Specific features of the behavior of α- and β-glycosylfluoride tetraacetates during chemical ionization in isobutane and tetramethylsilane

Reactions of - and -glycosylfluoride tetraacetates with trimethylsilicenium ion in the gas phase during chemical ionization have been studied. The [M+SiMe₃]⁺ ions formed from the glycosylfluorides are more stable than the corresponding [M+H]⁺ ions. The cleavage of the weakest glycosidic bond leading to the generation of glycosidic ions is not dominant for the trimethylsilylated ions, as it has been observed in the corresponding protolytic reactions. The ratio of the intensities of the [M+SiMe₃]⁺ and [M–F]⁺ ions characterizes the probability of the initial localization of the trimethylsilyl ion at the glycosidic center; the equatorial orientation of fluorine at C(1) makes it possible for the electrophile to bond with this substituent. Generation of the glycosidic ions is rather weakly affected by increasing temperature, whereas [M-AcO]⁺ formation is significantly intensified.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 629–633, April, 1994.This work was carried out with financial support from the Russian Foundation for Basic Research.     

Calculated vertical ionization energies of the common α-amino acids in the gas phase and in solution

The vertical ionization energies of the low-lying conformers of the α-amino acids found in proteins have been calculated. Geometry optimizations were first performed at the B3LYP/6-311G(d,p) level of theory, and then reoptimized at the MP2/6-311G(d,p) level of theory. Vertical ionization energies were then computed by three methods, electron propagator in the partial third-order (P3) approximation, Outer-Valence-Green's Functions, and by evaluating the difference in the total energy between the cation radical and the neutral amino acid in the geometry of the neutral species. When available, the results are compared to the experimental vertical ionization energies. The vertical ionization energies calculated using the MP2/P3 method gave the best overall agreement with the experimental results. Next, the ionization energies in solution are calculated for the zwitterionic forms of the α-amino acids by using IEFPCM methods. To obtain the vertical ionization energy in solution, it is necessary to use the nonequilibrium polarizable continuum model (NEPCM), the results of which are reported here for the α-amino acids.     

Study of the fragmentation pathway of α-aminophosphonates by chemical ionization and fast atom bombardment mass spectrometry

The diastereoisomers of α-aminophosphonates are key intermediates in the synthesis of enantiomerically pure α-aminophosphonic acids, which are analogs of α-amino acids. Although several methods have been reported for the diastereoselective synthesis of α-aminophosphonates, their mass spectrometry (MS) fragmentation patterns have not yet been fully investigated. The work described here involved a detailed study of the fragmentation of enriched α-aminophosphonate diastereoisomers by chemical ionization (CI-MS) and fast atom bombardment (FAB)-MS. The complete characterization of the different conventional MS fragmentation pathways is represented and this intriguing exercise required the use of tandem mass spectrometry (MS/MS) experiments and high-resolution accurate mass measurements. All α-aminophosphonates gave prominent pseudomolecular ions, protonated molecules [MH](+) , and their fragmentations mainly showed a loss of dimethyl phosphite to give the corresponding iminium ions as base peaks for α-aminophosphonates bearing methylbenzyl and 2,2-dimethylbutyl fragments. The loss of the chiral fragment from the iminium ions bearing the (S)-1-(1-naphthyl)ethyl group gave rise to a base peak due to aryl cations. The nature of all fragment ions were confirmed by high-resolution mass spectrometry (HRMS).     

The thermodynamics of ionization of α-alanine in methanol + water mixtures and the determination of single ion thermodynamics

In order to understand the effect of solvents on the thermodynamic parameters of amino acids, the thermodynamic dissociation constants, k₁ and K₂, for the reactions

and

(where RH^± = α-alanine) have been determined pH-metrically in methanol + water mixtures. The measurements were carried out in dilute solutions and in the absence of neutral electrolytes to minimize the “salt effect” as far as practicable so that the “medium effects” on the dissociation constants of the α-alanine can be properly understood.The enthalpy values for reactions (1) and (2) (up to 44.14 wt% of methanol) have been determined calorimetrically. It has been observed that the conversion of α-alanine into cations and anions is favourable both from enthalpic and entropic considerations, so that the reverse reactions of (1) and (2) are spontaneous. Attempts have been made to interpret the thermodynamics properties of α-alanine in terms of hydrophilic and hydrophobic interactions and other structural changes of the solvent molecules.In order to get a better insight into the nature of specific solute-solvent interactions, we have analysed the results in terms of single ion values using the thermodynamic values from the present work and other relevant data from previous work in our laboratory.     

