Formation of charged clusters during electrospray ionization of organic solute species

Electrospray dispersion of solutions into a bath gas can produce solute ions with many charges per molecule. In contrast to such multiple charging, it can also result in ‘fractional charging’ by which cluster ions are formed with more than one solute molecule per charge. Examples of such fractional charging or clustering are presented and discussed. In the case of arginine solutions, which are reported in some detail, ion masses corresponding to clusters containing as many as 24 solute molecules and up to four charges were identified. The number and size of these cluster ions increased markedly with increase in the initial solute concentration. When co-solutes consisting of acids, salts or bases were present, at up to millimolar concentrations in the initial solution, clustering was often strongly suppressed, especially by strong acids and their salts. This clustering phenomenon not only provides insight into the chemistry of solute precipitation in solution, but also offers a means of producing new kinds of clusters for experimental studies.     

Study of the electrospray ionization tandem mass spectrometry of sildenafil derivatives

A detailed analysis of the product ion spectrum generated from the protonated molecule of sildenafil (Viagra(R)) under multiple tandem mass spectrometry (ESI-MS(n)) conditions using an ion-trap mass spectrometer is reported. Molecular composition data of the fragment ions were obtained with the aid of comparisons of the multiple tandem mass spectra of eight sildenafil derivatives in two series, the structures of which are identical except for some substituted alkyl groups. Attempts have been made to provide rational pathways for the formation of the fragment ions from these protonated sildenafil derivatives. The structure-fragmentation relationships will facilitate the characterization of the structures of other sildenafil analogs.     

Detection of binuclear copper-caffeine complexes under electrospray ionization condition

A number of copper salts, Cu(OOCCH(3))(2), Cu(ClO(4))(2), Cu(NO(3))(2), CuCl(2) and CuSO(4) have been tested for their ability to form binuclear copper-caffeine complexes. The electrospray ionization (ESI) mass spectra of methanol solution containing caffeine and CuCl(2) or CuSO(4) show signals of two copper atom containing ions, so the signals correspond to binuclear complexes: [2Caf + Cu(2)SO(4)](+), [2Caf + Cu(2)](+), [2Caf + Cu(2)Cl](+), [2Caf + Cu(2)Cl(2)](+) and [2Caf + Cu(2)Cl(3)](+). Sulfate and chloride anion are characterized by charge densities higher than those of the carboxylate, nitrate and perchlorate anion. Thus, due to the electrostatic forces, the binuclear complexes containing SO(4)(2-) or Cl(-) can survive the transfer from solution to the gas phase and then can successfully be observed on ESI mass spectra. The ion [2Caf + Cu(2)Cl(3)](+) is present in solution and could be detected when using methanol/chloroform as solvent. The ions [2Caf + Cu(2)](+), [2Caf + Cu(2)Cl](+) and [2Caf + Cu(2)Cl(2)](+) are formed from the [2Caf + Cu(2)Cl(3)](+) ion (by subsequent loss of Cl atoms) on transfer from the solution to the gas phase or in the gas phase. The ion [2Caf + Cu(2)](+) does not contain a bridging agent, thus it is reasonable to assume that it contains a Cu-Cu bond.     

Gas chromatography electrospray ionization mass spectrometry analysis of trimethylsilyl derivatives

A method based on the analysis of trimethylsilyl (TMS) derivatives by capillary gas chromatography electrospray ionization mass spectrometry (GC–ESI/MS) was proposed. To improve separation, analytes were derivatized to their TMS derivative. During ESI analysis, TMS derivatives may hydrolyze back to their polar native form and are thus suitable for ESI analysis. Several types of analytes were studied to investigate the potential of the approach. Not all TMS derivatives hydrolyzed back to their native form as anticipated. Incomplete hydrolysis was observed for TMS‐organic acids and TMS‐nonchlorinated phenols. For TMS‐chlorophenols, the observation of only the [M ? H]^? ion suggested that these phenols were hydrolyzed back to their native form. For TMS‐beta agonists, the hydrolysis rate was low; therefore, the hydrolysis product was not detected. Both TMS‐chlorophenols and TMS‐beta agonists provide a sensitivity in the range of low parts per billion (0.25–5 ng/ml and 0.5–10 ng/ml respectively). Copyright © 2016 John Wiley & Sons, Ltd.     

