Detection of low molecular weight aldehydes in aqueous solution by membrane introduction mass spectrometry

Membrane introduction mass spectrometry (MIMS) is used to detect low molecular weight aldehydes in aqueous solutions. The best sensitivity was obtained by aqueous phase derivatization of aldehydes with O-(2,3,4,5,6-pentafluorobenzyl)-hydroxylamine (PFBOA) and electron capture detection. This negative chemical ionization mass spectrometry procedure allowed the measurement of C(1)C(6) aldehydes at low concentrations in mixtures. The characteristic ion signals in the mass spectrum of the mixture were verified by examining the full mass spectra and product ion MS/MS spectra of the derivatives of individual aldehydes. A reaction scheme is proposed to explain the fragmentation pattern of the molecular anions (M(-.)) of the derivatives. The processes observed include loss of HF to form (MHF)(-.) ions which then competitively fragment by elimination of H(R)CN and NO(.) to produce ions of m/z 178 and (M-50)(-.), respectively. Multiple reaction monitoring was applied to establish the lower limits of detection. Formaldehyde could be detected without preconcentration at 1 ppb with S/N = 3/1. The detection limits of acetaldehyde, propanal and butanal were found to be 10 ppb and that of pentanal and hexanal were found to be 20 ppb. Response curves vs. concentration are linear in the ppb range. This method is not as readily applicable to the corresponding ketones.     

Fragmentation of organic ions bearing fixed multiple charges observed in MALDI MS

Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI TOF MS) was used to analyze a series of synthetic organic ions bearing fixed multiple charges. Despite the multiple intrinsic charges, only singly charged ions were recorded in each case. In addition to the pseudo‐molecular ions formed by counterion adduction, deprotonation and electron capture, a number of fragment ions were also observed. Charge splitting by fragmentation was found to be a viable route for charge reduction leading to the formation of the observed singly charged fragment ions. Unlike multivalent metal ions, organic ions can rearrange and/or fragment during charge reduction. This fragmentation process will evidently complicate the interpretation of the MALDI MS spectrum. Because MALDI MS is usually considered as a soft ionization technique, the fragment ion peaks can easily be erroneously interpreted as impurities. Therefore, the awareness and understanding of the underlying MALDI‐induced fragmentation pathways is essential for a proper interpretation of the corresponding mass spectra. Due to the fragment ions generated during charge reduction, special care should be taken in the MALDI MS analysis of multiply charged ions. In this work, the possible mechanisms by which the organic ions bearing fixed multiple charges fragment are investigated. With an improved understanding of the fragmentation mechanisms, MALDI TOF MS should still be a useful technique for the characterization of organic ions with fixed multiple charges.     

Generation of multiple charged ions: Photoemission electron impact ionization

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Structural characterization of poly(N-alkyl-4-vinylpyridinium) triflates using pyrolysis/tandem mass spectrometry

Desorption chemical ionization (DCI) and desorption electron ionization (DEI) of homo- and co-polymers of N-alkyl-4-vinylpyridinium triflates having ethyl, n-hexyl and n-dodecyl groups as N-alkyl substituents, produce mass spectra that display oligomeric ions. These positively charged ions are singly-charged and result from cleavage of the polymer into neutral oligomers and the loss of a single triflate anion per oligomer. Analogous negatively charged ions, in which each neutral oligomer carries an extra triflate anion, are observed in the desorption chemical ionization mass spectra. Each oligomer within the available mass range is represented in the mass spectra. The formation of cluster ions in which a single, multiply-charged cation is associated with a number of singly-charged anions, as observed for these ammonium polysalts, is unusual. Five major and three minor series of positively charged ions are observed in DCI and DEI methods of ionization. Ions in the different series correspond either to cleavage at different bonds between the constituent monomers or to hydrogen transfer in different directions. Unique and structurally diagnostic fragmentation processes are observed in tandem mass spectrometry (MS/MS) experiments performed using collision activated dissociation of mass-selected oligomeric ions.     