A simple algorithm for the calculation of the π ionization energies of substituted benzenes

A simple algorithm, based on HMO approach is used for calculating Π-ionization potentials (Π-IP's) of polysubstituted benzenes. The parameters required for the method are determined from Π-IP's of monosubstituted benzenes. For 50 disubstituted benzenes the first three values of the Π-IPś are obtained with root mean square (rms) error of 0.19 eV. Some examples of application are presented, illustrating the possibility of fast and quite accurate estimation of Π-IP's for polysubstituted benzenes.     

Comparison of electrospray ionization and atmospheric pressure chemical ionization for multi-residue analysis of biocides,UV-filters and benzothiazoles in aqueous matrices and activated sludge by liquid chromatography–tandem mass spectrometry

This paper describes the development of a multi-residue method for the determination of 36 emerging organic pollutants (26 biocides, 5 UV-filters and 5 benzothiazoles) in raw and treated wastewater, activated sludge and surface water using liquid chromatography–tandem mass spectrometry (LC–MS/MS). The target analytes were enriched from water samples adjusted to pH 6 by solid-phase extraction (SPE) on Oasis HLB 200 mg cartridges and eluted with a mixture of methanol and acetone (60/40, v/v). Extraction of freeze-dried sludge samples was accomplished by pressurized liquid extraction (PLE) using a mixture of methanol and water (50/50, v/v) as extraction solvent followed by SPE. LC–tandem MS detection was compared using electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) in positive and negative ionization mode. ESI exhibited strong ion suppression for most target analytes, while APCI was generally less susceptible to ion suppression but partially leading to ion enhancement of up to a factor of 10. In general, matrix effects could be compensated using stable isotope-labeled surrogate standards, indicated by relative recoveries ranging from 70% to 130%. In wastewater, activated sludge and surface water up to 33 analytes were detected. Maximum concentrations up to 5.1 and 3.9 μg L⁻¹ were found in raw wastewater for the water-soluble UV-filters benzophenone-4 (BZP-4) and phenylbenz-imidazole sulfonic acid (PBSA), respectively. For the first time, the anti-dandruff climbazole was detected in raw wastewater and in activated sludge with concentrations as high as 1.4 μg L⁻¹ and 1.2 μg g TSS⁻¹, respectively. Activated sludge is obviously a sink for four benzothiazoles and two isothiazolones, as concentrations were detected in activated sludge between 120 ng g TSS⁻¹ (2-n-octyl-4-isothiazolin-3-one, OIT) to 330 ng g TSS⁻¹ (benzothiazole-2-sulfonic acid, BTSA).     

Comparison of negative chemical ionization and electron impact ionization in gas chromatography–mass spectrometry of endocrine disrupting pesticides

The study of pesticide residues belonging to endocrine disrupting chemicals (EDCs) (23 analytes of different chemical classes – organochlorines, organophosphates, pyrethroids, dicarboximides, phtalamides, dinitroanilines, pyrazole, triazinone) in apple matrix with conventional capillary GC–NCI-MS (with methane as reagent gas) in comparison to EI ionization is presented. For sample preparation QuEChERS method was applied. The lowest calibration levels (LCLs) for all pesticides were determined in both modes. Calibration in the NCI mode was performed at the concentration levels from 0.1 to 500 μg kg⁻¹ (R² > 0.999) and for EI in the range from 5 to 500 μg kg⁻¹ (R² > 0.99). From LCLs the instrumental limits of detection (LODs) and quantification (LOQs) were calculated. Chemometric study of pesticide signals in two MS modes was performed. Repeatability of all measurements, expressed by the relative standard deviations of absolute peak areas was better than 10% for the majority of compounds. Significantly lower values were obtained for the NCI mode.     

Analysis of ancient Greco–Roman cosmetic materials using laser desorption ionization and electrospray ionization mass spectrometry

