Structural characterization of the GM1 ganglioside by infrared multiphoton dissociation, electron capture dissociation, and electron detachment dissociation electrospray ionization FT-ICR MS/MS

Gangliosides play important biological roles and structural characterization of both the carbohydrate and the lipid moieties is important. The FT-ICR MS/MS techniques of electron capture dissociation (ECD), electron detachment dissociation (EDD), and infrared multiphoton dissociation (IRMPD) provide extensive fragmentation of the protonated and deprotonated GM1 ganglioside. ECD provides extensive structural information, including identification of both halves of the ceramide and cleavage of the acetyl moiety of the N-acetylated sugars. IRMPD provides similar glycan fragmentation but no cleavage of the acetyl moiety. Cleavage between the fatty acid and the long-chain base of the ceramide moiety is seen in negative-ion IRMPD but not in positive-ion IRMPD of GM1. Furthermore, this extent of fragmentation requires a range of laser powers, whereas all information is available from a single ECD experiment. However, stepwise fragmentation by IRMPD may be used to map the relative labilities for a series of cleavages. EDD provides the alternative of electron-induced fragmentation for negative ions with extensive fragmentation, but suffers from low efficiency as well as complication of data analysis by frequent loss of hydrogen atoms. We also show that analysis of MS/MS data for glycolipids is greatly simplified by classification of product ion masses to specific regions of the ganglioside based solely on mass defect graphical analysis.     

Rapid method for the determination of alachlor, atrazine and metolachlor in groundwater by solid-phase extraction

A solid-phase extraction method is described for the separation of alachlor, atrazine and metolachlor from groundwater using solid-phase disposable columns. The method is rapid, reproducible and uses considerably fewer reagents than classical liquid-liquid methods. The average recoveries were greater than 90% for all three compounds.     

Elucidation of the decomposition pathways of protonated and deprotonated estrone ions: application to the identification of photolysis products

With the future aim of elucidating the unknown structures of estrogen degradation products, we characterized the dissociation pathways of protonated estrone (E1) under collisional activation in liquid chromatography/tandem mass spectrometry (LC/MS/MS) experiments employing a quadrupole time‐of‐flight mass spectrometer. Positive ion and negative ion modes give information on the protonated and deprotonated molecules and their product ions. The mass spectra of estrone methyl ether (CH₃‐E1) and estrone‐d₄ (E1‐d₄) were compared with that of E1 in order (i) to elucidate the dissociation mechanisms of protonated and deprotonated molecules and (ii) to propose likely structures for each product ions. The positive ion acquisition mode yielded more fragmentation. The mass spectra of E1 were compared with those of estradiol (E2), estriol (E3) and 17‐ethynylestradiol (EE2). This comparison allowed the identification of marker ions for each ring of the estrogenic structure. Accurate mass measurements have been carried out for all the identified ions. The resulting ions revealed to be useful for the characterization of structural modifications induced by photolysis on each ring of the estrone molecule. These results are very promising for the determination of new metabolites in the environment. Copyright © 2010 John Wiley & Sons, Ltd.     

Quantum chemical mass spectrometry: ab initio prediction of electron ionization mass spectra and identification of new fragmentation pathways

The electron ionization mass spectra of four organic compounds are predicted based on the results of quantum chemical calculations at the DFT/B3LYP/6‐311 + G* level of theory. This prediction is performed ‘ab initio’, i.e. without any prior knowledge of the thermodynamics or kinetics of the reactions under consideration. Using a set of rules determining which routes will be followed, the fragmentation of the molecules' bonds and the complete resulting fragmentation pathways are studied. The most likely fragmentation pathways are identified based on calculated reaction energies ΔE when bond cleavage is considered and on activation energies ΔE^? when rearrangements are taken into account; the final intensities of the peaks in the spectrum are estimated from these values. The main features observed in the experimental mass spectra are correctly predicted, as well as a number of minor peaks. In addition, the results of the calculations allow us to propose fragmentation pathways new to empirical mass spectrometry, which have been experimentally verified using tandem mass spectrometry measurements. Copyright © 2016 John Wiley & Sons, Ltd.     