Analysis of aldehydes and ketones from beer as O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine derivatives

O-(2,3,4,5,6-pentafluorobenzyl)Hydroxylamine (PFBOA) was used as a derivatization reagent for carbonyl compounds in beer. Derivatization was carried out in aqueous solution without extraction or concentration of the sample. The effects of antifoam agent, reaction time and pH on the reaction efficiency were studied. Antifoam RD, a silicone polymer-based antifoam reagent, was the best antifoaming agent since it did not cause interferences. Reaction time studies showed that the yield of aldehydes increased up to 12 hr and then decreased slightly. The yield of 3-hydroxybutanone, a test compound for ketones, increased throughout the 48 hr test period. The natural pH of beer (ca. 4.5) was favourable for the determination of carbonyl compounds as PFBOA derivatives. Higher pH values caused yield losses and some compounds, such as butanedione, 2,3-pentanedione and 5-hydroxymethyl-2-furfural, could not be measured at all in neutral or basic conditions. Carbonyl compounds were identified by GC-MS, using three different ionization techniques, electron impact ionization, chemical ionization, and negative chemical ionization. The formation of the protonated molecules by ammonia chemical ionization and formation of the negative molecular ions and [M - HF](-.) ions by negative chemical ionization permitted reliable identification of the various carbonyl compounds studied. Sixteen carbonyl compounds from the 32 standard compounds were identified in beer and 11 of the most significant were quantitated using GC-ECD. Reproducibility of quantitation for beer samples was good, the relative standard deviations varied between 2.7 and 6.7 %. The estimated detection limits of the PFBOA derivatives of the carbonyl compounds in beer varied in the range of 0.01-1 microg/dm(3).     

The nature of collision-induced dissociation processes of doubly protonated peptides: comparative study for the future use of matrix-assisted laser desorption/ionization on a hybrid quadrupole time-of-flight mass spectrometer in proteomics.

Comparative MS/MS studies of singly and doubly charged electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) precursor peptide ions are described. The spectra from these experiments have been evaluated with particular emphasis on the data quality for subsequent data processing and protein/amino acid sequence identification. It is shown that, once peptide ions are formed by ESI or MALDI, their charge state, as well as the collision energy, is the main parameter determining the quality of collision-induced dissociation (CID) MS/MS fragmentation spectra of a given peptide. CID-MS/MS spectra of singly charged peptides obtained on a hybrid quadrupole orthogonal time-of-flight mass spectrometer resemble very closely spectra obtained by matrix-assisted laser desorption/ionization post-source decay time-of-flight mass spectrometry (MALDI-PSD-TOFMS). On the other hand, comparison of CID-MS/MS spectra of either singly or doubly charged ion species shows no dependence on whether ions have been formed by ESI or MALDI. This observation confirms that, at the time of precursor ion selection, further mass analysis is effectively decoupled from the desorption/ionization event. Since MALDI ions are predominantly formed as singly charged species and ESI ions as doubly charged, the associated difference in the spectral quality of MS/MS spectra as described here imposes direct consequences on data processing, database searching using ion fragmentation data, and de novo sequencing when ionization techniques are changed.     

Unimolecular and collision-induced reactions of doubly charged porphyrins

Results from a tandem mass spectrometry (MS/MS) study, obtained with a reverse-geometry mass spectrometer, of the unimolecular and collision-induced reactions of doubly charged free-base and metal containing alkyl-substituted porphyrins formed by electron ionization are reported. These doubly charged porphyrin ions dissociate to yield both singly and doubly charged product ions via a number of reactions. This article classifies the major reactions observed, illustrating each with the appropriate spectra. Supplementary data from the same porphyrins, acquired with a tandem quadrupole MS/MS instrument, are also presented. The potential utility of some of these reactions as new methods for porphyrin analysis is discussed.     

超高效液相色谱-电喷雾串联质谱法分离鉴定烟草中5种糖苷类香味前体物质

采用超高效液相色谱-电喷雾串联质谱法(UPLC-ESI MS/MS)并结合气相色谱-质谱法分离鉴定了烟草中5种主要的糖苷类香味前体物质。烟样经甲醇提取、XAD-2柱净化,得到初步纯化的糖苷,在pH 5条件下将其酶解,释放出糖苷配基。采用气相色谱-质谱分析并通过标准谱库检索确定了5种挥发性苷元;然后通过电喷雾质谱(负离子模式)确定糖苷母离子并作碎片离子扫描(MS2),确定了5种糖苷类香味前体物质的存在形式;最后采用UPLC-ESI MS/MS,以甲醇和乙酸-乙酸铵水溶液为流动相,通过RP-C18柱分离,在多反应监测(MRM)模式下,鉴定了烟草中5种主要的糖苷类香味前体物质,为应用液相色谱-质谱分析缺乏标准样品的糖苷类香味前体物质奠定了基础。     

