Generation of CsI cluster ions for mass calibration in matrix-assisted laser desorption/ionization mass spectrometry

A simple method was developed for the generation of cesium iodide (CsI) cluster ions up to m/z over 20,000 in matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS). Calibration ions in both positive and negative ion modes can readily be generated from a single MALDI spot of CsI₃ with 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene] malononitrile (DCTB) matrix. The major cluster ion series observed in the positive ion mode is [(CsI)_nCs]⁺, and in the negative ion mode is [(CsI)_nI]⁻. In both cluster series, ions spread evenly every 259.81 units. The easy method described here for the production of CsI cluster ions should be useful for MALDI MS calibrations.     

Secondary‐Ion Mass Spectrometry Images Cardiolipins and Phosphatidylethanolamines at the Subcellular Level

Millions of diverse molecules constituting the lipidome act as important signals within cells. Of these, cardiolipin (CL) and phosphatidylethanolamine (PE) participate in apoptosis and ferroptosis, respectively. Their subcellular distribution is largely unknown. Imaging mass spectrometry is capable of deciphering the spatial distribution of multiple lipids at subcellular levels. Here we report the development of a unique 70 keV gas‐cluster ion beam that consists of (CO₂)_n⁺(n>10 000) projectiles. Coupled with direct current beam buncher‐time‐of‐flight secondary‐ion mass spectrometry, it is optimized for sensitivity towards high‐mass species (up to m/z 3000) at high spatial resolution (1 μm). In combination with immunohistochemistry, phospholipids, including PE and CL, have been assessed in subcellular compartments of mouse hippocampal neuronal cells and rat brain tissue.     

Salicylaldehyde azine cluster formation observed by cold‐spray ionization mass spectrometry

We installed a cold‐spray ionization (CSI) source on a mass spectrometer to investigate the self‐assembly behavior of an aggregation‐induced emission enhancement system. Using a CSI source and the three‐dimensional platform, a self‐assembly system of a salicylaldehyde azine (SAA) was studied in mixture solution. This method permitted the determination of the structural information of the solution state, which cannot be detected by conventional mass spectrometry. In addition to the [M+H]⁺ ion (M is the SAA molecule), many major ion clusters such as [2M+Na]⁺ at m/z 503, [3M+Na]⁺ at m/z 743, [4M+Na]⁺ at m/z 983 and higher order aggregates were observed in the CSI mass spectra. However, many fragment ions, with the exception of cluster ions, appeared with high abundance when the ESI ion source was used due to the desolvation chamber temperature, suggesting that some aggregation can be detected at low temperatures. To investigate the effect of solvent on the aggregation, the CSI‐mass spectrometry (MS) experiments of SAA in absolute ethanol solution and ethanol/water (good/poor solvent) mixture solution were conducted. The most abundant ion peak was protonated SAA (m/z 241) in absolute ethanol, but many cluster ions and some multiple charged ion peaks were observed after adding a small amount of water into the ethanol solution. The results showed good agreement with that inferred by the combinational analysis of scanning electron microscope and fluorescence microscopy, indicating that CSI‐MS is capable of providing self‐assembly information of labile molecules in the solution state. Copyright © 2013 John Wiley & Sons, Ltd.     

Chemical discrimination of multilayered paint cross sections for potential forensic applications using time‐of‐flight secondary ion mass spectrometry

Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) equipped with a bismuth imaging source and an argon gas cluster ion beam (GCIB) was used to image polished cross‐sections of four automotive multilayer paint samples. Secondary ion mass spectrometry chemical imaging of the individual layers was possible after a GCIB sputter ion dose of (7 × 10¹⁵) ions/cm² was applied for the removal of polishing residue, at which point the chemical composition of the individual clear coats could be distinguished using principal components analysis. For the differentiation of the four clear coat chemistries, only four secondary ion peaks were necessary; C₂H₅O⁺ (m/z 45.04), C₉H₉NO₂⁺ (m/z 163.09), and C₁₀H₁₁NO₂⁺ (m/z 177.10) that appeared to be fragments of the carbamate‐based clear coat, and C₇H₁₁⁺ (m/z 95.09) that was strongly associated with the polyurethane‐based clear coat. Clear identification of the four paint samples based on this short peak list highlights the strength of the SIMS technique as a potential forensic approach to discriminate automotive paints and suggests that many more variables could be included in the multivariate and statistical analysis to differentiate a wider range of clear coat chemistries.     