Microsamples of pink cosmetic powders from the Greco–Roman period were analyzed using two complementary analytical approaches for identification of the colouring agents (lake pigments originally manufactured from madder plants with an inert binder, usually a metallic salt) present in the samples. The first technique was a methanolic acidic extraction of the archaeological samples with an additional ethyl acetate extraction of the anthraquinone-type colouring agents which were identified using high performance liquid chromatography coupled to electrospray ionization with high resolution mass spectrometry (LC–ESI–HRMS), and the second was direct analysis of a microsample by laser desorption ionization–mass spectrometry (LDI–MS). The latter technique is well suited when the quantity of samples is very low. This soft ionization technique enables the detection of very small quantities of compounds using the combination of positive and negative-ion modes. It was also successfully applied for the direct analysis of some laboratory-made reference compounds. However, the presence of lead in one of these ancient samples induced a spectral suppression phenomenon. In this case and conditional on a sufficient quantity of available sample, the former method is better adapted for the characterization of these anthraquinone-type molecules. This study also confirmed that purpurin, munjistin, and pseudopurpurin are the principal colouring agents present in these ancient cosmetic powders constituted from madder plants. Presented at the Annual French National Symposium on Mass Spectrometry, Electrophoresis and Proteomics, 20–23 September 2007 in Pau, France.     

Laser ionization spectra and electronic structure of Kn and NaKn−1 clusters

Photoionization of small K_n and NaK_n−1 clusters with n ⩽ 34 has been achieved in a crossed laser-molecular-beam experiment. The relative ion peak intensities in mass spectra reveal fragmentation effects which are wavelength-dependent. The vertical ionization profiles in the threshold region give some insight into the electronic structure.     

Complexation of antibiotic levomycetin (Chloroamphenicol) with α-hederin and hederasaponin c under the conditions of electrospray ionization

The complexation of antibiotic levomycetin (chloroamphenicol) with triterpene glycoside α-hederin and hederasaponin C was investigated by electrospray ionization mass spectrometry. The glycosides form complexes with levomycetin in the molar ratio 1: 1. The molecular complex of α-hederin is more stable than the one of hederasaponin C because of the presence of a free carboxyl group in its aglycone fragment. The carboxyl group of hederasaponin C is involved into the formation of a glycosidic bond with the trisaccharide unit and is not available for complex formation.     

Photofragmentations, state interactions, and energetics of Rydberg and ion-pair states: two-dimensional resonance enhanced multiphoton ionization of HBr via singlet-, triplet-, Ω = 0 and 2 states

Mass spectra were recorded for one-colour resonance enhanced multiphoton ionization (REMPI) of H(i)Br (i = 79, 81) for the two-photon resonance excitation region 79,040-80,300 cm(-1) to obtain two-dimensional REMPI data. The data were analysed in terms of rotational line positions, intensities, and line-widths. Quantitative analysis of the data relevant to near-resonance interactions between the F(1)Δ(2)(v' = 1) and V(1)Σ(+)(v' = m + 7) states gives interaction strengths, fractional state mixing, and parameters relevant to dissociation of the F state. Qualitative analysis further reveals the nature of state interactions between ion-pair states and the E(1)Σ(+) (v' = 1) and H(1)Σ(+)(v' = 0) Rydberg states in terms of relative strengths and J' dependences. Large variety in line-widths, depending on electronic states and J' quantum numbers, is indicative of number of different predissociation channels. The relationship between line-widths, line-shifts, and signal intensities reveals dissociation mechanisms involving ion-pair to Rydberg state interactions prior to direct or indirect predissociations of Rydberg states. Quantum interference effects are found to be important. Moreover, observed bromine atom (2 + 1) REMPI signals support the importance of Rydberg state predissociation channels. A band system, not previously observed in REMPI, was observed and assigned to the k(3)Π(0)(v' = 0) ←← X transition with band origin 80,038 cm(-1) and rotational parameter B(v('))=7.238 cm(-1).     

Effect of the Charge and the Nature of Both Cations and Anions on the Solubility of Zwitterionic Amino Acids,Measurements and Modeling

The effect of the charge and the nature of both the cations and the anions of some electrolytic salts: sodium fluoride (NaF), potassium fluoride (KF), sodium bromide (NaBr), potassium bromide (KBr), sodium iodide (NaI), potassium iodide (KI), sodium sulfate (Na₂SO₄), potassium sulfate (K₂SO₄), calcium chloride (CaCl₂), and barium chloride (BaCl₂), on the solubility of zwitterionic amino acids (glycine, DL-alanine, DL-valine, and DL-serine) in aqueous solutions at 298.15 K are studied and discussed. A salting-in effect is observed for all amino acids under investigation with all electrolytes used in the present study, except for DL-alanine and DL-valine in aqueous solutions containing sodium fluoride where a salting-out effect was observed. The orders of the effect of the nature and the charge of both the anions and the cations are: F^- < Cl^- < Br^- < I^- < NO₃^- < SO₄^2-\mathrm{F}^{-}<{}\mathrm{Cl}^{-}<{}\mathrm{Br}^{-}<{}\mathrm{I}^{-}<\mathrm{NO}_{3}^{-}<{}\mathrm{SO}_{4}^{2-} with both sodium and potassium cations; Na⁺<K⁺<Ca²⁺<Ba²⁺ with chloride anion.     