Comparison of CID versus ETD‐based MS/MS fragmentation for the analysis of doubly derivatized steroids

Electrospray ionization coupled with collision‐induced dissociation (CID) and tandem mass spectrometry (MS/MS) is a commonly used technique to analyze the chemical composition of steroids. However, steroids are structurally similar compounds, making it difficult to interpret their product‐ion spectra. Electron transfer dissociation (ETD), a relatively new technique for protein and peptide fragmentation, has been shown to provide more detailed structural information. In this study, we compared the ability of CID with that of ETD to differentiate between eight 3,20‐dioxosteroids that had been derivatizated with a quaternary ammonium salt, Girard reagent P (GirP), at room temperature or after exposure to microwave irradiation to generate doubly charged ions. We found that the derivatization of steroid with GirP hydrazine occurred in less than 10 min when the reaction was carried out in the presence of microwave irradiation compared to 30 min when the reaction was carried out at room temperature. According to the MS/MS spectra, CID provided rich, structurally informative ions; however, the spectra were complex, thereby complicating the peak assignment. In contrast, ETD generated simpler spectra, making it easier to recognize individual peaks. Remarkably, both CID and ETD were allowed to differentiate of steroid isomers, 17α‐hydroxyprogesterone (17OHP) and deoxycorticosterone (DOC), but the signature ions obtained from CID were less intense than those generated by ETD, which generated much clearer spectra. These results indicate that ETD in conjunction with CID can provide more structural information for precise characterization of steroids. Copyright © 2013 John Wiley & Sons, Ltd.     

Field ionization mass spectrometry as a facile method for the study of the thermal formation of radicals

Electron impact spectra of thermolysis products of organic salts heated in the ion source of a mass spectrometer may give rise to organic ions corresponding to the cation of the salt. Field ionization mass spectrometry has been used as a facile method for detemining whether such an ion is due to ionization of the corresponding radical present in the gas phase, or to an electron impact induced fragmentation of a reaction product of higher mass. By comparison of the electron impact and field ionization spectra of a series of N-methyl pyridinium, tropylium and 1,2-dithiolylium salts it has been found possible to identify the radicals formed thermolytically, when present.     

Mass Spectra Electron Ionization of Tetraalkyl Pyrophosphates in the Mode of Registration of Positive Ions and Negative Ions of Resonance Electron Capture. Group Identification and Determination of the Number of Carbon Atoms in Alkyl Radicals of These Compounds

he decomposition paths of tetraalkyl pyrophosphates under conditions of electron ionization in the mode of registration of positive ions correspond to the general pattern of fragmentation of phosphoric acids esters elucidated earlier by the authors. Electron ionization mass spectra in the mode of registration of negative ions by resonant electron capture are an effective means for extracting information on the number of carbon atoms in the alkyl radicals of tetraalkyl pyrophosphates and full esters of other phosphoric acids     

Electron ionization and electrospray mass spectra of diaryl-substituted enaminoketones and their thio analogs

Electron ionization (EI) and positive electrospray ionization (ESI) mass spectra of selected diaryl enaminoketones and enaminothiones have been studied. In the EI mass spectra of both classes of compound, molecular ion peaks are accompanied by the peaks corresponding to the [M-H](+) ions. The formation of these ions can be rationalized by a cyclization reaction resulting in the formation of the respective isoxazolium and isothiazolium cations. Under positive ESI conditions, in the spectra recorded for the enaminoketones peaks corresponding to the [M+H](+), [M+Na](+) and [2M+Na](+) ions appeared, while in the spectra recorded for the enaminothiones, peaks corresponding to the [M-H](+) ions were dominant. These ions are most likely formed by oxidation of the neutral enaminothione molecules on the surface of the positively charged stainless steel capillary in the ESI ion source (anodic oxidation).     

