Alkene Loss Research Articles

Chemical Composition of Larch Oleoresin before and during Thermal Modification

Larch is a strong timber, which grows rapidly in the UK climate, but can contain abundant resin pockets. To address the resin exudation issue, a mild thermal modification process has been developed, promoting the curing of the resin. This paper reports a series of studies which characterised the chemical profile of larch oleoresin before and after the mild thermal treatment, explaining the changes which occur when resin is dried. Further experiments were used to simulate specific points in time during the mild treatment process. The non-polar components of the fresh (untreated) and treated larch oleoresin were profiled using gas chromatography mass spectrometry (GC-MS). Fresh larch oleoresin was also subjected to isothermal experiments at different temperatures in a thermogravimetric analyser–differential scanning calorimeter (TGA/DSC), followed by re-analysing the resin composition. This demonstrated the loss of monoterpenes at temperatures of 120 °C and above, with complete loss by isothermal conditions of 150 °C and 60 min. The partial loss of sesquiterpene alkanes and alkenes were also observed at all temperatures, although completeness of this loss was achieved at isothermal temperatures of 150 °C and above. The diterpene composition was seen to change for isothermal experiments conducted at 150 °C and above, with a dehydration of terpenols to form the equivalent terpene alkenes. The observed physical changes in the TGA/DSC experiment were in good agreement with observations of the oleoresin sampled from thermally modified larch planks.

The importance of photochemical loss to source analysis and ozone formation potential: Implications from in-situ observations of volatile organic compounds (VOCs) in Guangzhou, China

The chemical loss of volatile organic compounds (VOCs) is influenced by atmospheric and is closely related to the photochemical formation of ozone (O3). Therefore, research on the atmospheric chemical transformation of VOCs is essential to understand their contribution to O3 formation. This study collected data on photochemical-related atmospheric components and meteorological factors in spring at a typical comprehensive observation platform in Guangzhou, China. The photochemical age method was used to evaluate the loss of VOCs due to atmospheric photochemical transformation and to analyze the contribution of such transformation to the source apportionment of VOCs and to the ozone formation potential (OFP). The results show that the mean photochemical loss of total volatile organic compounds (TVOCs) during the observation period was 4.1 ppbv, accounting for approximately 15% of the initial TVOC content. Alkenes had the highest consumed concentration/initial concentration (C/I ratio) of 37.3%. A comparison of the source apportionment results obtained using observational positive matrix factorization (Ob-PMF) and in situ positive matrix factorization (In-PMF) revealed that photochemical losses significantly affected VOCs emitted from industrial and biological sources. Furthermore, neglecting the photochemical loss of VOCs in PMF source apportionment could lead to an underestimation of the contribution of VOCs from industrial and biological sources. The calculated values of OFP under VOC loss (OFPC–VOCs) showed that OFPC-VOCs constituted approximately 30% of the OFP of the initial VOCs. 93.2% of the OFP was attributed to the photochemical loss of alkenes and aromatic hydrocarbons, indicating their significant influence on O3 formation. This study emphasizes the importance of fully considering the photochemical loss of VOCs for accurately assessing their sources and contributions to ozone pollution. It also provides theoretical support for formulating VOC emission reduction strategies in the Pearl River Delta region.

Selective deep desulfurization of liquefied petroleum gas on Ni/ZnO-based catalyst

Process of selective deep desulfurization of liquefied petroleum gas (LPG) was developed using a Ni/ZnO-based catalyst. Ethyl mercaptan, dimethyl disulfide and methyl disulfide were used as model compounds in LPG to investigate sulfides conversion over different catalysts. All three model sulfides were totally removed over the composite catalyst at above 300 °C in hydrogen atmosphere. Desulfurization of LPG from FCC unit were studied in a micro-fixed bed reactor, and scaled up in a fixed-fluidized bed reactor. Results showed that highly selective desulfurization of LPG could be achieved using the composite catalyst when the sulfur loading of catalyst was in a particular range. The sulfur content of LPG could be decreased to lower than 2.0 μg·g−1 with an olefin loss ratio of less than 1.5 %. X-ray photoelectron spectroscopy (XPS) characterization indicated that the interaction of Ni with S strengthened with the increasing of sulfur loading. Density functional theory calculations showed that the pre-coated S on Ni (111) surface increased the adsorption energy difference between propylene and sulfides which was helpful for the retention of propylene during desulfurization on Ni/ZnO-based catalyst.