Proton transfer and isotope‐induced reaction in aniline cluster ions

The proton transfer (PT) and other intraclusters reactions occurring after electron ionization of aniline clusters (PhNH₂)_N are investigated by the time‐of‐flight mass spectrometry. The mass spectra are recorded for different expansion conditions leading to the generation of different cluster sizes. Several fragment ions are shown to originate from intracluster reactions, namely, [Ph]⁺, [PhNH₃]⁺ and [Ph–N–Ph]⁺. Reaction schemes are proposed for these ions starting with the PT process. The mass region beyond the monomer mass is dominated by cluster ions (PhNH₂)_n⁺ accompanied by satellites with ±H and +2H. In experiments with deuterated species, new fragment ions are identified. The aniline isotopomer d₅‐PhNH₂ yields the fragment ions (PhNH₂)_n?(N–Ph–NH₂)⁺. Analogical series is observed in experiments with d₇‐PhND₂, and additional fragments occur corresponding to (PhND₂)_n?(D₂N–ND–Ph–ND–ND₂)⁺ ions. The possible reaction pathways to these ions and the unusual isotope effects are discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

Role of the UV absorber as a matrix in matrix‐assisted laser desorption/ionization mass spectrometric analysis of a mixture of a UV absorber and a stabilizer

A mixture of a UV absorber (Tinuvin 234 or Tinuvin 329) and a UV stabilizer (Tinuvin 770) was analyzed using matrix‐assisted laser desorption/ionization mass spectrometry (MALDI‐MS) without any matrix. Fragmentation patterns of the UV absorbers and stabilizer were also investigated. The mass spectra showed the [M+H]⁺ ions and some fragment ions. Tinuvin 234, Tinuvin 329, and Tinuvin 770 generated three (m/z 119, 370, 432), one (m/z 252), and two (m/z 124 and 140) fragment ions, repectively. These fragment ions can be used to identify the chemical structures of the UV absorbers and stabilizer. Since the UV absorber performed a role as the matrix, the ion abundance of the UV stabilizer was enhanced by mixing with the UV absorber. When organic materials extracted from polypropylene (PP) containing the UV absorber and stabilizer were directly analyzed using MALDI‐MS without any matrix, the protonated molecule of the UV stabilizer was detected in abundance but the product ions of the UV absorber were not observed. When 2,5‐dihydroxybenzoic acid was used as a matrix, the protonated molecule of the UV absorber was observed. Copyright © 2010 John Wiley & Sons, Ltd.     

Mass spectrometric evidence for collisionally induced removal of H2 from monoanions of 10B nido‐carborane derivatives investigated by electrospray ionization quadrupole linear ion trap and Fourier transform ion cyclotron resonance mass spectrometry

Some newly synthesized ¹⁰B nido‐carborane derivatives, i.e., 7,8‐dicarba‐nido‐undecaborane monoanions ([7‐Me‐8‐R‐C₂B₉H₁₀]^‐K⁺, R = H, butyl, hexyl, octyl and decyl), have been fully characterised and examined by electrospray ionization and Fourier transform ion cyclotron resonance mass spectrometry with liquid chromatographic separation (LC/ESI‐FTICR‐MS). These boron‐containing compounds exhibit abundant molecular ions ([M]^?) at m/z 140.22631 [CB₉H₁₄]^?, m/z 196.28883 [CB₉H₂₂]^?, m/z 224.32032 [CB₉H₂₆]^?, m/z 252.35133 [CB₉H₃₀]^? and m/z 280.38354 [CB₉H₃₄]^? at the normal tube lens voltage setting of ?90 V, which was an instrumental parameter value selected in the tuning operation. Additional [M–nH₂]^? (n = 1?4) ions were observed in the mass spectra when higher tube lens voltages were applied, i.e., ?140 V. High‐resolution FTICR‐MS data revealed the accurate masses of fragment ions, bearing either an even or an odd number of electrons. Collision‐induced dissociation of the [M–nH₂]^? ions (n = 0–4) in the quadrupole linear ion trap (LTQ) analyzer confirmed the loss of hydrogen molecules from the molecular ions. It is suggested that the loss of H₂ molecules from the alkyl chain is a consequence of the stabilization effect of the nido‐carborane charged polyhedral skeleton. Copyright © 2009 John Wiley & Sons, Ltd.     