Direct analysis of oligosaccharides and alpha hydroxy acids in fruits using electrosonic spray ionization mass spectrometry

Electrosonic spray ionization (ESSI) is a derivative technique of electrospray ionization (ESI) for mass spectrometry (MS) in which droplets are charged in the course of sonic spray. In this study, we applied ESSI MS to direct analysis of oligosaccharides and alpha hydroxy acids (AHAs) in fruits. The components were extracted from fruit fleshes by a feasible method prior to ESSI MS analysis, but the fruit juices were analyzed without further pretreatment. The results demonstrate that mainly alkali metal adducts of oligosaccharides are favorably produced in positive ion mode, while deprotonated AHAs and oligosaccharides are produced in negative ion mode. Compared with mass spectra obtained using electrospray droplet impact/secondary ion mass spectrometry (EDI/SIMS), mass spectra using ESSI make the identification of oligosaccharides more straightforward in positive ion mode than in negative ion mode.     

Electron impact and chemical ionization mass spectra of some tetra-acetylated anomeric glycosides

Electron impact and chemical ionization (CH₄, iso-C₄H₁₀ and NH₃) mass spectra of some tetra-acetylated anomeric glycosides have been examined with a view to the characterization of anomeric pairs. Minor differences observed in the relative intensities of common ions in the anomeric pairs in the electron impact mass spectra are found to be enhanced in the methane and isobutane chemical ionization mass spectra. In the absence of thermal decomposition, the β-anomers show greater ion abundances of common glycosyl ions than the α-anomers. Ammonia chemical ionization mass spectra show complementary behaviour indicating strong adduct ions and practically no fragmentation.     

Negative-ion electron capture dissociation: radical-driven fragmentation of charge-increased gaseous peptide anions

The generation of gaseous polyanions with a Coulomb barrier has attracted attention as exemplified by previous studies of fullerene dianions. However, this phenomenon has not been reported for biological anions. By contrast, electron attachment to multiply charged peptide and protein cations has seen a surge of interest due to the high utility for tandem mass spectrometry (MS/MS). Electron capture dissociation (ECD) and electron transfer dissociation (ETD) involve radical-driven fragmentation of charge-reduced peptide/protein cations to yield N-C(α) backbone bond cleavage, resulting in predictable c'/z(?)-type product ions without loss of labile post-translational modifications (PTMs). However, acidic peptides, e.g., with biologically important PTMs such as phosphorylation and sulfonation, are difficult to multiply charge in positive ion mode and show improved ionization in negative-ion mode. We found that peptide anions ([M - nH](n-), n ≥ 1) can capture electrons within a rather narrow energy range (~3.5-6.5 eV), resulting in charge-increased radical intermediates that undergo dissociation analogous to that in ECD/ETD. Gas-phase zwitterionic structures appear to play an important role in this novel MS/MS technique, negative-ion electron capture dissociation (niECD).     

Analysis of hydroxy fatty acids as pentafluorobenzyl ester,trimethylsilyl ether derivatives by electron ionization gas chromatography/mass spectrometry

Electron ionization (EI) gas chromatography/mass spectrometry (GC/MS) analysis of pentafluorobenzyl ester-trimethyl sllyl ether (PFB-TMS) derivatives of hydroxy-subshtuted fatty acids provides structural information comparable to that obtained in analysis of methyl ester-trimethyl silyl ether (Me-TMS) derivatives. Use of this derivative eliminates the need to prepare two separate derivatives, the PFB-TMS derivative for molecular weight determination by electron capture ionization (negative ions) analysis and the Me-TMS derivative for structural determination by EI GC/MS analysis. The relative abundance of fragment ions observed during EI GC/MS analysis of these derivatized unsaturated fatty acids indicates the location of the —OTMS substituents relative to double bond positions in those cases studied. The most abundant fragment ions are observed when the compound contains an unsaturation two carbon atoms removed from the —OTMS ether carbon (the β-OTMS position). The “saddle effect” observed in the GC/MS analyses of some derivatized monohy— droxy unsaturated fatty acids is suggested to be due to a thermally allowed pericyclic double bond rearrangement and indicates the presence of a conjugated diene one carbon atom removed from the —OTMS ether carbon (the α-OTMS position). The saddle effect is most prominent for fatty acids that contain additional unsaturation separated by a single methylene unit from the conjugated diene moiety.     

