An electrospray ionization Fourier transform ion cyclotron resonance mass spectrometric study of the gas-phase stabilities and fragmentation of N-alkylammonium resorcarenes

Five N-alkylated ammonium resorcarene derivatives, differing either in the nitrogen atom or in the lower rim alkyl chain, were studied using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The main purpose was to clarify the fragmentation and the relative stabilities of the protonated molecules by collision-induced dissociation (CID) experiments. The results indicate that fragmentation of the upper rim alkyl amino substituents at the 2-position occurs readily. Variation in the gas-phase stabilities of these molecules was also observed. In addition, the proposed fragmentation pathways are depicted.     

Structural features of polyacylated anthocyanins using matrix-assisted laser desorption/ionization and electrospray ionization time-of-flight mass spectrometry

In our continuing studies to isolate water-soluble vacuolar pigments, we expect to elucidate more structural details using mass spectrometry (MS). Because of its sensitivity, only a small amount of pigment extracted from natural plants is required for MS measurement. Nuclear magnetic resonance is also a useful spectroscopic method for structural determination. In this study, two soft ionization techniques, electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI), on time-of-flight (TOF) mass spectrometers, were used to analyze five polyacylated anthocyanins with more than two aromatic acid molecules in the side chains. ESI is advantageous for the detection of individual molecular ions, while MALDI is essential for the detection of characteristic fragment ions originating from the anthocyanidin. Although 2,5-dihydroxybenzoic acid (DHBA) is an effective matrix in MALDI-TOFMS to obtain informative fragment ions of polyacylated anthocyanins, α-cyano-4-hydroxycinnamic acid (CHCA) is the preferred matrix for the identification of aglycones. In particular, in measurements of polyacylated anthocyanins with two acylated glycoside chains, fragment ions originating from anthocyanidin can only be observed in MALDI-TOFMS using CHCA as the matrix.     

Noncovalent complex formation of tetratosylated resorcarene with aromatic,aliphatic, and cyclic alkyl ammonium ions: a mass spectrometric study of complex properties

The ability of tetratosylated resorcarene to form complexes with aromatic ammonium ions was investigated by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The formation of noncovalent complexes, [1+guest]⁺ and [1 · 1+guest]⁺, as observed with singly charged aromatic anilinium and phenylene aminoammonium guests. Comparison of the complexation efficiencies of the aromatic and aliphatic ammonium ions showed the importance of proton affinity of conjugate amines in complex formation. In collision‐induced dissociation experiments, gas‐phase stability was found to be lower for complexes formed with aromatic ions and this behavior was not found to depend on the proton affinity of conjugate amines. Fast oxidation of the para and ortho aminoammonium ions and complexation of these ions with tetratosylated resorcarene was observed. Copyright © 2005 John Wiley & Sons, Ltd.     

Fourier transform ion cyclotron resonance mass spectrometry in the speciation of high molecular weight sulfur heterocycles in vacuum gas oils of different boiling ranges

The analysis of sulfur aromatics in vacuum gas oils (VGO) distilled from an Iranian light crude oil is discussed. The VGOs were fractionated into three boiling ranges, 390–460, 460–520, and 520–550 °C, and were analyzed using liquid chromatographic separation on a Pd(II)-bonded stationary phase followed by identification with electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). It was possible to detect a large number of thiophenes, including a substantial number of isomers, in the three VGO fractions. Separation on the palladium phase and inclusion of sulfur-selective derivatization makes electrospray ionization of these nonpolar compounds possible. An elemental composition can be assigned to a large number of S₁ compounds without ambiguity in the presence of abundant hydrocarbons. With an increase in boiling temperature, an increase in the size of the aromatic system and the number of side chain carbon atoms was observed. In addition, the masses of higher magnitude shifted toward larger aromatic systems with an increase in boiling range. A comparison of FT-ICR MS and comprehensive gas chromatography is also given. This work is dedicated to Prof. B. Krebs on the occasion of his 70th birthday.     