Differentiation of diastereomeric N-aryltetrahydropyrano/tetrahydrofuranochromenylamines under electron ionization and chemical ionization conditions

A series of diastereomeric 4S,5S,6R/S-tetrahydropyrano- and 3S,4S,5R/S-tetrahydrofuranochromenylamine derivatives (a/b isomers; 1-26) has been studied under electron ionization (EI) and chemical ionization (CI) conditions. The EI mass spectra of all diastereomeric compounds show two characteristic fragment ions, of which one is formed by retro-Diels-Alder (RDA) reaction from the molecular ion, retaining the charge on the diene fragment, and the other [M-(HNAr)]+ ion by a simple radical loss. The RDA process is more favorable in all b isomers, whereas the radical loss is dominant in all a isomers; based on these two ions it is easy to differentiate the two diastereomers. The collision-induced dissociation (CID) spectra of all the molecular ions also show the same trend, which reflects the stereoselectivity in the formation of the two characteristic fragment ions. The results of theoretical calculations performed are in accordance with the experimental observations. The CI experiments (methane and isobutane) on all the diastereomeric compounds also enabled the differentiation of the isomers.     

Molecular spectra and optical investigation of dissociation and ionization of simple gases by electron collisions

This paper reports investigations of the VUV emission spectra which are produced by electron impact of simple molecules. Some new features in the emission spectrum of the N₂ molecule are given. Furthermore, new measurements of the dissociative excitation of N₂ and H₂O are reported and the excitation mechanisms are discussed.     

Differentiation of diastereomeric conduramine derivatives under electron ionization and chemical ionization mass spectral conditions

Diastereomeric conduramine derivatives, i.e., (1R,2S,3R/S,6S)-6-(N-carbomethoxyamino) 1,2-O-isopropylidenecyclohex-4-ene-1,2,3-triol (1 and 2) and their O-acetyl derivatives (3 and 4), were studied using gas chromatography (GC) with electron ionization (EI) and chemical ionization (CI). The EI mass spectra of diastereomeric pairs show consistent differences in the relative abundances of characteristic ions. The EI fragmentation patterns are based on precursor/product ion spectra, high-resolution mass spectrometry (HRMS) and deuterium labelling. The CI spectra show differences from the EI spectra, and the isobutane/CI spectra are much simpler than the methane/CI spectra. The differences shown in the CI spectra are similar to those shown in the product ion spectra of [M+H](+) ions generated under electrospray ionization (ESI) conditions. Theoretical calculations are performed to understand the observed differences. The differences in the relative stabilities of molecular ions, or protonated molecules at different sites, can explain the observed differences in the spectra.     

Differentiation of isomeric 1-, 3- and 7-deazaadenosines by electron ionization mass spectrometry

The low and high resolution electron ionization mass spectra of 1-deazaadenosine, 3-deazaadenosine and 7-deazaadenosine are reported. Fragmentation pathways and ion structures are proposed with the aid of linked-scan, daughter-ion spectra. Results indicate that the N-3 position of the purine ring serves as an important acceptor site in fragmentation processes involving hydrogen transfer from the sugar to the base. A mixture analysis of the trimethylsilyl derivatives of adenosine, 1-, 3- and 7-deazaadenosine by combined gas chromatography/mass spectrometry is also described.     

