[M + 11]+˙ and [M + 11]−˙ ions in the nitrogen positive and negative ion chemical ionization mass spectra of aromatic amines

The N₂ negative ion chemical ionization (NICI) mass spectra of aniline, aminonaphthalenes, aminobiphenyls and aminoanthracenes show an unexpected addition appearing at [M + 11]^?˙. This addition is also observed in the N₂ positive chemical ionization (PCI) mass spectra. An ion at [M – 15]^? is found in the NICI spectra of aminoaromatics such as aniline, 1- and 2-aminonaphthalene and 1- and 2-aminoanthracene. Ion formation was studied using labeled reagents, variation of ion source pressure and temperature and examination of ion chromatograms. These experiments indicate that the [M + 11]^?˙, [M – 15]^?˙ and [M + 11]^+˙ ions result from the ionization of analytes altered by surface-assisted reactions. Experiments with ¹⁵N₂, [¹⁵N] aniline, [2,3,4,5,6-²H₅] aniline and [¹³C₆] aniline show that the [M + 11]^?˙ ion corresponds to [M + N – 3H]^?˙. The added nitrogen originates from the N₂ buffer gas and the addition occurs with loss of one ring and two amino group hydrogens. Fragmentation patterns in the N₂ PCI mass spectrum of aniline suggest that the neutral product of the surface-assisted reaction is 1,4-dicyanobuta-1,3-diene. Experiments with diamino-substituted aromatics show analogous reactions resulting in the formation of [M – 4H]^?˙ ions for aromatics with ortho-amino groups. Experiments with methylsubstituted aminoaromatics indicate that unsubstituted sites ortho to the amino group facilitate nitrogen addition, and that methyl groups provide additional sites for nitrogen addition.     

Origin of the [M − H]+ ion in flavanone

Studies of the genesis of the [M ? H]⁺ ion in flavanone and 2′-hydroxychalcone, performed with the aid of metastable decompositions and deuterium labelling, allow new structural notations to be postulated for the [M ? H]+ ions, which in turn provide evidence for the pathways in the [M ? H – ketene]+ fragmentation routes for these compounds.     

Collisionally activated mass spectra of [M?H] ions of polyhydroxy and hydroxy ether compounds

Collision-induced decomposition/mass-analyzed ion kinetic energy or collisionally activated mass spectra of [M ? H]^? ions of polyhydroxy compounds and other alcohols and ethers are reported. The [M ? H]^? ion of each compound is produced under OH^? negative ion chemical ionization mass spectrometric conditions. Characteristic fragmentations are observed that include production of [M ? H ? 2]^?, [M ? H ? 18]^? and [M ? H ? 32]^? ions. Certain other fragment ions in the collisionally activated mass spectra make it possible to distinguish among structural isomers. In polyhydroxy compounds, fragmentation increases as the number of hydroxyl groups increase, and carbon-carbon bond cleavage becomes favored.     

FAB and chemical ionization in the production of [M+ H]+ and [M− H]− species from 2-aryl-2-methyl-1,3-dithianes

The unimolecular fragmentations of [M + H]⁺ and [M – H]^? ions from four 2-aryl-2-methyl-1,3-dithianes are described and clarified with the aid of deuterated derivatives. Comparison of the MIKE spectra of [M + H]⁺ species obtained under chemical ionization and fast atom bombardment (FAB) conditions reveals differences which are attributed to the different energetics involved in the two ionization processes. It is suggested that FAB is a ‘softer’ ionization technique but, at the same time, it provides, for the possibility of solvation, reaction sites not available in gas-phase protonation. [M – H]^? species and anionic fragments thereof were generally not obtained under FAB(?) conditions. [M – H]^? ions are readily produced in gas-phase reactions with OH^? via proton abstraction from C(4) or C(5), and from the 2-methyl substituent; and they fragment according to several reaction pathways.     

Structural determination of oxofatty acids by charge-remote fragmentations

A strategy is described to locate the carbonyl position in oxofatty acids by utilizing charge-remote fragmentations of various molecular ions that are desorbed by fast atom bombardment (FAB). Oxofatty acids were cationized with alkali metal ions (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) to form [M+2Met?H]⁺ or alkaline earth metal ions (Mg²⁺, Ca²⁺, Sr²⁺ or Ba²⁺) to form [M+Met?H]⁺ in the gas phase. The cationized acids undergo charge-remote fragmentations upon high-energy activation, giving a product-ion pattern that has a gap corresponding to the oxo position and bordered by two high-intensity peaks. One of the peaks corresponds to an ion that is formed by the cleavage of the C-C bond β to the oxo position and proximal to the charge (β ion), whereas the other is formed from the cleavage of the C-C bond γ to the oxo position and distal to the charge (γ′ ion). The oxo position is easily determined by identifying the gap and the β and γ′ ions. Furthermore, there are two competing patterns of fragments in a CAD spectrum of an oxofatty acid or ester [M+Li]⁺ ion. These arise because Li⁺ attaches to either the oxo or the carboxylic end, as was confirmed by ab initio molecular orbital calculations. The results demonstrate that control of the fragmentation can be guided by an understanding of metal-ion affinities. Collisional activation of the anionic carboxylates gives results that are similar to those for positive ions, showing that the process is not related to the charge status. Collisional activation of [M+H]⁺ ions does not give structural information because the charge migrates, leading to charge-mediated fragmentations.     

