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.     

Facile synthesis and altered ionization efficiency of diverse Nε-alkyllysine-containing peptides

Protein lysine methylation is a biologically-important posttranslational modification. Given the increased use of alkyllysine-containing peptides to study protein methylation, we describe an efficient solid-phase synthesis to access various alkyllysine-containing peptides. As an exemplary application, we evaluated how ionization efficiency of the alkyllysine peptides affects mass spectrometric analysis.     

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.     

O−• chemical ionization of carbonyl compounds

The negative ion chemical ionization mass spectra of twentyeight C₄ to C₇ carbonyl compounds were recorded using the oxide radical anion O^?? as reagent ion. As noted earlier, the reactions occurring include H⁺ abstraction, H ₂ ^+? abstraction, H? atom displacement, and alkyl radical displacement. In addition, the [M?2H]^? ions fragment further by alkyl radical elimination. The relative importance of these reactions depends strongly on molecular structure, with the result that isomer distinction frequently is possible. Where this is not possible, as for isomeric aldehydes, the collisional charge inversion mass spectra of common product ions provides isomer distinction. The H ₂ ^+? abstraction reaction is shown to involve abstraction not only of two hydrogens from the same α-carbon but also, in part, abstraction of one hydrogen from each α-carbon.     

Comparison of the effects of ionization mechanism, analyte concentration, and ion “cool-times” on the internal energies of peptide ions produced by electrospray and atmospheric pressure matrix-assisted laser desorption ionization

The propensities of a series of peptide ions produced by both electrospray and atmospheric pressure matrix assisted laser desorption ionization (AP-MALDI) to fragment in an ion trap mass spectrometer under various conditions were studied in detail by measuring the extent of fragmentation of precursor ions by collision induced dissociation (CID) as a function of applied resonance excitation RF voltage. For the most basic peptides, the energy required to fragment MH+ ions generated by electrospray exceeded that required to fragment equivalent AP-MALDI ions under identical instrumental conditions; the reverse was observed for a peptide incorporating no basic residues, while peptides of intermediate basicity showed little difference between the ionization methods. This correlation between peptide basicity and the difference in the energy required to induce fragmentation of MH+ ions generated by AP-MALDI and electrospray is attributed primarily to a trend in the internal energies of the ions generated by AP-MALDI (the greater the difference in gas-phase basicities between the matrix and the analyte the greater the internal energy of the analyte ions produced). Furthermore the internal energies of ions produced by AP-MALDI, but not the equivalent ions formed by electrospray, were observed to decrease with decreasing analyte concentration. We attribute this finding to the cooling effect of endothermic dissociation of analyte ion/matrix molecule clusters following the matrix assisted laser desorption step. Time-resolved analyses (measurement of extent of fragmentation of precursor ions by CID as a function of pre-CID "cool times") revealed that cooling periods in excess of 250 ms were required to achieve internal energy equilibrium through cooling collisions with the helium buffer gas. Furthermore, these analyses demonstrated that, even after these extended cooling times, equivalent ions formed by the two ionization techniques showed different propensities to fragment. We conclude that the two different ionization techniques produce ion populations that may differ in their three-dimensional structure.     

Detection and Characterization of Non-covalent Complex between Lappaconitine and β-Cyclodextrin by Electrospray ionization Tandem Mass Spectrometry

Lappaconitine(LA), exhibits antiarrhythm1 and analgesic actions2-5, and has been used to release pain for cancer patients. However, its application is limited due to poor water solubility. But its non-covalent complexes with CDs can solve this problem, th…     

Peculiarities of dissociative ionization of alkylcyclopentadienyl nitrosyl π-complexes of nickel

The decomposition of alkylcyclopentadienyl nitrosyl -complexes of nickel, (C₅H₄R)(NO)Ni (R=H, Et,i-Pr, CH₂Ph), under the action of electron impact has been studied. The nature of the nitrosyl ligand has been shown to be the factor determining the main fragmentation pathway which involves the abstraction of an NO molecule. The effect of the nature of the ligand on the ability of the molecular ion (C₅H₄R)LNi⁺ (L=C₅H₄R, C₅H₅, C₃H₅, NO) to rearrange with hydrogen atom migration from one ligand to another has been considered. The structure of the alkyl group R determines a competing fragmentation pathway involving cleavage of the -C-C bond with respect to the cyclopentadienyl ring in the substituent.Translated fromIzyestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1985–1988, November, 1993.     

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.     

Two-photon laser-induced fluorescence and “2+1” multiphoton ionization of silicon atoms

Two-photon laser-induced fluorescence and “2+1” multiphoton ionization detection of silicon (³P_^j″,¹D₂) atoms was accomplished using the 4 ³P_J'←3 ³P_j″, 4 ³S₁→ 3 ³P_J′, and the 4 ¹D₂→ 3 ¹D₂ resonant two-photon transitions. Silicon atoms were produced in their ground ³P_J″ (J″ = 0,1,2) and first singlet ¹D₂ states by the multiphoton dissociation of Si(CH₃)₄ in the wavelength region 400–460 nm. Line strength intensities for the two-photon transitions originating from the 3 ³P_J″ (J″ = 0,1,2) states were calculated and compared to experimental values determining that the silicon (³P_J″) atoms were produced in a statistical population distribution.     

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).     