Chiral recognition of borneol by association with zinc(II) and l-tryptophan in the gas phase

Chiral recognition of the racemic borneol was obtained in the gas phase by using electrospray ionization time-of-flight mass spectrometry (ESI-TOFMS) and electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTMS). Both single stage MS and tandem MS were employed for investigating the chiral recognition depending strongly on the stereochemistry of the ligands in the zinc(II)-l-tryptophan-borneol multimeric cluster ions. It was found that the type of acid which was used for adjusting the pH of the mixed solution played an important role in the chiral recognition, which was obtained when acetic acid or propanoic acid was used as an additive. No chiral recognition was observed using hydrochloric acid or formic acid. Furthermore, the nozzle potential was an important parameter for optimizing the discrimination of chirality. In tandem MS, the difference in intensity between two diastereomeric complex ions showed chiral recognition behavior. Such a difference could be determined by two approaches. One was using the fragment ion as an internal standard, the other was a new approach of using the isotopic form of parent complex ion as an internal standard.     

Redox transformations in desorption electrospray ionization

Redox changes occur in some circumstances when organic compounds are analyzed by desorption electrospray ionization mass spectrometry (DESI-MS). However, these processes are limited in scope and the data presented here suggest that there are only limited analogies between the redox behavior in DESI and the well-known solution-phase electrochemical processes in standard electrospray ionization (ESI). Positive and negative ion modes were both investigated and there is a striking asymmetry between the incidence of oxidation and of reduction. Although in negative ion mode DESI experiments, some aromatic compounds were ionized as odd-electron anion radicals, examples of full reduction were not found. By contrast, oxidation in the form of oxygen atom addition (or multiple oxygen atom additions) was observed for several different analytes. These oxidation reactions point to chemically rather than electrochemically controlled processes. Data is presented which suggests that oxidation is predominantly caused by reaction with discharge-created gas-phase radicals. The fact that common reducing agents and known antioxidants such as ascorbic acid are not modified, while a saturated organic acid like stearic acid is oxidized in DESI, indicates that the usual electrochemical redox reactions are not significant but that redox chemistry can be induced under special experimental conditions.     

Oxidation of a laccase mediator ABTS as studied by ESI-FTICR mass spectrometry

The oxidation reaction of a laccase mediator ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) was studied by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICRMS). Oxidation products of ABTS were measured after reaction times that varied from a few minutes up to several days and both positive and negative ionization modes were employed. Exact mass measurements and collision-induced dissociation (CID) experiments were used to characterize the structures of the ions formed. After reacting with Trametes versicolor laccase (TvL), the radical cation form of ABTS was the main product observed by the positive ionization mode. Negative ionization mode experiments revealed that a degradation product from ABTS was formed.     

Generation and detection of multiply-charged peptides and proteins by matrix-assisted laser desorption electrospray ionization (MALDESI) fourier transform ion cyclotron resonance mass spectrometry

We report the coupling of a hybrid ionization source, matrix-assisted laser desorption electrospray ionization (MALDESI), to a Fourier transform-ion cyclotron resonance mass spectrometer (FT-ICR MS). The details of the source design and initial data are presented. Analysis of peptides and proteins ranging from 1 to 8.6 kDa resulted in high resolving power single-acquisition FT-ICR mass spectra with average charge-states highly correlated to those obtained by nanoESI, thus, providing strong evidence that the ESI process dictates the observed charge-state distribution. Importantly, unlike the recently introduced electrospray assisted laser desorption ionization (ELDI) source reported by Shiea and coworkers [1, 2], the data we have obtained to date rely on the use of an organic acid matrix. The results presented herein provide insight into the charging mechanism of this emerging ionization approach, while also expanding the utility of FT-ICR MS for top-down protein and complex mixture analysis.     