Electron impact,chemical ionization and negative chemical ionization mass spectra,and mass-analysed ion kinetic energy spectrometry–collision-induced dissociation fragmentation pathways of some deuterated 2,4,6-trinitrotoluene (TNT) derivatives

Mass-analysed ion kinetic energy spectrometry (MIKES) with collision-induced dissociation (CID) has been used to study the fragmentation processes of a series of deuterated 2,4,6-trinitrotoluene (TNT) and deuterated 2,4,6-trinitrobenzylchloride (TNTCI) derivatives. Typical fragment ions observed in both groups were due to loss of OR′ (R′ = H or D) and NO. In TNT, additional fragment ibns are due to the loss of R₂′O and 3NO₂, whilst in TNTCI fragment ions are formed by the loss of OCI and R₂′OCI. The TNTCI derivatives did not produce molecular ions. In chemical ionization (Cl) of both groups. MH⁺ ions were observed, with [M – OR′]⁺ fragments in TNT and [M – OCI]⁺ fragments in TNTCI. In negative chemical ionization (NCI) TNT derivatives produced M^?′, [M–R′]^?, [M–OR′]^? and [M–NO]^? ions, while TNTCI derivatives produced [M–R]^?, [M–Cl]^? and [M – NO₂]^? fragment ions without a molecular ion.     

Structural characterization of isomeric triazolocoumarins by high- and low-energy collision spectrometry of M+˙, [M + H]+ and [M − H]− species

Two monometayl- and four dimethyl-triazolocoumarin isomers were characterized by their electron impact mass spectra and by low-energy collision experiments performed on molecular ions M⁺˙ and other fragment ions with an ion-trap mass spectrometer. High-energy collision-activated dissociation measurements were performed on the protonated [M + H]⁺ and deprotonated [M ? H]^? molecular ion obtained by fast atom bombardment and M⁺˙ species produced by electron impact ionization on a double-focusing, reverse-geometry instrument. The data obtained allowed unequivocal structural identification of all the compounds investigated.     

Mass spectra of aliphatic dicarboxylic acids and their dimethyl esters: Cyclic structures for the [M − H2O]+˙ ions from the diacids and [M − MeOH]+˙ ions from the dimethyl esters

Methyl 2-oxocycIoalkane carboxylate structures are proposed lor the [M ? MeOH]^+˙ ions from dimethyl adipate, pimelate, suberate and azelate. This proposal is based on a comparison of the metastable ion mass spectra and the kinetic energy releases for the major fragmentation reaction of these species with the same data for the molecular ions of authentic cyclic β-keto esters. The mass spectra of α,α,α′,α′-d₄-pimelic acid and its dimethyl ester indicate that the α-hydrogens are involved only to a minor extent in the formation of [M ? ROH]^+˙ and [M ? 2ROH]^+˙ ions, while these α-hydrogens are involved almost exclusively in the loss of ROH from the [M ? RO˙]⁺ ions (R = H or CH₃). The molecules XCO(CH₂)₇COOMe (X = OH, Cl) form abundant ions in their mass spectra with the same structure as the [M ? 2MeOH]^+˙ ions from dimethyl azelate.     

Competition in the formation of ion–neutral complexes. Expulsion of methane from the molecular ion of acetamide

An ion–neutral complex is a non-covalently bonded aggregate of an ion with one or more neutral molecules in which at least one of the partners rotates freely (or nearly so) in all directions. A density-of-states model is described, which calculates the proportion of ion–neutral complex formation that ought to accompany simple bond cleavages of molecular ions. Application of this model to the published mass spectrum of acetamide predicts the occurrence of ions that have not hitherto been reported. Relative intensities on the order of 0.1 (where the abundance of the most intense fragment ion = 1) ere predicted for [M – HO]⁺ and [M – CH₄]⁺˙ ions, which have the same nominal masses as the prominent [M – NH₃]⁺˙ and [M – NH₂]⁺ fragments. High-resolution mass spectrometric experiments confirm the presence of the predicted fragment ions. The [M – HO]⁺ and [M – CH₄]⁺˙ fragments were observed with relative abundances of 0.02 and 0.04, respectively. Differences between theory and experiment may be ascribed to effects of competing distonic ion pathways.     