Structural determination by atmospheric pressure photoionization tandem mass spectrometry of some compounds isolated from the SARA fractions obtained from bitumen

We have identified compounds obtained from the SARA fractions of bitumen by using atmospheric pressure photoionization mass spectrometry and low‐energy collision tandem mass spectrometric analyses with a QqToF‐MS/MS hybrid instrument. The identified compounds were isolated from the maltene saturated oil and the aromatic fractions of the SARA components of a bitumen. The QqToF instrument had sufficient mass resolution to provide accurate molecular weight information and to enhance the tandem mass spectrometry results. The APPI‐QqToF‐MS analysis of the separated compounds showed a series of protonated molecules [M + H]⁺ and molecular ions [M]^+? of the same mass but having different chemical structures, in the maltene saturated oil and the aromatic SARA fractions. These isobaric ions were a molecular ion [M₂]^+? at m/z 418.2787 and a protonated molecule [M₅ + H]⁺ at m/z 287.1625 in the saturated oil fraction, and molecular ions [M₆]^+? at m/z 418.1584 and [M₇]^+? at m/z 287.1285 in the aromatic fraction. The identification of this series of chemical compounds was achieved by performing CID‐MS/MS analyses of the molecular ions [M]^+? ([M₁]^+? at m/z 446. 2980, [M₂]^+? at m/z 418.2787, [M₃]^+? at m/z 360.3350 and [M₄]^+? at m/z 346.2095) in the saturated oil fraction and of the [M₅ + H]⁺ ion at m/z 287.1625 also in the saturated oil fraction. The observed CID‐MS/MS fragmentation differences were explained by proposed different breakdown processes of the precursor ions. The presented tandem mass spectrometric study shows the capability of MS/MS experiments to differentiate between different classes of chemical compounds of the SARA components of bitumen and to explain the reasons for the observed mass spectrometric differences. However, greater mass resolution than that provided by the QqToF‐MS/MS instrument would be required for the analysis of the asphaltene fraction of bitumen. Copyright © 2011 John Wiley & Sons, Ltd.     

The renaissance of high-energy CID for structural elucidation of complex lipids: MALDI-TOF/RTOF-MS of alkali cationized triacylglycerols

Triacylglycerols were analyzed as cationized species (Li⁺, Na⁺, K⁺) by high-energy CID at 20 keV collisions utilizing MALDI-TOF/RTOF mass spectrometry. Precursor ions, based on [M+Li]⁺-adduct ions exhibited incomplete fragmentation in the high and low m/z region whereas [M+K]⁺-adducts did not show useful fragmentation. Only sodiated precursor ions yielded product ion spectra with structurally diagnostic product ions across the whole m/z range. The high m/z region of the CID spectra is dominated by abundant charge-remote fragmentation of the fatty acid substituents. In favorable cases also positions of double bonds or of hydroxy groups of the fatty acid alkyl chains could be determined. A-type product ions represent the end products of these charge-remote fragmentations. B- and C-type product ions yield the fatty acid composition of individual triacylglycerol species based on loss of either one neutral fatty acid or one sodium carboxylate residue, respectively. Product ions allowing fatty acid substituent positional determination were present in the low m/z range enabling identification of either the sn-1/sn-3 substituents (E-, F-, and G-type ions) or the sn-2 substituent (J-type ion). These findings were demonstrated with synthetic triacylglycerols and plant oils such as cocoa butter, olive oil, and castor bean oil. Typical features of 20 keV CID spectra of sodiated triacylglycerols obtained by MALDI-TOF/RTOF MS were an even distribution of product ions over the entire m/z range and a mass accuracy of ±0.1 to 0.2 u. One limitation of the application of this technique is mainly the insufficient precursor ion gating after MS1 (gating window at 4 u) of species separated by 2 u.     

Multicyclic polyethers by the polycondensation of 1,2‐ or 1,3‐dicyanotetrafluorobenzene with flexible diphenols

1,2‐Dicyanotetrafluorobenzene (1,2‐DCTB) was polycondensed with various flexible diphenols in a molar ratio of 1:2, and experimental parameters such as the concentration and temperature were varied. Certain diphenols allowed a complete substitution of all C? F bonds, so perfect multicyclic polyethers (B_nCN, where B stands for bridge units, C represents cycles, and N is the degree of polymerization) were the main reaction products. Despite complete conversion, gelation was avoidable under optimized reaction conditions. However, in the case of 1,3‐dicyanotetrafluorobenzene (1,3‐DCTB), complete tetrasubstitution was not feasible with a feed ratio of 1:2. Yet, because of the inductive and mesomeric electronic interactions of all substituents in 1,3‐DCTB, the three C? F groups in the ortho position with respect to the cyano groups were significantly more reactive than the fourth C? F bond. Therefore, polycondensations with diphenols in a 3:2 feed ratio showed a relatively clean course, yielding soluble multicycles of structure B_{n /2}CN. All the multicyclic polyethers were amorphous and possessed molar mass distributions with polydispersities greater than 2. Heating with Cu²⁺ salts caused crosslinking of the multicycles derived from 1,2‐DCTB because of the formation of phthalocyanine complexes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5546–5556, 2006     