False positives and the detection of cyclodextrin inclusion complexes by electrospray mass spectrometry

The results of previous works that have claimed to detect cyclodextrin inclusion complexes via the “soft” ionization technique of electrospray ionization mass spectrometry are revisited. A more extensive study of cyclodextrin mixtures with amino acids and small peptides demonstrates that amino acid and peptide “complexes” are detected by electrospray mass spectrometry regardless of the presence (or not) of an aromatic moiety on the side chain. Amino acids that may be least likely to form hydrophobic inclusion complexes with cyclodextrin in solution generally show the most intense complex ions. The data suggest that these “complexes” are, in all likelihood, electrostatic adducts formed during the electrospray process. Systematic controls are suggested to ensure that “false positives” do not negate many of the claims concerning the detection of solution-derived noncovalent compounds.     

蛋白质分子量测定过程中的酸效应

在基质辅助激光解吸电离飞行时间质谱（MALDI-TOF-MS）和电喷雾质谱（ESI-MS）测定蛋白质分子分子量的过程中,适当提高样品的酸度,可提高分析测试的灵敏度。在选定最佳样品分子浓度的基础上,通过适当加入三氟乙酸（TFA）来调整测试样品的酸度,准确测定了标准蛋白质-溶菌酶（lysozyme）的分子量,并且对蛋白质分子在“软电离”质谱中,受酸效应的影响进行了初步探讨。     

Ozonolysis of phospholipid double bonds during electrospray ionization: a new tool for structure determination

Ozonolysis of double bonds is observed during the negative ion electrospray ionization of unsaturated phospholipids under conditions that produce a corona discharge. Ionic products of the ozonolysis are detected and characterized by mass spectrometry, and the mass-to-charge ratio of these species can be used to unambiguously assign the double bond position within the unsaturated fatty acid radyls. The reaction products are consistent with the gas-phase ozonolysis of desolvated phospholipid ions in an atmosphere rich in volatilized solvent. Reactions may be carried out in a conventional electrospray ionization mass spectrometer and provide a new method for the structural characterization of phospholipids.     

Atmospheric pressure laser ionization (APLI) coupled with Fourier transform ion cyclotron resonance mass spectrometry applied to petroleum samples analysis: comparison with electrospray ionization and atmospheric pressure photoionization methods

The analysis of crude oil samples remains a tough challenge due to the complexity of the matrix and the broad range of physical and chemical properties of the various individual compounds present. In this work, atmospheric pressure laser ionization (APLI) is utilized as a complementary tool to other ionization techniques for crude oil analysis. Mass spectra obtained with electrospray ionization (ESI) and atmospheric pressure photoionization (APPI) are compared. APLI is primarily sensitive towards non‐polar aromatic hydrocarbons, which are generally present in high amounts especially in heavy crude oil samples. The ionization mechanisms of APLI vs. APPI are further investigated. The results indicate the advantages of APLI over established methods like ESI and APPI. The application of APLI in combination with Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR MS) is thus demonstrated to be a powerful tool for the analysis of aromatic species in complex crude oil fractions. Copyright © 2011 John Wiley & Sons, Ltd.     

Structural analysis of oligosaccharides by a combination of electrospray mass spectrometry and bromine isotope tagging of reducing-end sugars with 2-amino-5-bromopyridine

Model reducing-end oligosaccharides were successfully labeled by a brominated aromatic amine reagent, 2-amino-5-bromopyridine (ABP), through reductive amination. Using either a combination of liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) with in-source fragmentation or liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS), sequence information corresponding to the model oligosaccharides was revealed with little ambiguity via the diagnostic unique twin peaks arising from the bromine isotopes, for both the molecular ions of the derivatized oligosaccharides and their fragments. No fragment ions arising from loss of the bromine atom were observed.     