The unusual fragmentation of long‐chain feruloyl esters under negative ion electrospray conditions

Long‐chain ferulic acid esters, such as eicosyl ferulate ( 1 ), show a complex and analytically valuable fragmentation behavior under negative ion electrospay collision‐induced dissociation ((?)‐ESI‐CID) mass spectrometry, as studied by use of a high‐resolution (Orbitrap) mass spectrometer. In a strong contrast to the very simple fragmentation of the [M + H]⁺ ion, which is discussed briefly, the deprotonated molecule, [M – H]^?, exhibits a rich secondary fragmentation chemistry. It first loses a methyl radical (MS²) and the ortho‐quinoid [M – H – Me]^‐? radical anion thus formed then dissociates by loss of an extended series of neutral radicals, C_nH_2n + 1^? (n = 0–16) from the long alkyl chain, in competition with the expulsion of CO and CO₂ (MS³). The further fragmentation (MS⁴) of the [M – H – Me – C₃H₇]^? ion, discussed as an example, and the highly specific losses of alkyl radicals from the [M – H – Me – CO]^‐? and [M – H – Me – CO₂]^‐? ions provide some mechanistic and structural insights.     

[C]+· and [CH]+ from doubly charged ions formed from malononitrile

The metastable ions [M]²⁺, [M – H]²⁺· and [M – H₂]²⁺ from malononitrile fragment by loss of [CH]⁺, [C]⁺· and [C]⁺·, respectively. The reaction of the molecular ion involves the methylene and nitrile carbon atoms in the statistical probability ratio, while that of [M – H]²⁺· involves exclusively the nitrile carbon and that of [M ? H₂]²⁺ involves an approximately equal contribution, from both sources. It is suggested that the metastable molecular ion fragments through a bipyrimidal intermediate.     

Laser-Induced Hydrogen Radical Removal in UV MALDI-MS Allows for the Differentiation of Flavonoid Monoglycoside Isomers

Negative-ion matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectra and tandem mass spectra of flavonoid mono-O-glycosides showed the irregular signals that were 1 and/or 2 Da smaller than the parent deprotonated molecules ([M – H]^–) and the sugar-unit lost fragment ions ([M – Sugar – H]^–). The 1 and/or 2 Da mass shifts are generated with the removing of a neutral hydrogen radical (H*), and/or with the homolytic cleavage of the glycosidic bond, such as [M – H* – H]^–, [M – Sugar – H* – H]^–, and [M – Sugar – 2H* – H]^–. It was revealed that the hydrogen radical removes from the phenolic hydroxy groups on the flavonoids, not from the sugar moiety, because the flavonoid backbones themselves absorb the laser. The glycosyl positions depend on the extent of the hydrogen radical removals and that of the homolytic cleavage of the glycosidic bonds. Flavonoid mono-glycoside isomers were distinguished according to their TOF MS and tandem mass spectra.

The fragmentation of 5-phenyl-1,4-benzodiazepin-2-ones. Mechanism for the formation of [M ? H]+

The fragmentations under electron impact of 5-phenyl-1,4-benzodiazepin-2-ones are investigated with the aid of high resolution, metastable decompositions and deuterium labeling. Based on our data a mechanism for the formation of the [M – H]⁺ ion is proposed. It is shown that the [M – CHO]⁺ ion is probably formed by two different pathways. Data on two minor fragment ions give support to the structure proposed for the [M – CHO]⁺ ion.     

Estimation of peptide N–Cα bond cleavage efficiency during MALDI‐ISD using a cyclic peptide