Characterization of photochemical losses of volatile organic compounds and their implications for ozone formation potential and source apportionment during summer in suburban Jinan, China

Volatile organic compounds (VOCs) undergo substantial photochemical losses during their transport from emission sources to receptor sites, resulting in serious implications for their source apportionment and ozone (O3) formation. Based on the continuous measurements of VOCs in suburban Jinan in August 2022, the effects of photochemical losses on VOC source contributions and O3 formation were evaluated in this study. The observed and initial concentrations of total VOCs (TVOC) were 12.0 ± 5.1 and 16.0 ± 7.4 ppbv, respectively. Throughout the observation period, alkenes had the most prominent photochemical losses (58.2%), followed by aromatic hydrocarbons (23.1%), accounting for 80.6% and 6.9% of the total losses, respectively. During high O3 episodes, the photochemical loss of VOCs was 6.9 times higher than that during the cleaning period. Alkene losses (exceeding 67.3%), specifically losses of isoprene, propylene, ethylene, and n-butene, dominated the total losses of VOCs during the O3 increase period. Eight sources of VOCs were identified by positive matrix factorization (PMF) based on the observed and initial concentration data (OC-PMF and IC-PMF, respectively). Concentrations of all emission sources in the OC-PMF were underestimated by 2.4%–57.1%. Moreover, the contribution of each emission source was over- or underestimated compared with that in case of the IC-PMF. The contributions of biogenic and motor vehicle exhaust emissions were underestimated by 5.3 and 2.8 percentage points, respectively, which was associated with substantial oxidation of the emitted high-reactive species. The contributions of coal/biomass burning and natural gas were overestimated by 2.4 and 3.9 percentage points, respectively, which were related to the emission of low-reactive species (acetylene, ethane, and propane). Based on our results, the photochemical losses of VOCs grossly affect their source apportionment and O3 formation. Thus, photochemical losses of VOCs must be thoroughly accounted to establish a precise scientific foundation for air-pollution control strategies.

Reactive adsorption desulfurization of FCC gasoline over self-sulfidation adsorbent

NiSO4/ZnO-Al2O3-SiO2 adsorbent, as a novel reactive adsorption desulfurization (RADS)adsorbent, has self-sulfidation ability and high desulfurization performance when treating model fuel. However, the RADS performance of the adsorbent under FCC gasoline feedstock is not well understood. Here, we compare the performance of the adsorbent under FCC gasoline with model fuel to test the effect of the hydrocarbon composition of the feedstock. The complex composition of FCC gasoline makes it sensitive to the nickel loading amount of the adsorbent. Under the influence of specific surface area and pore structure, the adsorbent loaded with 3 wt% nickel shows better desulfurization activity when treating FCC gasoline. As hydrocarbons in FCC gasoline compete with sulfur compounds for active Ni sites, the reaction conditions have been tested and optimized to achieve deep desulfurization while maintaining low olefin loss to ensure the octane number of the product. By experimental tests and characterization, it has been proved that NiSx and NiO acted as active Ni sites of the adsorbent. Based on these observations, the mechanism of RADS of FCC gasoline over NiSO4/ZnO-Al2O3-SiO2 adsorbent has been proposed, which provides theoretical support for the development of new adsorbents and the optimization of RADS process.

Effect of photochemical losses of ambient volatile organic compounds on their source apportionment