The complex formation of tetracyclohexylammonium C1-resorcinarene with various guests - an electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry study

The complex formation of a tetraammonium C1-resorcinarene (R+4HCl) was studied using electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry. Although R+4HCl easily loses its counter ions in the ESI process, a neutral self-assembled structure with an intramolecular circular hydrogen-bonded 16-membered -N(+)-H ... X(-) ... H-N(+)- array with ammonium ion as the charge-giving species was observed in the gas phase. In addition to chloride, several other counter ions were also studied. The size and basicity of the counter ion as well as the size of the charge-giving cation strongly affected the gas-phase stability of the self-assembled system. H/D exchange experiments showed that the ammonium substituents in the apical position of R affect the hydrogen-bonding system in the resorcinarene. The complexation of the saturated dicarboxylic acids was found to depend on the length of the carbon chain. The rigidity of the molecular skeleton of the acid improved the complexation considerably. The orientation and position of the carboxylic groups also had an effect on the complexation and consequently enabled stereochemical differentiation of the acids. Mass spectrometric observations were supported by theoretical calculations.     

Electrospray ionization for determination of non‐polar polyaromatic hydrocarbons and polyaromatic heterocycles in heavy crude oil asphaltenes

Electrospray ionization (ESI) is the most common ionization method in atmospheric pressure ionization mass spectrometry because of its easy use and handling and because a diverse range of components can be effectively ionized from high to medium polarity. Usually, ESI is not employed for the analysis of non‐polar hydrocarbons, but under some circumstances, they are effectively ionized. Polyaromatic hydrocarbons and aromatic heterocycles can form radical ions and protonated molecules after ESI, which were detected by Fourier transform ion cyclotron resonance mass spectrometry. The highly condensed aromatic structures are obtained from a heavy crude oil, and the results show class distribution from pure hydrocarbons up to more non‐basic nitrogen‐containing species. By using different solvent compositions [toluene/methanol (50/50 v/v), dichloromethane/methanol (50/50 v/v), dichloromethane/acetonitrile (50/50 v/v) and chloroform], the results show that the lack of proton donor agent helps to preserve the radical formation that was created at the metal/solution interface inside the electrospray capillary. The results demonstrate that with an appropriate selection of solvent and capillary voltage, the ratio between the detected radical ion and protonated molecule form can be manipulated. Therefore, ESI can be expanded for the investigation of asphaltene and other polyaromatic systems beyond the polar constituents as non‐polar hydrocarbons can be efficiently analyzed. Copyright © 2015 John Wiley & Sons, Ltd.     

Continuous‐flow extractive desorption electrospray ionization coupled to normal phase separations and for direct lipid analysis from cell extracts

Normal phase liquid chromatography is a common mode for chiral separations. Many chiral amines are used as drugs or are important intermediates for drug synthesis. Electrospray ionization mass spectrometry is well known for its high sensitivity. However, when using normal phase liquid chromatography, electrospray ionization is hampered by the poor ionization efficiency of analytes from organic eluents. Continuous‐flow extractive desorption electrospray ionization, which introduces the eluents through a hypodermic needle into the electrospray plume is demonstrated here for its success to interface normal phase liquid chromatography to mass spectrometry detection. Such an approach was shown to be as or more sensitive than ultraviolet detection for a selected set of aromatic amine‐functionalized enantiomers. Also demonstrated is the direct infusion of cell extracts to monitor phospholipids from three different bacterial cells. Despite their presence in non‐electrospray‐ionization‐friendly extraction solvents, continuous‐flow extractive desorption electrospray ionization enabled the sensitive detection of phospholipids and the ability to tune ion forms through incorporation of different spray modifiers.     