Soft or hard ionization of molecules in helium nanodroplets? An electron impact investigation of alcohols and ethers

Electron impact (70 eV) mass spectra of a series of C1-C6 alcohols encased in large superfluid liquid helium nanodroplets (approximately 60,000 helium atoms) have been recorded. The presence of helium alters the fragmentation patterns when compared with the gas phase, with some ion product channels being more strongly affected than others, most notably cleavage of the C(alpha)-H bond in the parent ion to form the corresponding oxonium ion. Parent ion intensities are also enhanced by the helium, but only for the two cyclic alcohols studied, cyclopentanol and cyclohexanol, is this effect large enough to transform the parent ion from a minor product (in the gas phase) into the most abundant ion in the helium droplet experiments. To demonstrate that these findings are not unique to alcohols, we have also investigated several ethers. The results obtained for both alcohols and ethers are difficult to explain solely by rapid cooling of the excited parent ions by the surrounding superfluid helium, although this undoubtedly takes place. A second factor also seems to be involved, a cage effect which favors hydrogen atom loss over other fragmentation channels. The set of molecules explored in this work suggest that electron impact ionization of doped helium nanodroplets does not provide a sufficiently large softening effect to be useful in analytical mass spectrometry.     

Differentiation of rum and Brazilian artisan cachaça via electrospray ionization mass spectrometry fingerprinting

Rum and cacha?a are sugarcane distillates produced on large scales and of similar composition, and their differentiation is currently a subject of commercial dispute and a challenging analytical task. We have investigated the ability of direct-infusion electrospray ionization mass spectrometry in the negative ion mode, i.e. ESI(-)-MS, to distinguish between samples of these distillates. ESI(-)-MS fingerprints were collected for some samples of Brazilian artisan cacha?a, aged in two types of wooden casks, i. e. amburana (Amburana cearensis) and jequitibá (Cariniana estrellensis), and of commercial rum. The mass spectra were found to be very distinctive, showing sets of diagnostic ions for each type of sample, i. e. (1) cacha?a aged in amburana (m/z 271, 313, 377) and jequitibá (m/z 171, 255, 455) casks; and (2) commercial rum (m/z 89, 97, 179, 255, 283). When applied to the ESI(-)-MS data, principal component analysis and hierarchical cluster analysis split rum and cacha?a samples into well-defined groups. Moreover, the two types of cacha?a samples aged in wooden casks of amburana or jequitibá were also split into two distinct groups. Direct-infusion ESI(-)-MS can therefore be potentially applied to the rapid, simple, and accurate differentiation of these commercially important sugarcane distillates.     

Investigation of noncovalent complexes between β-cyclodextrin and polyamide acids containing N-methylpyrrole and N-methylimidazole by electrospray ionization mass spectrometry

Electrospray ionization (ESI) mass spectrometry was utilized to investigate noncovalent complexes between beta-cyclodextrin (beta-CD) and five novel polyamide acids containing N-methylpyrrole and N-methylimidazole. The 1:1 binding mode was specified by examining the binding stoichiometry from ESI mass spectra. It found that polyamide acids with beta-CD have binding affinities in the order: ImImImbetaCOOH > ImPyImbetaCOOH > ImPyPybetaCOOH > PyPyPybetaCOOH > NO(2)PyPyPybetaCOOH. The method gives, simultaneously, the binding constants between beta-CD and polyamide acids based on a novel linear equation.     

Determination of enthalpy of ionization of water from 250 to 350° C

The enthalpy changes at zero ionic strength (H°) for the ionization of water (H₂O=H⁺+OH^–) were determined by flow calorimetry from the heats of mixing of aqueous NaOH and HCl solutions in the temperature range 250 to 350°C. Pitzer ion-interaction models developed by other workers were used to calculate enthalpies of dilution of aqueous NaOH, HCl, and NaCl solutions for the extrapolation of H values from the conditions of the experiment to infinite dilution. Equations are derived for thermodynamic quantities (log K, H°, S°, C _p ^° and V°) for the ionization of water using the H° values determined in this study from 250 to 350°C and literature log K and H° values from 0 to 225°C. Smoothed values of log K, H°, S°, C _p ^° , and V° are presented at rounded temperatures from 0 to 350°C and at the saturation pressure of water for each temperature. The equations in the present study provide a better representation of experimental thermodynamic data from 0 to 350°C than the Marshall-Franck equation.     

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.     

Detection and quantification of 3,5,3′-triiodothyronine and 3,3′,5′-triiodothyronine by electrospray ionization tandem mass spectrometry

A novel and rapid method for identifying and quantifying 3,5,3'-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3; reverse T3) has been introduced using electrospray ionization tandem mass spectrometry (ESI-MS/MS). MS(2) spectra in either negative ionization mode or positive ionization mode can be used to differentiate T3 and rT3. Quantification of the T3 and rT3 isomers under the negative ionization mode is also achieved without prior separation by HPLC.     

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.     

Preservation and detection of specific antibody—peptide complexes by matrix-assisted laser desorption ionization mass spectrometry

The direct detection of an antibody-peptide complex is reported by matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). Experimental conditions have been found in which specific, noncovalent interactions in solution are maintained throughout the sample preparation and ionization process. Mass measurements based on the ion signals for the intact antibody and 1:1 antibody-peptide complex reveal that specific noncovalent associations between a monoclonal antibody and a peptide, which comprises the determinant of the corresponding antigen, are maintained in the gas phase. These results support the wider application of MALDI-MS to studies of the structure and specificity of macromolecular complexes important to immune and other biological function.