Photochemical losses of ambient volatile organic compounds (VOCs) substantially affect source apportionment analysis. Hourly speciated VOC data measured from April to August 2020 in Tianjin, China were used to analyze the photochemical losses of VOC species and assess the impacts of photochemical losses on source apportionment by comparing the positive matrix factorization (PMF) results based on observed and initial concentration data (OC-PMF and IC-PMF). The initial concentrations of the VOC species were estimated using a photochemical age-based parameterization method. The results suggest that the average photochemical loss of total VOCs (TVOCs) during the ozone pollution period was 2.4 times higher than that during the non-ozone pollution period. The photochemical loss of alkenes was more significant than that of the other VOC species. Temperature has an important effect on photochemical losses, and different VOC species have different sensitivities to temperature; high photochemical losses mainly occurred at temperatures between 25 °C and 35 °C. Photochemical losses reduced the concentrations of highly reactive species in the OC-PMF factor profile. Compared with the IC-PMF results, the OC-PMF contributions of biogenic emissions and polymer production-related industrial sources were underestimated by 73 % and 50 %, respectively, likely due to the oxidation of isoprene and propene, respectively. The contribution of diesel and gasoline evaporation was underestimated by 39 %, which was likely due to the loss of m,p-xylene. Additionally, the contributions of liquefied petroleum gas, vehicle emissions, natural gas, and oil refinery were underestimated by 31 %, 29 %, 23 %, and 13 %, respectively. When the O3 concentrations were higher than 140 μg m−3 or the temperatures were higher than 30 °C, the photochemical losses from most sources increased substantially. Additionally, solar radiation produced different photochemical losses for different source types.

Reactivation of NiSO4/ZnO-Al2O3-SiO2 adsorbent for reactive adsorption desulfurization

NiSO4/ZnO-Al2O3-SiO2 adsorbent as a novel adsorbent for the reactive adsorption desulfurization (RADS) process performs a higher activity and lower nickel loading than NiO/ZnO-Al2O3-SiO2 adsorbent. Under reaction conditions with high temperature and high pressure, the reactivity of adsorbents inevitably decreases after long-term use. Therefore, the deactivation and reactivation of the adsorbent need to be well understood. Here, we choose model fuel and FCC gasoline as the feedstock to observe the different deactivation causes. The ZnS formation and carbon deposition are the main reasons for the deactivation. Then, we conduct different regeneration methods and conditions to optimize for different kinds of deactivation. Segmenting regeneration is adopted to ensure the effective removal of carbon deposition while avoiding excessive temperature rise. The effectiveness of the regeneration method was verified by multiple cycle experiments. The analysis of the product shows that the loss of olefin is lower here than that of NiO/ZnO-Al2O3-SiO2 adsorbent. At last, the reactivation mechanism of NiSO4/ZnO-Al2O3-SiO2 adsorbent has been studied. Of note is that since the main active site NiSx is stable during regeneration, reactivated NiSO4/ZnO-Al2O3-SiO2 adsorbent has higher initial RADS activity than NiO/ZnO-Al2O3-SiO2 adsorbent. These results guide the continuous long-term use of NiSO4/ZnO-Al2O3-SiO2 adsorbent and provide a new strategy for the design of RADS adsorbent.

Synthesis of a Fe3-Carbyne Motif by Oxidation of an Alkyl Ligated Iron-Sulfur (WFe3S3) Cluster.

The presence of a carbide ligand in the active site of nitrogenases remains an unusual example of organometallic chemistry employed by a protein. Carbide incorporation into the MFe7S9C cluster involves complex biosynthesis, but analogous synthetic methodologies are limited. Herein, we present a new synthetic strategy for incorporating carbon based bridging ligands into iron-sulfur clusters. Starting from a halide precursor, a WFe3S3 cluster displaying three terminal alkyl ligands and an open Fe3 face was prepared. Oxidation results in loss of alkane and formation of a μ3-carbyne. Characterization of these clusters and mechanistic studies are presented.

Improved positive matrix factorization for source apportionment of volatile organic compounds in vehicular emissions during the Spring Festival in Tianjin, China

Photochemical losses of volatile organic compounds (VOCs) and uncertainties in calculated factor profiles can reduce the accuracy of source apportionment by positive matrix factorization (PMF). We developed an improved PMF method (termed ICLP-PMF) that estimated the reaction-corrected (“initial”) concentrations of VOCs. Source profiles from literature provided constraints. ICLP-PMF evaluated the vehicular emission contributions to hourly speciated VOC data from December 2020 to March 2021 and estimated gasoline and diesel vehicles contributions to Tianjin's VOC concentrations around the Chinese Spring Festival (SF). The average observed and initial total VOCs (TVOCs) concentrations were 24.2 and 42.9 ppbv, respectively. Alkanes were the highest concentration VOCs while aromatics showed the largest photochemical losses during the study period. Literature gasoline and diesel profiles from representative Chinese cities were constructed and provided constraints. Source apportionment was performed using ICLP-PMF method and three other PMF approaches. Photochemical losses of alkenes and aromatic hydrocarbons induced differences between calculated factor profiles and literature profiles. Using observed concentrations and unconstrained profiles produced underestimated SF contributions (∼121% and 72% for gasoline and diesel vehicles, respectively). According to the ICLP-PMF results, the contributions of gasoline and diesel vehicles during the SF were 25.6% and 23.2%, respectively, lower than those before and after the SF. No diel diesel vehicle contribution variations were found during the SF likely due to the decline in truck activity north of the study site during the holiday period.

