Mechanistic study of thermal decomposition of isoprene (2-methyl-1,3-butadiene) using flash pyrolysis supersonic jet VUV photoionization mass spectrometry

The thermal decomposition of isoprene up to 1400 K was performed by flash pyrolysis with an approximately 100 mus time scale. This pyrolysis was followed by supersonic expansion to isolate the reactive intermediates and initial products, and detection was accomplished by vacuum ultraviolet single photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS) at lambda = 118.2 nm. Products CH(3), C(2)H(4), C(3)H(3), C(3)H(4), C(4)H(4), C(4)H(5), C(5)H(6), C(5)H(7), and C(6)H(6) were directly observed and provide mechanistic insights to the isoprene pyrolysis. At temperatures >or= approximately 1200 K, the molecular elimination of ethene to form C(3)H(4) and sigma bond homolysis producing C(4)H(5) and CH(3) radicals are competitive reaction pathways. The molecular elimination of acetylene to form C(3)H(6) was minimal and direct C(2)-C(3) sigma bond homolysis was not observed. The C(3)H(3) radicals are also observed, as a result of hydrogen loss of C(3)H(4) by pyrolysis or hydrogen abstraction by the CH(3) radical from C(3)H(4). Above approximately 1250 K, production of C(6)H(6) was observed and identified as the combination product of the C(3)H(3) radicals.     

Photofragment translational spectroscopy of 1,3-butadiene and 1,3-butadiene-1,1,4,4-d(4) at 193 nm

The photodissociation dynamics of 1,3-butadiene at 193 nm have been investigated with photofragment translational spectroscopy coupled with product photoionization using tunable VUV synchrotron radiation. Five product channels are evident from this study: C(4)H(5) + H, C(3)H(3) + CH(3), C(2)H(3) + C(2)H(3), C(4)H(4) + H(2), and C(2)H(4) + C(2)H(2). The translational energy (P(E(T))) distributions suggest that these channels result from internal conversion to the ground electronic state followed by dissociation. To investigate the dissociation dynamics in more detail, further studies were carried out using 1,3-butadiene-1,1,4,4-d(4). Branching ratios were determined for the channels listed above, as well as relative branching ratios for the isotopomeric species produced from 1,3-butadiene-1,1,4,4-d(4) dissociation. C(3)H(3) + CH(3) is found to be the dominant channel, followed by C(4)H(5) + H and C(2)H(4) + C(2)H(2), for which the yields are approximately equal. The dominance of the C(3)H(3) + CH(3) channel shows that isomerization to 1,2-butadiene followed by dissociation is facile.     

Formation of H and D atoms in pyrolysis of 1,3-butadiene and 1,3 butadiene-1,1,4,4,-d4 behind shock waves

Highly dilute mixtures of 1,3-butadiene and 1,3-butadiene-1,1,4,4-d₄ were pyrolyzed behind reflected and incident shock waves, respectively. Concentrations of H and D atoms were measured by resonance absorption spectroscopy. In the early stages of the reaction, nearly equal amounts of H and D were formed from CD₂CHCHCD₂, indicating that loss of H from C₂ followed by loss of D from C₁ is a more important reaction than breaking of the central C? C bond. Overall, rate constants for atom-forming reactions are much slower than rate constants for disappearance of butadiene in earlier experiments, suggesting that most of the butadiene disappears by processes that do not involve H or D atoms or by radicals that produce them rapidly.     

Synthesis of 1,1,4,4-tetrabromo-2-butenes and related compounds via desilylation-bromination of silylated 1,3-butadiene derivatives

The combination of zirconocene-mediated coupling of silylated alkynes with a protonation-desilylation or bromination-desilylation process afforded otherwise unavailable butadiene derivatives. When (E,E)-2,3-dialkyl-1,4-bis(trimethylsilyl)-1,3-butadienes were treated with 3 equiv of Br(2) in CH(2)Cl(2), (E)-2,3-dialkyl-1,1,4,4-tetrabromo-2-butenes were obtained in excellent yields with perfect stereoselectivity.     

Selective determination of 2,4-xylenol by gas chromatography/supersonic jet/resonance-enhanced multiphoton ionization/time-of-flight mass spectrometry

Gas chromatography/supersonic jet/resonance-enhanced multiphoton ionization/time-of-flight mass spectrometry (GC/SSJ/REMPI/TOF-MS) was employed for isomer-selective determination of 2,4-xylenol in river and seawater samples. The sample containing 2,4-xylenol was measured using argon, rather than helium, as the GC carrier gas to cool the analyte molecule sufficiently. The instrumental detection limit (IDL) achieved at a flow rate of 1 mLmin(-1) was 14 pg. Although this value was comparable to the value (ca. 10 pg) obtained by gas chromatography/electron impact/quadrupole mass spectrometry (GC/EI/QMS). When the flow rate was increased to 8 mLmin(-1), interference from the 2,5-xylenol isomer was completely suppressed. The IDL was degraded to 83 or 160 pg at a flow rate of 5 or 8 mLmin(-1), respectively. The recovery of 2,4-xylenol from the river and the seawater samples was 85 and 93%, respectively. The time for analysis was only 10 min per one sample in GC/SSJ/REMPI/TOF-MS. These results suggest that GC/SSJ/REMPI/TOF-MS is useful for the selective measurement of 2,4-xylenol, which has been designated a Class I chemical substance in the Pollutant Release and Transfer Register (PRTR).     

