+) (m/z=74), (CH(3))(2)(CH)(2)(+) (m/z=56), C(4)H(7)(+) (m/z=55), NH(2)CHOH(+) (m/z=46), NH(2)CH(2)(+) (m/z=30) and m/z=18, species are also briefly described.     

A vacuum ultraviolet photoionization time-of-flight mass spectrometer with high sensitivity for study of gas-phase radical reaction in a flow tube

Photoionization mass spectrometry as a powerful analytical method has been widely utilized and provided valuable insight in the field of gas-phase reactions. Here, a highly sensitive vacuum ultraviolet (VUV) photoionization time-of-flight mass spectrometer combined with a microwave discharge generator and a fast flow tube reactor has been developed to study radical reactions of atmospheric and combustion interests. Two kinds of continuous light sources, the tunable VUV synchrotron radiation at Hefei, China for isomer-specific product detection and a commercial krypton discharge lamp for time-consuming kinetic measurements, are employed as photoionization sources in the apparatus. A multiplexed detection with high sensitivity (the limit of detection ∼0.8 ppb) and high mass resolution (M/ΔM ∼ 2100) has been approached. As representative examples, the self-reaction of the methyl radical, CH₃, and the reaction of the methyl radical with molecular oxygen are studied and multiple species including reactive radicals and isomeric/isobaric products are detected and identified. In addition, some preliminary results related to the reaction kinetics are also presented.