电喷雾离子源中样品离子化能量转移理论的初探

电喷雾离子源（electrospray ionization,ESI）不仅可以用于小分子的检测,也能够用于蛋白质、多肽等大分子的研究。本文通过对离子化过程的系统分析,提出了基于能量最低原理的离子化过程能量转移理论。样品分子在由液相转移到气相形成离子化气体的过程中受到静电力、分子间的范德华力等多种力的作用。样品的离子化是多种力共同作用的结果,在不同的离子化阶段,不同形式的力的作用也不尽相同。电荷在样品表面蒸发和多电荷离子的形成之间存在竞争。对不同结构的分子,分子形态、构象改变导致的两相间转移Gibbs自由能变化不同,可能导致离子蒸发、大分子形成多电荷离子、产生链弹射等行为。离子化能量转移理论不仅能够对已有的3种理论加以简化统一,也可以说明溶剂、电解质离子等在离子化过程中的作用,为优化不同结构与形态样品的质谱检测、了解离子化的真实过程提供了一种可能的依据。     

Supermetallization of peptides and proteins during electrospray ionization

The formation of metal‐peptide complexes during electrospray ionization (ESI) is a widely known phenomenon and is often considered to be undesirable. Such effect considerably limits the use of ESI mass spectrometry for the investigation of biologically relevant metal‐peptide compounds that are present in the solution and play critical roles in many bioprocesses such as progression of neurodegenerative diseases. In the article, it is demonstrated that under specific conditions such as high temperature of the desolvating capillary, an interesting effect, which can be called as ‘supermetallization’, occurs. Using a model peptide Αβ amyloid domain 1–16, it was observed that an increase in the temperature of the desolvating capillary results in multiple substitutions of hydrogen atoms by Zn atoms in this peptide. At high temperatures (T ~ 400 °C), up to 11 zinc atoms can be covalently bound to (1–16) Αβ. It was observed that supermetallization of (1–16) Αβ depends on the solvent composition and pH. Supermetallization was also demonstrated for proteins, such as ubiquitin and cytochrome C. That proves that the supermetallization is a general phenomenon for peptides and proteins. For the structural investigation of supermetallized complexes, electron‐capture dissociation (ECD) fragmentation was applied. The effect of hydrogen rearranging during ECD was observed. In addition, quantum chemical calculations were used to estimate the possible structures of different supermetallized complexes. These results allow a more deep understanding of the limitations of the use of ESI mass spectrometry for the investigation of biologically relevant metal‐peptide complexes. Copyright © 2015 John Wiley & Sons, Ltd.     

The 'apparent' hydrolysis of alkyl esters during electrospray ionization

Electrospray ionization (ESI) mass spectrometry was employed to obtain both molecular weight confirmation and structural information for a series of novel alkenyldiarylmethane (ADAM) analogs. The mass spectral data were intended for use during the structure elucidation of ester hydrolysis products formed during an in vitro metabolism study of a series of novel ADAM analogs. The data on the precursor molecules show the presence of the molecular ion peak, [M+H](+), as well as a peak consistent with the hydrolysis product of the original ester ([MH-ROH+H(2)O](+)). However, chemical ionization mass spectrometry, elemental analysis and (1)H NMR data indicated the presence of only the intact diester compounds, suggesting that the formation of the hydrolysis product was an instrumental artifact, i.e., in-beam hydrolysis during ESI or a result of longer ion residence times of the ion trap mass analyzer.     

A fragmentation study of podophyllotoxin and its 4'-demethyl-4beta-substituted derivatives by electrospray ionization ion-trap time-of-flight tandem mass spectrometry

High-resolution electrospray ionization multistage tandem mass spectrometry (MS(1-9)) was used to determine the accurate masses and the fragmentation pathways of protonated podophyllotoxin (1) and its corresponding 4'-demethyl-4beta-substituted derivatives (2-4). The protonated molecules, [M + H](+), of all the four compounds were observed in the conventional single-stage mass spectra. Two fragmentation pathways, that appear to be characteristic of the four compounds, are proposed on the basis of their multistage tandem mass spectrometric data. The characteristic elimination, from the precursor protonated ions, of the neutral groups 4-R(1)H, 1-ArH, CO, CH(2)O and C(4)H(4)O(2), in which R is located on C-4, is the common elimination, and the product ions at m/z 267, 239, 229, 181, 173, 153, 143 and 115 are the common diagnostic masses. The elimination of the R(1) group substituent located on the C-4 position of compounds 1-4 has a significant influence on the fragmentation pathway obtained in the conventional single-stage mass spectra. A large R(1) group would be unfavorable for this elimination, unless the collision energy is raised. Apart from the common fragmentations obtained for the protonated molecules 1-4, significant additional product ions were detected in the various multistage tandem mass spectrometric analyses, particularly in the case of the product ions derived initially from the phenolic hydroxyl group of 2-4, which are different from those of 1. Based on these additional formed product ions, several additional fragmentation pathways for 1 or 2-4 are also presented.     