Characterization of ice‐nucleating bacteria using on‐line electron impact ionization aerosol mass spectrometry

The mass spectral signatures of airborne bacteria were measured and analyzed in cloud simulation experiments at the AIDA (Aerosol Interaction and Dynamics in the Atmosphere) facility. Suspensions of cultured cells in pure water were sprayed into the aerosol and cloud chambers forming an aerosol which consisted of intact cells, cell fragments and residual particles from the agar medium in which the bacteria were cultured. The aerosol particles were analyzed with a high‐resolution time‐of‐flight aerosol mass spectrometer equipped with a newly developed PM_2.5 aerodynamic lens. Positive matrix factorization (PMF) using the multilinear engine (ME‐2) source apportionment was applied to deconvolve the bacteria and agar mass spectral signatures. The bacteria mass fraction contributed between 75 and 95% depending on the aerosol generation, with the remaining mass attributed to agar. We present mass spectra of Pseudomonas syringae and Pseudomonas fluorescens bacteria typical for ice‐nucleation active bacteria in the atmosphere to facilitate the distinction of airborne bacteria from other constituents in ambient aerosol, e.g. by PMF/ME‐2 source apportionment analyses. Nitrogen‐containing ions were the most salient feature of the bacteria mass spectra, and a combination of C₄H₈N⁺ (m/z 70) and C₅H₁₂N⁺ (m/z 86) may be used as marker ions. Copyright © 2015 John Wiley & Sons, Ltd.     

Negative ion mode electrospray ionization mass spectrometry study of ammonium-counter ion clusters

Electrospray ionization mass spectrometry (ESI-MS) was used to examine clusters of protonated amine salt solutions with chloride counter ions in the negative ion mode. These ions have the general formula [(RNH₃)_xCl_x+1]⁻. Primary amines generate a wide cluster distribution with clusters up to 14 mer for methylamine hydrochloride clusters. Secondary and quaternary amines only generate the monomer ion under identical conditions. Collision induced dissociation (CID) of the cluster ions generates cluster ions of lower m/z with the next lower cluster being the most abundant. The product ions from MeNH₃Cl₂⁻, Me₂NH₂Cl₂⁻ and (MeNH₃)₂Cl₃⁻ have low threshold appearance energies of 1. 24 to 2. 22 eV center-of-mass frame. Secondary amine monomer ions have lower threshold CID energies than primary amine monomer ions. The amine threshold CID energy decreases as the carbon chain length increases. As an electrospray solvent, isopropyl alcohol (IPA) promotes the formation of counter ions and clustering.     

Laser desorption ionization of red phosphorus clusters and their use for mass calibration in time‐of‐flight mass spectrometry

Phosphorus clusters P_n (n = 1–89) are easily formed from red phosphorus by laser desorption ionization (LDI) and they cover a range of up to approx. m/z 3000 in both positive and negative ion mode. The clusters are singly charged and the spectra are simple because phosphorus is monoisotopic. The mass spectra can be measured with an acceptable resolution and intensity. The use of positively charged P_n clusters for calibration in mass spectrometry was examined and it was demonstrated that in external calibration a standard deviation of ±0.04 m/z units can be achieved even when using a common commercial matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF) instrument. When used as internal standards the P_n clusters react with some analytes – C₆₀ and C₇₀ fullerenes and cucurbituril[8], for example. It was also found that red phosphorus is a suitable MALDI matrix for peptides and proteins, illustrated by the examples of a Calmix mixture of bradykinin, angiotensin, renin, adrenocorticotropic hormone ACTH fragment 18‐359 and insulin, and of insulin alone. Copyright © 2009 John Wiley & Sons, Ltd.     