Characterization of the photodegradation products of metolachlor: structural elucidation,potential toxicity and persistence

Aqueous solutions of metolachlor and metolachlor‐d₆ were photolyzed with UV‐visible radiations. The structures of 15 by‐products of metolachlor were determined through gas chromatography‐mass spectrometry analyses using electron and chemical ionization combined with multistage mass spectrometry. The photolysis by‐products of metolachlor resulted mainly from dehalogenation and hydroxylation, in some cases accompanied by cyclization. In silico tests for toxicity prediction showed that the toxicity of some photolysis products is expected to be greater than that of metolachlor. Persistence studies showed that the by‐product relative abundances vary in large amounts with the irradiation time. The post‐photolysis evolution of the solution was also studied, in order to determine the persistence of the main by‐products. It allowed to establish that most of the by‐products can be found more than 12 h after the end of the photolysis, which is of a great concern as treated water is generally available for consumption only a few hours after treatment in most of industrial processes. Copyright © 2012 John Wiley & Sons, Ltd.     

Sequencing of peptide‐derived Amadori products by the electron capture dissociation method

The electron capture dissociation (ECD) of peptide‐derived Amadori products has been successfully applied for their sequencing. In contrast to the collision induced dissociation (CID), based on the vibrational excitation of peptides, the ECD method does not produce ions formed by fragmentation of the hexose moiety, that facilitates interpretation of the obtained spectra. The fragmentation spectrum is dominated by c_n and z·_n ions, providing the sufficient information for sequencing of peptides and establishing the location of glycated Lys residues in the peptide chain. The ECD experiments were conducted on a series of synthetic peptides and unseparated digests of glycated ubiquitin. Copyright © 2009 John Wiley & Sons, Ltd.     

Ionization and dissociation of C6F6 isomers under electron impact

The mass spectra of hexafluorobenzene, hexafluorobicyclo[2.2.0]hexa-2,5-diene (perfluoro-Dewar benzene) and 1,1,1,6,6,6-hexafluorohexa-2,4-diyne, and the fragmentation mechanisms of their parent ions are reported. The behaviour of the two cyclic isomers under electron impact is very similar; the linear one behaves quite differently. The ionization potentials of the molecules and the appearance potentials of the fragment ions (both normal and metastable) have been measured. The heats of formation of [C₆F₅]⁺ and [C₅F₃]⁺ are calculated. A value for the heat of formation of 1,1,1,6,6,6-hexafluorohexa-2,4-diyne is proposed.     

Mass spectra of new heterocycles: XIV. Investigation of 2-(alkyl- and propargylsulfanyl)-3-(ferrocenylmethoxy)pyrroles by electron ionization and laser desorption/ionization

By mass spectrometry methods applying electron ionization (70 eV) and laser desorption/ionization (NALDI, MALDI) 1-(alkyl-, cycloalkyl-, vinyloxyethyl-, and aryl)substituted 2-(alkyl- and propargylsulfanyl)-3-(ferrocenylmethoxy)pyrroles were studied and characterized. At the electron ionization the peak of the molecular ion possesses very low intensity. The main fraction ions are the ferrocenylmethyl ion (the most abundant peak) and its decomposition ions. Under the NALDI conditions molecular ions and protonated molecules [M + H]⁺ are generated. The general fragmentation rules in the field-free region are determined. By an example of 1-phenyl derivative of pyrrole the MALDI procedure was also demonstrated to be suitable for investigation of 2-(alkylsulfanyl)-3-(ferrocenylmethoxy)pyrroles.     

Fragmentation of pyrazolecarbaldehyde thio- and dithioacetals under electron impact and chemical ionization

The mass spectra of 1-substituted 3,5-dimethyl-1H-pyrazole-4-carbaldehyde bis(2-hydroxyethyl) dithioacetals and thioacetals were studied for the first time. The main fragmentation pathways of their molecular ions generated under electron impact and chemical ionization were similar. Primary decomposition of the molecular ions of bis(2-hydroxyethyl) dithioacetals involves elimination of 2-sulfanylethanol molecule with formation of the corresponding 1,3-oxathiolane radical cation. Fragmentation of the molecular ions [M]⁺ · and [M + H]⁺ derived from 2-(3,5-dimethyl-1H-pyrazol-4-yl)-1,4,6-oxadithiocanes includes cleavage of the eight-membered heteroring and elimination of C₄H₉OS ·. Substituents in the heteroring of pyrazolecarbaldehydes inhibit decomposition processes related to the aldehyde group.     