Reactive extractive electrospray ionization tandem mass spectrometry for sensitive detection of tetrabromobisphenol A derivatives

Sensitive detection of tetrabromobisphenol A (TBBPA) and its derivatives, a group of emerging toxic contaminants, is highly necessitated in environmental investigation. Herein a novel analytical strategy based on reactive extractive electrospray ionization (EESI) tandem mass spectrometry for detection of tetrabromobisphenol A bis(2-hydroxyethyl ether) (TBBPA-BHEE), tetrabromobisphenol A bis(glycidyl ether) (TBBPA-BGE), tetrabromobisphenol A bis(allylether) (TBBPA-BAE), and tetrabromobisphenol S bis(allylether) (TBBPS-BAE) in industrial waste water samples was developed. Active silver cations (Ag⁺), generated by electrospraying a silver nitrate methanol solution (10 mg L⁻¹), collides the neutral TBBPA derivatives molecules in the EESI source to form [M + Ag]⁺ complexes of the analytes under the ambient conditions. Upon collision-induced dissociation (CID), characteristic fragments of the [M + Ag]⁺ complexes were identified for confident and sensitive detection of the four TBBPA derivatives. Under the optimized experimental conditions, the instrumental limits of detection (LODs) of TBBPA-BHEE, TBBPA-BGE, TBBPA-BAE and TBBPS-BAE were 0.37, 0.050, 0.76, and 4.6 μg L⁻¹, respectively. The linear ranges extended to 1000 μg L⁻¹ (R² ≥ 0.9919), and the relative standard deviations (RSDs), inter-day variation and intra-day variation were less than 7.8% (n = 9), 10.0% (n = 5), and 14.8% (n = 1 per day for 5 days) for all derivatives. TBBPA derivative manufacturing industrial waste water, river water and tap water samples were fast analyzed with the proposed method. The contents of TBBPA derivatives were various in the collected samples, with the highest 19.9 ± 0.3 μg L⁻¹ of TBBPA-BAE in the waste water samples.     

Detection of intermediates for the Eschweiler-Clarke reaction by liquid-phase reactive desorption electrospray ionization mass spectrometry

Desorption electrospray ionization mass spectrometry (DESI-MS) has been developed dramatically as a powerful tool for the rapid analysis of samples in their native environment. Here a novel application of DESI-MS was demonstrated for direct probing of the reactive intermediates in the liquid-phase Eschweiler-Clarke reaction, a reductive amination reaction whereby a primary (or secondary) amine is successively N-methylated using excess formaldehyde and formic acid. The intermediates ion species of sodiated amino alcohol ([I + Na](+)) and iminium ([II](+)), along with the corresponding protonated molecules of amine reactant ([M + H](+)) and end product ([III + H](+)), were simultaneously and unambiguously characterized by the positive ion DESI-MS in the native liquid-phase reactive condition. The operating variables were optimized for better analytical performance including the spray solvent composition (such as formic acid concentration, proportion of methanol-water), voltage applied, spray spatial distance and incident angle. The feasibility of the reactive DESI-MS detection of acid-formaldehyde methylations was further validated using amines of a large variety of chemical types (2 primary and 3 secondary amines). Thus, the liquid-phase reactive DESI-MS technique allows the direct analysis of reaction intermediates occurring in complex liquid solutions without sample preparation to provide a valuable insight into chemical reaction mechanisms.     

A simple model for the disintegration of highly charged solvent droplets during electrospray ionization

This work uses a minimalist model for deciphering the opposing effects of Coulomb repulsion and surface tension on the stability of electrosprayed droplets. Guided by previous observations, it is assumed that progeny droplets are ejected from the tip of liquid filaments that are formed as protrusions of an initially spherical parent. Nonspherical shapes are approximated as assemblies of multiple closely spaced beads. This strategy greatly facilitates the calculation of electrostatic and surface energies. For a droplet at the Rayleigh limit the model predicts that growth of a very thin filament is a spontaneous process with a negligible activation barrier. In contrast, significant barriers are encountered for the formation of larger diameter filaments. These different barrier heights favor highly asymmetric droplet fission because the dimensions of the filament determine those of the ejected droplet(s). Substantial charge accumulation occurs at the filament termini. This allows each progeny droplet to carry a significant fraction of charge, despite its very small volume. In the absence of a long connecting filament, relieving electrostatic stress through progeny droplet emission would be ineffective. The model predicts the prevalence of fission events leading to the formation of several progeny droplets, instead of just a single one. Ejection bursts are followed by collapse back to a spherical shape. The resulting charge depleted system is incapable of producing additional progeny droplets until solvent evaporation returns it to the Rayleigh limit. Despite the very simple nature of the model used here, all of these predictions agree with experimental data.     