Rapidly switchable matrix-assisted laser desorption/ionization and electrospray quadrupole-time-of-flight mass spectrometry for protein identification

We describe a new interface for a prototype quadrupole-quadrupole-time-of-flight (TOF) mass spectrometer (Centaur, Sciex) that allows rapid switching between electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) modes of operation. Instrument performance in both modes is comparable (i.e., resolution approximately 10,000 FWHM, mass accuracy <10 ppm, sensitivity approximately 1 fmol) because the ion source is decoupled from the TOF mass analyzer by extensive gas collisions in the quadrupole stages of the instrument. The capacity to obtain side-by-side high quality ESI and MALDI mass spectra from a single proteolytic mixture greatly facilitates the identification of proteins and elucidation of their primary structures. Improved strategies for protein identification result from this ability to measure spectra using both ionization modes in the same instrument and to perform MS/MS on singly charged as well as multiply charged ions. Examples are provided to demonstrate the utility and performance of the modified instrument.     

Polarization induced electrospray ionization mass spectrometry for the analysis of liquid,viscous and solid samples

In this study, a polarization‐induced electrospray ionization mass spectrometry (ESI‐MS) was developed. A micro‐sized sample droplet was deposited on a naturally available dielectric substrate such as a fruit or a stone, and then placed close to (~2 mm) the orifice of a mass spectrometer applied with a high voltage. Taylor cone was observed from the sample droplet, and a spray emitted from the cone apex was generated. The analyte ion signals derived from the droplet were obtained by the mass spectrometer. The ionization process is similar to that in ESI although no direct electric contact was applied on the sample site. The sample droplet polarized by the high electric field provided by the mass spectrometer initiated the ionization process. The dielectric sample loading substrate facilitated further the polarization process, resulting in the formation of Taylor cone. The mass spectral profiles obtained via this approach resembled those obtained using ESI‐MS. Multiply charged ions dominated the mass spectra of peptides and proteins, whereas singly charged ions dominated the mass spectra of small molecules such as amino acids and small organic molecules. In addition to liquid samples, this approach can be used for the analysis of solid and viscous samples. A small droplet containing suitable solvent (5–10 µl) was directly deposited on the surface of the solid (or viscous) sample, placed close the orifice of mass spectrometer applied with a high voltage. Taylor cone derived from the droplet was immediately formed followed by electrospray processes to generate gas‐phase ions for MS analysis. Analyte ions derived from the main ingredients of pharmaceutical tablets and viscous ointment can be extracted into the solvent droplet in situ and observed using a mass spectrometer. Copyright © 2015 John Wiley & Sons, Ltd.     

Characterization of Rhenium(V) Complexes with Phenols Using Mass Spectrometry with Selected Soft Ionization Techniques

The synthesis, characterization, and mass spectra of oxorhenium(V) complexes with 1,2-dihydroxybenzene, 1,2,3-trihydroxybenzene, and 2,3-dihydroxynaphtalene are reported. Electrospray ionization, atmospheric pressure photoionization, and laser desorption/ionization mass spectra of the complexes showed abundant negatively charged molecular anions and low fragmentation. Calculated similarity indexes showed significant conformity between the computed and experimental isotopic patterns of selected ions and confirmed correct assignment of elemental composition to m/z values. Electrospray tandem mass spectrometry provided essential information about fragments from molecular ions of studied complexes, making it possible to distinguish among fragment ions and the ions arising from compounds present in the reaction mixture. Based on the results, mass spectrometry utilizing soft common ionization techniques is useful for monitoring complex formation reaction kinetics and the stabilities of the complexes. Representative spectra were recorded for micromolar concentrations of the analytes.     

Mass spectral characteristics of okadaic acid and simple derivatives.

Electron ionization (EI), chemical ionization (CI) and fast-atom bombardment (FAB) mass spectra of the marine toxin okadaic acid and its synthetic methyl, pentafluorobenzyl, and trimethylsilyl ester and ether derivatives were generated. Several ionization conditions and ion-processing methods were used to obtain positive- and negative-ion conventional spectra and tandem (MS/MS) spectra. The EI and the positive-ion CI spectra provided fragment ions characteristic of the structure, and the negative-ion CI and FAB spectra provided molecular ions. The addition of alkali salts to the FAB matrix resulted in reduced fragmentation and the formation of intense alkali-metal-cationized molecules. Pentafluorobenzyl ester derivatives provided intense carboxylate ions under electron-capture ionization. Analytically useful MS/MS spectra were obtained by low-energy collision-induced decomposition of the carboxylate anion produced from the tetrasilylated pentafluorobenzylokadaate.