Rearrangement reaction of the hydroxyl group of ω-hydroxy-alkyltriphenyl phosphonium bromides

No molecular ion peak from the Electron Impact lonization of eight co-hydroxyalkyltriphenyl phosphonium bromides(Ph3P (CH2)nOHBr-,n=2-6,8-10)can be found,except a part of some relative powerful fragment ions can be observed only.Each compound forms a very characteristic ion(O=PPbj-1) at m/z 277 through hydroxyl rearrangement reaction.The intensity of this ion is closely related with the size of the carbon chain of hydroxyalkyl and with temperature of ion source and temperature of sample probe.The above rearrangement reaction and the reaction to form ion at m/z 262 take place simultaneously,thus leading to strong competition.At n=2,ion at m/z 277 is the most powerful and becomes continuously the base peak.At n=3 and n=4,the intensity of ion at m/z 262 reaches the maximum,and is always the base peak,and the relative abundance of m/z 277 is only around 2%.At n=5,6,8,9,10,m/z 277 becomes base peak when the temperature of probe is below 300℃.But,when the temperature increases from 300℃to 350℃,m/z 262 sud     

A MALDI‐TOF MS study of lanthanide(III)‐cored poly(phenylenevinylene) dendrimers

An extensive study by matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF) mass spectrometry (MS) of some first‐generation and second‐generation lanthanide(III)‐cored poly(phenylenevinylene) dendrimers is described. The complexes were obtained by self‐assembly of suitably functionalized carboxylate dendrons around the lanthanide ion (La³⁺, Er³⁺). Fourier transform infrared (FT‐IR) spectroscopy gave reasonable evidence for the proposed structures. However, MS was used to ascertain unequivocally the complex formation. The most reliable results were found in the negative reflector mode, using 2‐[(2E)‐3‐(4‐tert‐butylphenyl)‐2‐methylprop‐2‐enylidene]malononitrile (DCTB) as matrix. Well‐defined and highly resolved base peaks corresponding to negative ions of [Gn₄La]^? and [Gn₄Er]^? were found in all cases, with an excellent match between the theoretical and observed isotope distributions. However, the 3 : 1 stoichiometry used in the synthesis guarantees an empirical formula Gn₃Ln for the complexes. Copyright © 2008 John Wiley & Sons, Ltd.     

Sputtering properties for polyimide by vacuum electrospray droplet impact (V‐EDI) using size‐selected cluster ions

Recently, the vacuum electrospray droplet impact (V‐EDI) was developed as a cluster ion beam source in our laboratory. To attain the ion beam stability and compact design of the ion source, a silica nano‐capillary with 15 µm i.d. was used as the emitter of the beam. It was found that stable electrospray was generated from the capillary tip without the use of laser heating when aqueous solution of 20% ethanol was used. The m/z distribution of electrospray droplets was measured by pulsing the primary beam. By assuming that the charged droplets contain 50% of the excess charges defined by the Rayleigh limit equation, the average mass, and charge of the droplets generated by the present V‐EDI are estimated as 2.5 × 10⁸ u and + 625 charges, respectively, i.e. [(H₂O)_14,000,000 + 625H]⁶²⁵⁺. By chopping the primary cluster beam, clusters composed of smaller m/z clusters (group 1: G1, [(H₂O)_46,000 + 36H]³⁶⁺) and those of larger m/z clusters (group 2: G2, [(H₂O)_560,000 + 125H]¹²⁵⁺) were generated. Surface analysis for polyimide (PI) film by X‐ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) was made using G1, G2, and non‐selected cluster beams. No selective etching was observed when G1, G2, and non‐selected beams were used. However, larger surface roughening was observed when smaller size cluster beams were used. This suggests that larger size clusters cure the surface damage caused by the smaller ones. Copyright © 2016 John Wiley & Sons, Ltd.     

Analysis of Amaryllidaceae alkaloids from Crinum by high‐performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry

A high‐performance liquid chromatography/electrospray ionization multi‐stage tandem mass spectrometry (HPLC/ESI‐MSⁿ) method was developed to analyze two structurally related groups of Amaryllidaceae alkaloids (AmAs), crinane‐ and tazettine‐type alkaloids, in the species Crinum latifolium and C. asiaticum, as well as different organs of C. latifolium. In ESI‐MSⁿ spectra of the two types of alkaloids, characteristic fragmentation reactions were observed that allowed us to determine and differentiate them. Based on the fragmentation rules of reference standards, crinane‐type alkaloids displayed concurrent neutral loss of C₂H₅N (43 u) and C₂H₆N (44 u) as well as characteristic ions of m/z 213 and 211, whereas tazettine‐type alkaloids exhibited neutral loss of C₃H₇N (57 u) [or C₂H₅N (43 u), C₃H₇NO (73 u)] from the [M+H]⁺ and [M+H–H₂O]⁺ ions. These were supported by quadrupole time‐of‐flight (Q‐Tof)‐MS/MS analysis. The chemical complexity of the mixture was resolved by profiling. The compositions of the main crinane‐ and tazettine‐type alkaloids in the above‐mentioned species and organs were also compared. Overall, 28 AmAs comprising 14 crinane‐type and 14 tazettine‐type alkaloids were identified and studied by MS. Among them, 14 AmAs were tentatively characterized from the two species for the first time. This method allowed a rapid analysis of alkaloid distribution and composition of Crinum species, and may also be used for quality control and screening of extracts designated for pharmaceutical application. Copyright © 2009 John Wiley & Sons, Ltd.     