An electrospray ionization study of some novel alkylamine thiohydantoin amino acid derivatives

An electrospray ionization-mass spectrometric (ES-MS) study of some novel alkylamine thiohydantoin amino acid derivatives is presented. The alkylamine derivatives are being developed as part of an on-going effort to couple an Edmanlike protein sequencer to a bench-top electrospray ionization (ES) mass spectrometer. The ES-MS and capillary-skimmer collision-induced dissociation (CID) mass spectra of eight dimethylaminopropylthiohydantoin (DMAP-TH) amino acid derivatives as well as the trimethylaminopropylthiohydantoin (TMAP-TH), diethylaminopropylthiohydantoin, and dibutylaminopropylthiohydantoin (DBAP-TH) derivatives of Phe are presented. The spectra contain prominent [M + H]⁺ ions as well as fragment ions due to the loss of the respective neutral alkylamines. The CID spectrum of DMAP-TH-Phe also contains the dibutylaminoethyl cation. The relative responses of the alkylamine thiohydantoin derivatives obtained under identical solvent conditions are found to increase as the solvophobicity of the amino acid R group increases; the most solvophobic DBAP-TH-Phe gives rise to the highest overall response. DMAP-TH-Phe and the quaternary amine derivative TMAP-TH-Phe have comparable sensitivities when a sufficiently acidic solvent (pH = 3) is employed. Implications of these studies in the coupling of a protein sequencer to an ES mass spectrometer are discussed as are the requisite modifications of a single quadrupole mass spectrometer for ES analyses.     

Transformation of neutral rhenium compounds during electrospray ionization mass spectrometry

Four neutral rhenium compounds were examined by electrospray ionization mass spectrometry. Acetonitrile solutions of (Ind)Re(CO)₃ (Ind = indenyl) and (Cp)Re(CO)₃ (Cp = cyclopentadienyl) gave rise to [Re(CO)₃(CH₃CN)₃]⁺ ions. This is indicative of a reaction with the solvent, although these compounds do not react with acetonitrile under regular laboratory conditions. In contrast, (Ind)Re(CO)₂(butyne) and (Cp)Re(CO)₂(butyne) did not lose their aromatic hydrocarbon ligand upon ionization; the predominant product ions generated upon electrospray ionization were [(Ind)Re(CO)(CH₃CN)(butyne)]⁺ and [(Cp)Re(CO)(CH₃CN)(butyne)]⁺, respectively.     

Adduct formation in electrospray ionization mass spectrometry II. Benzoic acid derivatives

This work serves as a follow-up to Part I of experiments designed to determine the underlying principles in the formation of pseudomolecular, or adduct, ions during electrospray ionization. Aromatic acids were studied by flow injection analysis in the negative ionization mode of electrospray ionization mass spectrometry. Part I dealt with common acidic anti-inflammatory pharmaceuticals. such as ibuprofen and related analogues. Part II deals with functionally less complex molecules, namely benzoic acid (BA) and substituted benzoic acids. Halide-substituted molecules are investigated to deduce the effect of electron-withdrawing substituents (bromo-, chloro-, and fluoro-) and ring position (ortho-, meta- and para-) on the response of a traditional deprotonated molecular ion ([M-H]-) and a sodium-bridged dimer ion ([2M-2 H+Na]-). Amino-substituted benzoic acids are also analyzed in order to study the effect of an additional ionizable group on the molecule, and para-tert.-butyl-BA is analyzed to study the effect of increased hydrophobicity, as they relate to the formation of pseudomolecular ions. This study shows that solution character [octanol-water partition coefficient (or log P) and pKa] of the model compounds controls the relative efficiency of formation of [M-H]- and [2M-2H+Na]- ions. However the relative gas phase character (gas phase basicity and proton affinity) also has a significant effect on the formation of the sodium-bridged dimer ion. For the halide-substituted species, placement of the electron-withdrawing atom at the meta-position gives the greatest enhancement in sensitivity. Observations also show that as the structural complexity of the model compound increases, predictions relating analyte acidity to sodium-bridged dimer ion formation give way to a stronger dependence between log P values and ionization efficiency. Supporting this hypothesis is the nearly ten-fold enhancement in signal for tert.-butyl BA relative to BA. due to the greater hydrophobicity, and consequently, increased surface activity in an electrosprayed droplet of the analyte molecule.