Evaluation of offline sampling for atmospheric C3-C11 non-methane hydrocarbons

The concentration variation of C3-C11 non-methane hydrocarbons (NMHCs) collected in several types of commercial flexible bags and adsorption tubes was systematically investigated using a gas chromatography-flame ionization detector (GC-FID) system. The percentage loss of each NMHC in the polyvinyl fluoride (PVF) bags was less than 5% during a 7-hr storage period; significant NMHCs loss was detected in aluminum foil composite film and fluorinated ethylene propylene bags. The thermal desorption efficiency of NMHCs for adsorption tubes filled Carbopack B and Carboxen1000 sorbents was greater than 95% at 300℃, and the loss of NMHCs in the adsorption tubes during 20-days storage at 4℃ was less than 8%. The thermal desorption efficiency for C11 NMHCs in the adsorption tube filled with Carbograph 1 and Carbosieve SⅢ absorbents was less than 40% at 300℃, and pyrolysis of the absorbents at 330℃ interfered significantly with the measurements of some alkenes. The loss of alkenes was significant when NMHCs were sampled by cryo-enrichment at -90℃ in the presence of O3 for the online NMHC measurements, and negligible for enrichment using adsorption tubes at 25℃. Although O3 scrubbers have been widely used to eliminate the influence of O3 on NMHC measurements, the loss of NMHCs with carbon numbers greater than 8 was more than 10%. Therefore, PVF bags and adsorption tubes filled Carbopack B and Carboxen1000 sorbents were recommended for the sampling of atmospheric NMHCs.

A Series of Crystallographically Characterized Linear and Branched σ-Alkane Complexes of Rhodium: From Propane to 3-Methylpentane.

Using solid-state molecular organometallic (SMOM) techniques, in particular solid/gas single-crystal to single-crystal reactivity, a series of σ-alkane complexes of the general formula [Rh(Cy2PCH2CH2PCy2)(ηn:ηm-alkane)][BArF4] have been prepared (alkane = propane, 2-methylbutane, hexane, 3-methylpentane; ArF = 3,5-(CF3)2C6H3). These new complexes have been characterized using single crystal X-ray diffraction, solid-state NMR spectroscopy and DFT computational techniques and present a variety of Rh(I)···H–C binding motifs at the metal coordination site: 1,2-η2:η2 (2-methylbutane), 1,3-η2:η2 (propane), 2,4-η2:η2 (hexane), and 1,4-η1:η2 (3-methylpentane). For the linear alkanes propane and hexane, some additional Rh(I)···H–C interactions with the geminal C–H bonds are also evident. The stability of these complexes with respect to alkane loss in the solid state varies with the identity of the alkane: from propane that decomposes rapidly at 295 K to 2-methylbutane that is stable and instead undergoes an acceptorless dehydrogenation to form a bound alkene complex. In each case the alkane sits in a binding pocket defined by the {Rh(Cy2PCH2CH2PCy2)}+ fragment and the surrounding array of [BArF4]− anions. For the propane complex, a small alkane binding energy, driven in part by a lack of stabilizing short contacts with the surrounding anions, correlates with the fleeting stability of this species. 2-Methylbutane forms more short contacts within the binding pocket, and as a result the complex is considerably more stable. However, the complex of the larger 3-methylpentane ligand shows lower stability. Empirically, there therefore appears to be an optimal fit between the size and shape of the alkane and overall stability. Such observations are related to guest/host interactions in solution supramolecular chemistry and the holistic role of 1°, 2°, and 3° environments in metalloenzymes.