Detection of Radical Adducts with Small Molecular Weights by Matrix-Assisted Laser Desorption/Ionization with Fourier Transform Mass Spectrometry

As an alternative method, matrix-assisted laser desorption/ionization with Fourier transform mass spectrometry （MALDI-FTMS） has been successfully used to detect and identify free radical adducts with small molecular weights of hydroxyl and 2-cyano-2-propyl radicals trapped with 5,5-dimethylpyrroline N-oxide （DMPO）. The detection and identification by MS/MS experiments using sustained offresonance irradiation collision-induced dissociation （SORI-CID） of [（DMPO＋·OH-·H）＋H^＋] （m/z 130.0868） and [DMPO＋2 ·CH（CH3）2CN＋H^＋] （m/z 250.1917） have demonstrated that MALDI-FTMS could be an effective method for detection and identification of free radical adducts. Other radical adducts have been also detected and identified. The approach of MALDI-FTMS is simple, fast, and sensitive which has potential for high-throughput analysis.     

Identification of isomeric 5‐hydroxytryptophan‐ and oxindolylalanine‐containing peptides by mass spectrometry

Cells continuously produce reactive oxidative species that can modify all cellular components. In proteins, for example, cysteine, methionine, tryptophan (Trp), and tyrosine residues are particularly prone to oxidation. Here, we report two new approaches to distinguish two isomeric oxidation products of Trp residues, i.e. 5‐hydroxytryptophan (5‐HTP) and oxindolylalanine (Oia) residues, in peptides. First, 2‐nitrobenzenesulfenyl chloride, known to derivatize Trp residues in position 2 of the indole ring, was used to label 5‐HTP residues. The mass shift of 152.98 m/z units allowed identifying 5‐HTP‐ besides Trp‐containing peptides by mass spectrometry, whereas Oia residues were not labeled. Second, fragmentation of the Oia‐ and 5‐HTP‐derived immonium ions at m/z 175.08 produced ions characteristic for each residue that allowed their identification even in the presence of y₁ ions at m/z 175.12 derived from peptides with C‐terminal arginine residues. The pseudo MS³ spectra acquired on a quadrupole time‐of‐flight hybrid mass spectrometer displayed two signals at m/z 130.05 and m/z 132.05 characteristic for Oia‐containing peptides and a group of six signals (m/z 103.04, 120.04, 130.04, 133.03, 146.04, and 148.04) for 5‐HTP‐cointaining peptides. In both cases, the relative signal intensities appeared to be independent of the sequence providing a specific fingerprint of each oxidative modification. Copyright © 2012 John Wiley & Sons, Ltd.     

Selectivity of gas‐phase ion/molecule reaction of carbon dioxide with phenide ions

On contrary to the widely accepted conviction that the m/z 93 ion derived from phenol does not react with CO₂, we demonstrate that it makes an adduct with CO₂ to a small but demonstrable extent. For example, the product‐ion mass spectrum recorded for the m/z 98 ion derived from [²H₆]phenol showed a small peak at m/z 142 when CO₂ was used as the collision gas. The formation of an m/z 137 adduct ion from the m/z 93 ion (generated either directly from phenol, or indirectly from salicylic acid by in‐source decarboxylation) was demonstrated also by multiple‐reaction‐monitoring tandem mass spectrometric experiments. According to literature, the m/z 93 ion derived from salicylic acid does not undergo CO₂ addition because it is deemed to exist only in the phenoxide form. This reaction has been previously proposed as a method for differentiating phenoxide ion from its isomeric hydroxyphenide ions. We propose that the m/z 93 ion, albeit small, exists also as the phenide form together with the predominant phenoxide ion. Copyright © 2014 John Wiley & Sons, Ltd.