Investigation of the products of the ozonolysis of sclareol and the pathways for their formation

It has been established that the main products of the ozonolysis of sclareol (I) under various conditions are “sclareol oxide” (III), 8α-hydroxy-14,15-bisnorlabdan-13-ome (IV), and the new compound (13R,14R)-14-hydroperoxy-13,14-epoxy-14-nor-labdan-8α-ol (X), and structure of which has been shown on the basis of its transformations. The acid fraction of the ozonation products, the yield of which was low (<8%) included two compounds: sclareolic acid (85%) and 18α-hydroxy-14,15,-16-trisnorlabdan-13-oic acid (3%). The mechanism for the formation of the above-mentioned compounds is discussed. Institute of Chemistry, Academy of Sciences of the Moldavian SSR, Kishenev. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 502–511, July–August, 1991.     

Structural dependence of the fragmentation of naphthalenesulfonyl halide and naphthalenesulfonamide molecules under electron ionization

By mass spectrometry it is found that naphthalene, 1-naphthalenesulfonyl chloride, 2-naphthalenesulfonyl fluoride, 2-naphthalenesulfonyl chloride, 1,5-naphthalenedisulfonyl chloride, 1-naphthalenesulfonamide, and 2-naphthalenesulfonamide evaporate congruently on heating and their saturated vapor is presented by the corresponding monomeric molecular forms. The relationship is established between the character of the fragmentation of molecules during electron ionization and the specific features of their geometric and electronic structures.     

Investigation of the products of the ozonolysis of sclareol and the pathways for their formation

It has been established that the main products of the ozonolysis of sclareol (I) under various conditions are sclareol oxide (III), 8-hydroxy-14,15-bisnorlabdan-13-ome (IV), and the new compound (13R,14R)-14-hydroperoxy-13,14-epoxy-14-nor-labdan-8-ol (X), and structure of which has been shown on the basis of its transformations. The acid fraction of the ozonation products, the yield of which was low (<8%) included two compounds: sclareolic acid (85%) and 18-hydroxy-14,15,-16-trisnorlabdan-13-oic acid (3%). The mechanism for the formation of the above-mentioned compounds is discussed.Institute of Chemistry, Academy of Sciences of the Moldavian SSR, Kishenev. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 502–511, July–August, 1991.     

Comparison of EI and metastable atom bombardment ionization for the identification of polyurethane thermal degradation products

Polyurethanes are widely used in the manufacture of commercial products such as foams and paints. During combustion, these polymers can generate isocyanates, which induce adverse health effects. Polymer pyrolysis (Py) hyphenated with mass spectrometry (MS) allows the investigation of polymer thermal degradation over time/temperature. A diphenylmethanediisocyanate (MDI) polyurethane foam was analyzed with electron ionization (EI) and metastable atom bombardment (MAB) ionization at a pyrolysis temperature of 400 °C. The recently introduced MAB ionization source uses discrete energy stored in metastable atoms of gases to ionize the analytes. This characteristic allows modulation of the ionization energy by simply changing the ionization gas. The extensive fragmentation of molecular ions observed using EI 70 eV is not totally eliminated with EI 10 eV. However, only molecular ions are observed with MAB using N₂ as the ionization gas. Temperature gradients were used to separate the products generated during the thermal degradation of a 1,6-hexamethylenediisocyanate (HDI) polyurethane paint. The analysis of mass spectra was facilitated owing to a selective desorption of pyrolysis products. Furthermore, changing the MAB ionization gas allows elucidation of the structure of the pyrolysis products by controlling the extent of their fragmentation. During these experiments, isocyanic acid, methyleneisocyanate, ethyleneisocyanate, propylisocyanate and butylisocyanate were detected.