Characteristics of Ozone and Source Apportionment of the Precursor VOCs in Tianjin Suburbs in Summer

From June to August 2018, a 1-hr resolution concentration dataset of ozone and its gaseous precursors (volatile organic compounds(VOCs) and NOx), and meteorological parameters were synchronously monitored by online instruments of the Nankai University Air Quality Research Supersite. The relationships and variation characteristics between ozone and its precursors were analyzed. According to the photochemical age, the initial concentrations of VOCs were calculated, and the photochemical loss of the concentration of VOCs during the daytime (06:00-24:00) was corrected. The initial and directly monitored concentrations of VOCs were incorporated into the PMF model for source apportionment. The results indicated that the mean concentration of O3 in Tianjin in summer was (41.3±25.7)×10-9, while that of VOCs was (13.9±12.3)×10-9. The average concentration of alkane (7.0±6.8)×10-9 was clearly higher than that of other VOC species. The species with high concentrations of alkanes were propane and ethane, accounting for 47% of the total alkane concentration. The average ozone formation potential (OFP) in summer was 52.1×10-9, and the OFP value of alkene was the highest and its contribution reached 57%. During the daytime, alkene loss accounted for 75% of the total VOC loss. The major sources of VOCs that were calculated based on the initial concentration data were the chemical industry and solvent usage (25%), automobile exhaust (22%), combustion source (19%), LPG/NG (19%), and gasoline volatilization (15%), respectively. Compared with the apportionment results based on directly monitored concentrations, the contribution of the chemical industry and solvent usage decreased by 4%, while automobile exhaust decreased by 5%. By combining the results of PMF apportionment and the OFP model to analyze the relative contributions of emission sources to ozone formation, and we found that the highest contribution source of ozone was the chemical industry and solvent usage (26%) in summer. Compared with the analysis results based on the directly monitored concentrations, the OFP values of the chemical industry and solvent usage decreased by 7%, while that of NG/LPG apparently decreased by 13%.

Methanogenic crude oil-degrading microbial consortia are not universally abundant in anoxic environments

Crude oil-amended microcosms were prepared with inocula from eleven anoxic environments (4 river sediments, 3 lake sediments, and 4 sludges from wastewater treatment reactors) to determine their ability to produce methane from the biodegradation of crude oil. Over incubation periods of up to 1150 days, oil-stimulated methanogenesis and concomitant loss of alkanes occurred in microcosms prepared with five of the inocula whereas six of the inocula did not show oil-stimulated methane production. Bacterial and archaeal communities from microcosms exhibiting high levels of oil-stimulated methanogenesis were distinct from communities where methanogenic crude oil degradation was not detected. Successional changes were consistent with the quantitative enrichment of syntrophic hydrocarbon degrading bacteria and methanogens over time. In conclusion, in oil-impacted environments methanogenic crude oil-degrading microbial consortia are present in relatively low abundance and exhibit slow growth, and while they may be ubiquitously distributed they may not be present at sufficiently high abundance to be detected.

Chromatographic analyses and unambiguous identification of atranol‐like secondary metabolites in oakmoss absolute

AbstractOakmoss is a commercially significant natural extract used in exotic perfumes, cosmetics, and other personal care products. Unfortunately, oakmoss absolutes and oils also contain atranol and chloroatranols, well‐known potent skin sensitizers, and hence, their levels in personal care products are regulated. To address the presence of these metabolites and unravel the similar metabolites in the oakmoss absolute, two independent analytical methods were developed. As a proof of concept, one commercial absolute oakmoss was analyzed with complementary, orthogonal LC/MS, GC/MS methods and further confirmed with MS2 experiments. Sixteen secondary metabolites, including atranols, were identified by both GC and LC analyses. Many of the identified components appear to be the precursors of the atranols or process artifacts. One atranol analog, in particular, isopropyl haematommate, was investigated extensively with electron ionization mass spectrometry to show that fragmentation spectra can be applied for the unambiguous identification of related compounds. An intriguing fragmentation pattern was observed for a typical α‐hydroxybenzoate, involving sequential loss of alkene, alkanol, water, or carbon monoxide. A similar fragmentation pattern has been observed with ESI; however, the loss of CO2 was dominant, while the loss of CO was most commonly observed with the EI system independent of substituents on the aromatic ring. The identification of these metabolites was further confirmed with pure standards. Even though many of these unique secondary metabolites are structurally similar to atranols, their skin sensitization potential is unknown; hence, the developed methods can serve as an effective tool to assess the overall quality of various oakmoss‐based products.