Analysis of thermal decomposition products arising from polyvinyl chloride analogs by supersonic jet/multiphoton ionization/mass spectrometry

Polyvinyl chloride (PVC) and chlorinated polyvinyl chloride (CPVC) were thermally decomposed at 200-500 degrees C, and the reaction products measured by supersonic jet/multiphoton ionization/mass spectrometry. No precursor molecules of dioxins, such as chlorobenzene, were observed from PVC, although benzene was produced as one of the major components. On the other hand, a large peak corresponding to chlorobenzene was observed, when CPVC was used as a sample. These results suggest that the release of hydrogen chloride and aromatic ring formation occur efficiently and produce chlorinated aromatic hydrocarbons only when excess chlorine atoms are present in the chain of PVC. This method, which has very high selectivity is preferred for trace analysis of specific compounds such as dioxin precursors in a complex mixture. Isomer-selective analysis, e.g. detection of o-, m-, and p-dichlorobenzenes, is also demonstrated in this study.     

Analysis of polymers by pyrolysis/chemical ionization mass spectrometry

Two addition-type polymers, polystyrene and polypropylene, and two condensation-type polymers, poly(ethylene terephthalate) (Mylar) and poly(hexamethylene adipamide) (nylon-6,6), were pyrolyzed directly in the ion source of a mass spectrometer by chemical ionization (CI) to reduce fragmentation. The spectra are relatively easy to interpret because most ion peaks occur at a mass corresponding to the ion formula M±H or M + 3H, where M is the formula of a fragment of the original polymer chain. Oligomer peaks were observed for the addition-type polymers. The intensities of the protonated dimer and monomer were measured as a function of time for styrene (S) and propylene (P). It was found that essentially all the HS⁺2 was produced by the reaction of HS⁺ with S, but at least part of the HP⁺2 was formed by proton transfer to P₂. The absence of the analogous reaction in styrene is presumably a result of the reduced volatility of S₂.     

Non-statistical behavior in the dissociative photoionization of the 1,3-butadiene/sulfur dioxide van der Waals complex

Dissociative photoionization of the van der Waals complex of 1,3-butadiene and SO₂ to yield the fragment ion C₄H₆-SO⁺ is reported. From     

Photoionization studies on various quinones by an infrared laser desorption/tunable VUV photoionization TOF mass spectrometry

Photoionization and dissociative photoionization characters of six quinones, including 1,2-naphthoquinone (1,2-NQ), 1,4-naphthoquinone (1,4-NQ), 9,10-phenanthroquinone (PQ), 9,10-anthraquinone (AQ), benz[a]- anthracene-7,12-dione (BAD) and 1,2-acenaphthylenedione (AND) have been studied with an infrared laser desorption/tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry (IR LD/VUV PIMS) technique. Mass spectra of these compounds are obtained at different VUV photon energies. Consecutive losses of two carbon monoxide (CO) groups are found to be the main fragmentation pathways for all the quinones. Detailed dissociation processes are discussed with the help of ab initio B3LYP calculations. Ionization energies (IEs) of these quinones and appearance energies (AEs) of major fragments are obtained by measuring the photoionization efficiency (PIE) spectra. The experimental results are in good agreement with the theoretical data.     

Bridged silver(I) complexes of the polycyclic aromatic compounds tetraphenylethylene and 1,1,4,4-tetraphenyl-1,3-butadiene

For the purpose of investigating the coordination behavior of the sterically congested alkenes and exploring the possibility of cofacial complexation in the polycyclic aromatic system for formation of extended polymeric networks, tetraphenylethylene (tphe) and 1,1,4,4-tetraphenyl-1,3-butadiene (tphb) have been studied with regard to their complexation with a silver(I) ion. The crystal structures of [Ag(tphe)(ClO4)(p-xylene)], [Ag2(tphe)(ClO4)2], [Ag4(tphe)(CF3SO3)4], [Ag2(tphb)(ClO4)2], and [Ag2(tphb)(CF3SO3)2], together with the metal-free ligands tphe and tphb, have been determined by single-crystal X-ray diffraction. The pi-electron-rich cleft in organic components is found to offer a potential site for complexation, which can be utilized to generate an interesting array of organometallic compounds with one- and two-dimensional frameworks.     