Comparison of the isomeric α-amino acyl adenylates and amino acid phosphoramidates of adenosine by electrospray ionization tandem mass spectrometry

The isomeric α-amino acyl adenylates and amino acid phosphoramidates of adenosine were synthesized and analyzed in detail by electrospray ionization tandem mass spectrometry (ESI-MS(n)). In ESI-MS/MS of α-amino acyl adenylates, the novel rearrangement ion [cAMP-H](-) observed as the most intense signal was formed through the pentacoordinate phosphorus intermediate with a six-membered ring by nucleophilic attack of the 3'-hydroxyl group on the phosphorus atom. In contrast, for the amino acid phosphoramidate of adenosine, the phosphorus atom could be attacked not only by the carboxylic group to form the cyclic aminoacyl phosphoramidates (CAPAs), but also by the nitrogen atom on the nucleobase leading to intramolecular phosphoryl group migration. It was found that the sodium ion having multidentate binding ability played an essential role in this characteristic rearrangement. The proposed mechanisms were supported by the MS/MS study, deuterium-labeled experiments, high-resolution tandem mass spectrometry and moderate calculations at the B3LYP/6-31G* level. The characteristic fragmentation patterns of α-amino acyl phosphates and amino acid phosphoramidates allows identification of stereoisomers when either the phosphorylation is at the N-terminus or C-terminus of amino acids.     

Structural elucidation of natural 2-hydroxy di- and tricarboxylic acids and esters, phenylpropanoid esters and a flavonoid from Autonoë madeirensis using gas chromatographic/electron ionization, electrospray ionization and tandem mass spectrometric techniques

The chemical composition of Autonoë madeirensis bulbs was characterized as part of a systematic phytochemical study of this species. The compounds reported were mainly identified on the basis of gas chromatography/electron ionization, electrospray ionization and tandem mass spectrometry. The structures of the pure compounds were also characterized by means of other physical and spectroscopic data (m.p., IR, UV, NMR). The compounds identified were 2‐hydroxy di‐ and tricarboxylic acids and esters (malic acid, citric acid and their methyl and ethyl esters), cis‐ and trans‐hydroxycinnamic esters (methyl and ethyl p‐coumarate and methyl ferulate) and a new flavone diglucoside, 7‐O‐[β‐glucosyl‐(1→2)‐O‐β‐glucosyl]apigenin, the interglucosidic linkage (1→2) of which is, to the best of our knowledge, reported for the first time in a diglucoside of apigenin. The results may contribute to the chemotaxonomy of the Autonoë genus and lead to a rapid tool for the systematic characterization of these compounds in plant extracts. Copyright © 2003 John Wiley & Sons, Ltd.     

Structure and hydration of the C4H4●+ ion formed by electron impact ionization of acetylene clusters

Here we report ion mobility experiments and theoretical studies aimed at elucidating the identity of the acetylene dimer cation and its hydrated structures. The mobility measurement indicates the presence of more than one isomer for the C(4)H(4)(●+) ion in the cluster beam. The measured average collision cross section of the C(4)H(4)(●+) isomers in helium (38.9 ± 1 A?(2)) is consistent with the calculated cross sections of the four most stable covalent structures calculated for the C(4)H(4)(●+) ion [methylenecyclopropene (39.9 A?(2)), 1,2,3-butatriene (41.1 A?(2)), cyclobutadiene (38.6 A?(2)), and vinyl acetylene (41.1 A?(2))]. However, none of the single isomers is able to reproduce the experimental arrival time distribution of the C(4)H(4)(●+) ion. Combinations of cyclobutadiene and vinyl acetylene isomers show excellent agreement with the experimental mobility profile and the measured collision cross section. The fragment ions obtained by the dissociation of the C(4)H(4)(●+) ion are consistent with the cyclobutadiene structure in agreement with the vibrational predissociation spectrum of the acetylene dimer cation (C(2)H(2))(2)(●+) [R. A. Relph, J. C. Bopp, J. R. Roscioli, and M. A. Johnson, J. Chem. Phys. 131, 114305 (2009)]. The stepwise hydration experiments show that dissociative proton transfer reactions occur within the C(4)H(4)(●+)(H(2)O)(n) clusters with n ≥ 3 resulting in the formation of protonated water clusters. The measured binding energy of the C(4)H(4)(●+)H(2)O cluster, 38.7 ± 4 kJ/mol, is in excellent agreement with the G3(MP2) calculated binding energy of cyclobutadiene(●+)·H(2)O cluster (41 kJ/mol). The binding energies of the C(4)H(4)(●+)(H(2)O)(n) clusters change little from n = 1 to 5 (39-48 kJ/mol) suggesting the presence of multiple binding sites with comparable energies for the water-C(4)H(4)(●+) and water-water interactions. A significant entropy loss is measured for the addition of the fifth water molecule suggesting a structure with restrained water molecules, probably a cyclic water pentamer within the C(4)H(4)(●+)(H(2)O)(5) cluster. Consequently, a drop in the binding energy of the sixth water molecule is observed suggesting a structure in which the sixth water molecule interacts weakly with the C(4)H(4)(●+)(H(2)O)(5) cluster presumably consisting of a cyclobutadiene(●+) cation hydrogen bonded to a cyclic water pentamer. The combination of ion mobility, dissociation, and hydration experiments in conjunction with the theoretical calculations provides strong evidence that the (C(2)H(2))(2)(●+) ions are predominantly present as the cyclobutadiene cation with some contribution from the vinyl acetylene cation.     