Leaving Group Effects in a Series of Electrosprayed CcHhN1 Anthracene Derivatives.

We investigate the gas-phase structures and fragmentation pathways of model compounds of anthracene derivatives of the general formula CcHhN1 utilizing tandem mass spectrometry and computational methods. We vary the substituent alkyl chain length, composition, and degree of branching. We find substantial experimental and theoretical differences between the linear and branched congeners in terms of fragmentation thresholds, available pathways, and distribution of products. Our calculations predict that the linear substituents initially isomerize to form lower energy branched isomers prior to loss of the alkyl substituents as alkenes. The rate-determining chemistry underlying these related processes is dominated by the ability to stabilize the alkene loss transition structures. This task is more effectively undertaken by branched substituents. Consequently, analyte lability systematically increased with degree of branching (linear < secondary < tertiary). The resulting anthracen-9-ylmethaniminium ion generated from these alkene loss reactions undergoes rate-limiting proton transfer to enable expulsion of either hydrogen cyanide or CNH. The combination of the differences in primary fragmentation thresholds and degree of radical-based fragmentation processes provide a potential means of distinguishing compounds that contain branched alkyl chain substituents from those with linear ones.

Optimizing both the CoMo/Al2O3 catalyst and the technology for selectivity enhancement in the hydrodesulfurization of FCC gasoline

Herein, a series of alumina supports were prepared by rehydration-dehydration of the traditional γ-Al2O3 under different hydrothermal temperatures. After loading CoMo species, it was found the selectivity factor of corresponding CoMo/Al2O3 catalysts was improved greatly from 1.21 to 2.51 with the increased hydrothermal temperature of alumina supports in the hydrodesulfurization (HDS) of a model fluid catalytic cracking (FCC) naphtha. The improved HDS selectivity is attributed to the weakened metal-support interaction and lowered dispersion of MoS2 particles, resulting in catalysts with larger edge-to-corner ratios of CoMoS slabs ((fe/fc)CoMo). Besides, the recombination mercaptans formed by reaction of produced H2S and olefins were found as a big challenge to reach a high HDS conversion (S < 10 ppm) when only using the selective HDS catalyst (one-unit process). Therefore, a two-unit process has been developed by using a selective HDS catalyst to remove the most of the refractory sulfur compounds firstly, after which using another mercaptan-removing catalyst to deal with the recombination mercaptans specially. The catalytic results show that the two-unit process has a smaller olefin loss (4.38 v%) than that of one-unit process (7.08 v%) when reducing the sulfur concentration of a heavy FCC fraction to less than 10 ppm, thus resulting in a 0.7 unit less of the research octane number loss. The present work shows that high HDS conversion and low olefin hydrogenation conversion in selective HDS of FCC gasoline can be achieved through optimization design of both the selective HDS catalyst and the whole technology.

Rate Constants of the Reactions of O(3P) Atoms with Ethene and Propene over the Temperature Range 230–900 K

ABSTRACTThe kinetics of the reactions of ethene and propene with triplet oxygen atoms have been studied over a wide temperature range, T = 230–900 K, using a low‐pressure (P = 1 Torr) flow tube reactor coupled with an electron impact ionization quadrupole mass spectrometer: O + C2H4 → products (1) and O + C3H6 → products (2). The rate constants of the title reactions were determined under pseudo–first‐order conditions, either monitoring the kinetics of O‐atom consumption in excess of alkene or alkene loss in excess of oxygen atoms. The temperature dependence of the rate constant of reaction (1), k1 = 8.64 × 10−17 T1.70 exp(–206/T) cm3 molecule−1 s−1 (uncertainty of 20%), was found to be in excellent agreement with multiple previous data that can be considered as a validation of the experimental approach. The measurements of the rate constant of the reaction of O atoms with propene, k2 = 3.65 × 10−18 T2.20 exp(455/T) cm3 molecule−1 s−1 (uncertainty of 20%), allowed to harmonize the results of previous low‐ and high‐temperature measurements and to recommend the expression for k2 in a wide temperature range, 200–1200 K.