Analysis of acrylonitrile, 1,3-butadiene, and related compounds in acrylonitrile-butadiene-styrene copolymers for kitchen utensils and children's toys by headspace gas chromatography/mass spectrometry

A headspace gas chromatography/mass spectrometry method was developed for the simultaneous determination of the residual levels of acrylonitrile (AN), 1,3-butadiene (1,3-BD), and their related compounds containing propionitrile (PN) and 4-vinyl-1-cyclohexene (4-VC) in acrylonitrile-butadiene-styrene (ABS) copolymers for kitchen utensils and children's toys. A sample was cut into small pieces, then N,N-dimethylacetamide and an internal standard were added in a sealed headspace vial. The vial was incubated for 1 h at 90 degrees C and the headspace gas was analyzed by gas chromatography/mass spectrometry. The recovery rates of the analytes were 93.3-101.8% and the coefficients of variation were 0.3-6.5%. In ABS copolymers, the levels were 0.3-50.4 microg/g for AN, ND-4.5 microg/g for PN, 0.06-1.58 microg/g for 1,3-BD, and 1.1-295 microg/g for 4-VC. The highest level was found for 4-VC, which is a dimer of 1,3-BD, and the next highest was for AN, which is one of the monomers of the ABS copolymer. Furthermore, the method was also applied to acrylonitrile-styrene (AS) copolymers and polystyrenes (PS) for kitchen utensils, and nitrile-butadiene rubber (NBR) gloves. In AS copolymers, AN and PN were detected at 16.8-54.5 and 0.8-6.9 microg/g, respectively. On the other hand, the levels in PS and NBR samples were all low.     

Sensitive and selective analysis of polychlorinated dibenzo-p-dioxins/dibenzofurans and their precursors by supersonic jet/resonance-enhanced multiphoton ionization/time-of-flight mass spectrometry.

Polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs), some of which are extremely toxic, are present at low concentrations in incinerator flue gases. The on-line real-time measurement of these compounds is necessary, since these concentrations fluctuate drastically with the process conditions used. Supersonic jet/resonance-enhanced multiphoton ionization/time-of-flight mass spectrometry (SSJ/REMPI/TOF-MS), which provides both high selectivity and sensitivity, appears suitable for application to the on-line real-time monitoring of PCDD/Fs. In this review, the resonant ionization wavelengths of PCDD/Fs and their precursors are given. Moreover, improvements in SSJ/REMPI/TOF-MS that lead to enhanced sensitivity and selectivity are presented. That is, novel ionization sources and advanced ionization schemes are reported for the sensitive measurement of PCDD/Fs and their precursors. Those techniques are advantageous for the trace analysis of these compounds.     

Study of the mechanism of polysulfone decomposition by pyrolysis/gas chromatography and pyrolysis/gas chromatography/mass spectrometry

Both co- and terpolysulfones have been flash-pyrolyzed at high temperature followed by separation and identification of the products by gas chromatography and/or gas chromatography/mass spectrometry. As expected, most of the products were the corresponding olefin and SO₂. Additionally, higher molecular weight products, including aromatics, and olefin isomerization products, were produced. Mechanisms for initiation and formation of the higher molecular weight products are presented which include the back reaction of intermediate free radicals to abstract hydrogen or to form C? C bonds followed by expulsion of SO₂. The free-radical intermediates formed by the SO₂ expulsion undergo transformations to give the aromatic products. No breakdown products were found with either O or S present, nor was SO₂H found.     

四元共聚物的热裂解色谱-质谱分析

报道了2－丁烯、甲基丙烯酸甲酯、丙烯酸丁酯、丙烯酸辛酯四元共聚物在不同裂解温度下的热裂解行为。用热裂解色谱－质谱分析方法，研究了裂解温度对裂解产物的影响，并讨论了四元共聚物的热裂解规律。     

Determination of nivalenol and deoxynivalenol by liquid chromatography/atmospheric pressure photoionization mass spectrometry

Fusarium species, a plant pathogenic fungus of wheat and other cereals, produces toxic metabolites such as nivalenol (NIV) and deoxynivalenol (DON). Control of contamination by these toxins is very difficult, and a continuous survey of the occurrence is necessary for these toxins. Thus, the accurate and convenient determination of the cereals contaminated with these toxins is important for the supply of safe foods. A selective analytical method based on high‐performance liquid chromatography, combined with atmospheric pressure photoionization (APPI) mass spectrometry, has been developed for simultaneous determination of NIV and DON. The parameters investigated for the optimization of APPI were the ion source parameters fragmentor voltage, capillary voltage, and vaporizer temperature, and also mobile phase composition and flow rate. Furthermore, chemical noise and signal suppression of analyte signals due to sample matrix interference were investigated for APPI. The results indicated that APPI provides lower matrix effect and the correlation coefficient of NIV and DON in the range 0.2–100 ng · mL^?1 was above 0.999. Recoveries of NIV and DON in wheat ranged from 86 to 107% and limits of detection of NIV and DON were 0.20 ng · g^?1 and 0.39 ng · g^?1, respectively. In addition, the proposed method was applied for the analysis of naturally contaminated wheat samples. APPI was found to offer lower matrix effect and was a convenient technique for routine analysis of NIV and DON residues in wheat at trace levels. Copyright © 2009 John Wiley & Sons, Ltd.     

Heat of formation for the SH radical by photoionization mass spectrometry

Photoionization mass spectrometry has been used to measure the appearance energies for [C₂H₅]⁺ from ethanethiol, [C₃H₇]⁺ from 2-propanethiol and [C₃H₅]⁺ from 2-methylthiirane. From the known thermochemistry of these cations and their precursor molecules, a 298 K heat of formation of 138.6±0.4 kJ mol^?1 for the SH radical has been derived.