Cross-validation and evaluation of the performance of methods for the elemental analysis of forensic glass by μ-XRF, ICP-MS, and LA-ICP-MS

Elemental analysis of glass was conducted by 16 forensic science laboratories, providing a direct comparison between three analytical methods [micro-x-ray fluorescence spectroscopy (μ-XRF), solution analysis using inductively coupled plasma mass spectrometry (ICP-MS), and laser ablation inductively coupled plasma mass spectrometry]. Interlaboratory studies using glass standard reference materials and other glass samples were designed to (a) evaluate the analytical performance between different laboratories using the same method, (b) evaluate the analytical performance of the different methods, (c) evaluate the capabilities of the methods to correctly associate glass that originated from the same source and to correctly discriminate glass samples that do not share the same source, and (d) standardize the methods of analysis and interpretation of results. Reference materials NIST 612, NIST 1831, FGS 1, and FGS 2 were employed to cross-validate these sensitive techniques and to optimize and standardize the analytical protocols. The resulting figures of merit for the ICP-MS methods include repeatability better than 5 % RSD, reproducibility between laboratories better than 10 % RSD, bias better than 10 %, and limits of detection between 0.03 and 9 μg g^?1 for the majority of the elements monitored. The figures of merit for the μ-XRF methods include repeatability better than 11 % RSD, reproducibility between laboratories after normalization of the data better than 16 % RSD, and limits of detection between 5.8 and 7,400 μg g^?1. The results from this study also compare the analytical performance of different forensic science laboratories conducting elemental analysis of glass evidence fragments using the three analytical methods.

Exploration of the π-electronic structure of singlet, triplet, and quintet states of fulvenes and fulvalenes using the electron localization function

The singlet ground states and lowest triplet states of penta- and heptafulvene, their benzannulated derivatives, as well as the lowest quintet states of pentaheptafulvalenes, either the parent compound or compounds in which the two rings are intercepted by either an alkynyl or a phenyl segment, were investigated at the (U)OLYP/6-311G(d,p) density functional theory level. The influence of (anti)aromaticity was analyzed by the structure-based aromaticity index HOMA, the harmonic oscillator model of aromaticity. The extent of (anti)aromatic character was also evaluated in terms of the π-electron (de)localization as measured by the π component of the electron localization function (ELF(π)). The natural atomic orbital (NAO) occupancies were calculated in order to evaluate the degree of π-electron shift caused by the opposing electron-counting rules for aromaticity in the electronic ground state (S(0); Hückel's rule) and the first ππ* excited triplet state (T(1); Baird's rule). Pentaheptafulvalene (5) shows a shift of 0.5 π electrons from the 5-ring to the 7-ring when going from the S(0) state to the lowest quintet state (Qu(1)). The pentaheptafulvalene 5 and [5.6.7]quinarene 7 were also investigated in their 90° twisted conformations. From our study it is apparent that excitation localization in fulvalenes, but not in fulvenes, to a substantial degree is determined by aromaticity localization to triplet biradical 4n π-electron cycles. Isolated benzene rings in these compounds tend to remain as closed-shell 6π-electron cycles.