The mechanism of alkene elimination from protonated toluenesulphonamides generated by electrospray ionisation.

The positive ion electrospray mass spectra of a range of sulphonamides of general structure CH3C6H4SO2NHR1 [R1=CnH2n+1 (n=1-7), CnH2n-1 (n=3, 4), C6H5, C6H5CH2 and C6H5CH(CH3)] and CH3C6H4SO2NR1R2 [R1, R2=CnH2n+1 (n=1-8)] are reported and discussed. The protonated sulphonamides derived from saturated primary and secondary aliphatic amines generally fragment to only a limited extent unless energised by collision. Two general fragmentations are observed: firstly, elimination of an alkene, CnH2n, obtained by hydrogen abstraction from one of the CnH2n+1 alkyl groups on nitrogen; secondly, cleavage to form CH3C6H4SO2+. The mechanism by which an alkene is lost has been probed by studying the variation of the intensity of the [M+H-CnH2n]+ signal with the structure of the alkyl substituent(s) on nitrogen and by monitoring the competition between the loss of different alkenes from protonated unsymmetrical sulphonamides in which two different alkyl groups are attached to nitrogen. This fragmentation is favoured by branching of the alkyl group at the carbon atom directly attached to nitrogen, thus suggesting that it involves a mechanism in which the stability of the cation obtained by stretching the bond connecting the nitrogen atom to the alkyl group is critical. This interpretation also explains the competition between alkene elimination and cleavage to form CH3C6H4SO2+ (and, in some cases, cleavage to form C6H5CH2+ or [C6H5CHCH3]+).

Rapid Oxidation of Skin Oil by Ozone

The reaction of gas-phase ozone with human skin oil has been studied at room temperature. Skin oil was exposed to ozone at mixing ratios similar to those in the ambient environment and then analyzed for condensed-phase products using direct analysis in real time mass spectrometry (DART-MS). Prior to ozone exposure, skin oil gives rise to prominent mass spectral signals indicative of highly unsaturated alkenes, sterols, triglycerides, long-chain fatty acids, pyroglutamic acid, and probably waxy esters. Upon oxidation with 50 ppb ozone for 90 min, there is a rapid loss of alkene, fatty acid, and triglyceride signals resulting from efficient multiphase ozonolysis. Oxygenated products, including a variety of carboxylic acids, are identified via studies with pure compounds present in skin oil, i.e., squalene, cholesterol, and triolein. The chemistry is rapid, occurring on time scales of tens of minutes, implying that these highly oxygenated reaction products are always present on human skin both indoors and ou...

Competitive homolytic and heterolytic decomposition pathways of gas-phase negative ions generated from aminobenzoate esters.

An alkyl-radical loss and an alkene loss are two competitive fragmentation pathways that deprotonated aminobenzoate esters undergo upon activation under mass spectrometric conditions. For the meta and para isomers, the alkyl-radical loss by a homolytic cleavage of the alkyl-oxygen bond of the ester moiety is the predominant fragmentation pathway, while the contribution from the alkene elimination by a heterolytic pathway is less significant. In contrast, owing to a pronounced charge-mediated ortho effect, the alkene loss becomes the predominant pathway for the ortho isomers of ethyl and higher esters. Results from isotope-labeled compounds confirmed that the alkene loss proceeds by a specific γ-hydrogen transfer mechanism that resembles the McLafferty rearrangement for radical cations. Even for the para compounds, if the alkoxide moiety bears structural motifs required for the elimination of a more stable alkene molecule, the heterolytic pathway becomes the predominant pathway. For example, in the spectrum of deprotonated 2-phenylethyl 4-aminobenzoate, m/z 136 peak is the base peak because the alkene eliminated is styrene. Owing to the fact that all deprotonated aminobenzoate esters, irrespective of the size of the alkoxy group, upon activation fragment to form an m/z 135 ion, aminobenzoate esters in mixtures can be quantified by precursor ion discovery mass spectrometric experiments.