单糖衍生物的电喷雾质谱裂解规律研究

以1-(2-萘基)-3-甲基-5-吡唑啉酮(NMP)作单糖标识剂, 经在线串联的LC-ESI-MS建立了单糖衍生物的电喷雾质谱裂解方法.衍生物在质谱裂解中糖类化合物特有的规范信息.借助糖类化合物在ESI-MS条件下表现出的分子离子峰m/z [M H] , 及在ESI-MS/MS条件下呈现出的特征碎片离子峰m/z 473, 可有效地确定出单糖类化合物的组成. 尽管一些脂肪醛和芳香醛也能同时被标识, 然而在质谱条件下不产生m/z 473的特征碎片离子峰, 且它们的洗脱远在糖类组分之后, 因此不干扰糖类化合物的分离和结构确定.通过建立的LC-ESI-MS方法, 对水解蜂花粉中的单糖进行了分析.结果表明: 水解的蜂花粉中含甘露糖(Man)、半乳糖醛酸(GalUA)、葡萄糖醛酸(GlcUA)、鼠李糖(Rha)、葡萄糖 (Glc)、半乳糖(Gal)、阿拉伯糖(Ara)、木糖(Xyl)和岩藻糖(Fuc).本方法为环境样品中单糖类化合物的确定提供了准确、可靠的技术手段.     

Characterization of heavy petroleum fraction by positive-ion electrospray ionization FT-ICR mass spectrometry and collision induced dissociation: Bond dissociation behavior and aromatic ring architecture of basic nitrogen compounds

This paper examined the bond dissociation behavior and aromatic ring architecture of basic nitrogen compounds in Sudan heavy petroleum fraction. Both broadband and quadrupole isolation modes positive-ion electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with collision induced dissociation (CID) techniques were used to characterize a low sulfur crude oil derived vacuum residuum (VR). The appropriate CID operating condition was selected by comparing the molecular weight distributions of the basic nitrogen compounds under various CID operating conditions. Both oddand even-electron fragment ions were observed from the mass spectrum, indicating that the heterolytic and homolytic bond cleavages occurred simultaneously during the CID process. The odd-electron fragment ions were predominant in each class species, indicating preferential heterolytic bond cleavages. At the optimal CID condition, the alkyl groups decomposed deeply and just left the aromatic cores of the nitrogen compounds. No significant variation in double bond equivalent (DBE) value was observed between the fragment and parent ions, revealing that the domination of single core structure.     

Origin of 13C complexation shifts in the adduct formation of 2-butyl phenyl ethers with a dirhodium tetracarboxylate complex

Complexation of the oxygen atom in 2-butyl phenyl ethers to a rhodium atom of the dirhodium tetracarboxylate Rh(II) 2[(R)-(+)-MTPA]4(Rh*, MTPA-H = methoxytrifluoromethylphenylacetic acid identical with Mosher's acid) deshields an sp3-hybridized 13C nucleus directly bonded to the ether oxygen; apparently, the inductive effect of the oxygen is enhanced when it is complexed to the rhodium atom. On the other hand, deshielding complexation shifts of aromatic ipso-carbons (alpha-positioned) are minute but ortho- and para-carbon signals are influenced by the resonance effect of oxygen. This effect can be modulated by further substituents at the benzene ring. In turn, this modulation of the resonance correlates linearly ith the magnitude of the inductive effect exerted on the aliphatic alpha-carbon atoms. Diastereomeric dispersion effects at 13C signals can be observed for most compounds, indicating that enantiodifferentiation is possible in this class of ethers.     

Characterization of phosphatidylethanolamine as a lithiated adduct by triple quadrupole tandem mass spectrometry with electrospray ionization

Structural characterization of the glycerophosphoethanolamine (GPE) molecule as a lithiated adduct ion by collisionally activated dissociation (CAD) tandem mass spectrometry with electrospray ionization is described. Abundant fragment ions reflecting polar head group and fatty acid constituents were observed in the product ion spectrum of GPE, which permits an unambiguous structural determination, including the regiospecificity of fatty acyl substituents. The pathways leading to the formation of fragment ions are proposed. The suggested mechanisms are supported by the tandem mass spectra of various deuterated analogs and source CAD of GPE followed by CAD tandem mass spectrometry. Identification of GPE molecular species and specific GPE subclasses in a biological mixture by tandem mass spectrometry with various constant neutral loss scannings is also described.