Matrix‐assisted laser desorption/ionization in‐source decay (MALDI‐ISD) induces N–C_α bond cleavage via hydrogen transfer from the matrix to the peptide backbone, which produces a c′/z? fragment pair. Subsequently, the z? generates z′ and [z + matrix] fragments via further radical reactions because of the low stability of the z?. In the present study, we investigated MALDI‐ISD of a cyclic peptide. The N–C_α bond cleavage in the cyclic peptide by MALDI‐ISD produced the hydrogen‐abundant peptide radical [M + 2H]⁺? with a radical site on the α‐carbon atom, which then reacted with the matrix to give [M + 3H]⁺ and [M + H + matrix]⁺. For 1,5‐diaminonaphthalene (1,5‐DAN) adducts with z fragments, post‐source decay of [M + H + 1,5‐DAN]⁺ generated from the cyclic peptide showed predominant loss of an amino acid with 1,5‐DAN. Additionally, MALDI‐ISD with Fourier transform‐ion cyclotron resonance mass spectrometry allowed for the detection of both [M + 3H]⁺ and [M + H]⁺ with two ¹³C atoms. These results strongly suggested that [M + 3H]⁺ and [M + H + 1,5‐DAN]⁺ were formed by N–C_α bond cleavage with further radical reactions. As a consequence, the cleavage efficiency of the N–C_α bond during MALDI‐ISD could be estimated by the ratio of the intensity of [M + H]⁺ and [M + 3H]⁺ in the Fourier transform‐ion cyclotron resonance spectrum. Because the reduction efficiency of a matrix for the cyclic peptide cyclo(Arg‐Gly‐Asp‐D‐Phe‐Val) was correlated to its tendency to cleave the N–C_α bond in linear peptides, the present method could allow the evaluation of the efficiency of N–C_α bond cleavage for MALDI matrix development. Copyright © 2016 John Wiley & Sons, Ltd.     

Collisional activation of ions formed by [Li]+ ion attachment

Since no unimolecular fragmentation is observed with [M+Li]⁺ ions under normal operating conditions the collisional activation method was used to study the fragmentation behaviour of these ions. It was found that the liberation of the [Li]⁺ ion is a dominant process only with smaller molecules. In addition, direct bond cleavages and new types of rearrangement reactions lead to fragment ions in which the lithium is normally retained. The decomposition behaviour of [M+Li]⁺ ions represents an intermediate case between that of [M]+ ions and excited neutral molecules and is quite different from that of [M+H]⁺ ions.     

Influence of reversed- and normal-phase liquid chromatographic eluents in the fragmentation of selected chlorinated herbicides in thermospray liquid chromatography-mass spectrometry

The effect of two completely different mobile phase compositions, reversed-phase acetonitrile-water + ammonium acetate and normal-phase cyclohexane, were compared in filament-on thermospray liquid chromatography-mass spectrometry (LC-MS) for the determination of selected chlorinated herbicides such as chloroatrazines and chlorinated phenoxyacetic acids. By using acetonitrile-water + 0.05 M ammonium acetate mixtures in positive ion mode thermospray LC-MS, the chloroatrazine herbicides showed the acetonitrile adduct ion [M + (CH₃CN)H]⁺ as the base peak, whereas the chlorinated phenoxyacetic acids showed no signal. In contrast, when cyclohexane, which is reported for the first time as an eluent in the thermospray technique, was used as the mobile phase the chlorinated phenoxyacetic acid herbicides exhibited [M – H]⁺, [M – Cl]⁺ and M⁺˙ as the main ions. Negative ion mode thermospray LC-MS showed [M – H]^? as the base peak for the chloroatrazines in the different mobile phases, whereas the chlorinated phenoxyacetic acids exhibited [M + H]^?, [M + Cl]^? or [M – HCl]^? as the base peaks in cyclohexane and [M + acetate]^? in acetonitrile-water-ammonium acetate.     

The unexpected formation of [M − H]+ species during MALDI and dopant‐free APPI MS analysis of novel antineoplastic curcumin analogues

Unusual ionization behavior was observed with novel antineoplastic curcumin analogues during the positive ion mode of matrix‐assisted laser desorption ionization (MALDI) and dopant‐free atmospheric pressure photoionization (APPI). The tested compounds produced an unusual significant peak designated as [M ? H]⁺ ion along with the expected [M + H]⁺ species. In contrast, electrospray ionization, atmospheric pressure chemical ionization and the dopant‐mediated APPI (dopant‐APPI) showed only the expected [M + H]⁺ peak. The [M ? H]⁺ ion was detected with all evaluated curcumin analogues including phosphoramidates, secondary amines, amides and mixed amines/amides. Our experiments revealed that photon energy triggers the ionization of the curcumin analogues even in the absence of any ionization enhancer such as matrix, solvent or dopant. The possible mechanisms for the formation of both [M ? H]⁺ and [M + H]⁺ ions are discussed in this paper. In particular, three proposed mechanisms for the formation of [M ? H]⁺ were evaluated. The first mechanism involves the loss of H₂ from the protonated [M + H]⁺ species. The other two mechanisms include hydrogen transfer from the analyte radical cation or hydride abstraction from the neutral analyte molecule. Copyright © 2014 John Wiley & Sons, Ltd.     

Negative ion mass spectra of dicarboxylic acids

The negative ion mass spectra of dicarboxylic acids show [M]^?˙ and prominent [M – H]^?ions. These ions can therefore be used to determine the molecular weight of dicarboxylic acids which do not give positive molecular ions. The [C₂H₃]^? ion is a base peak in the spectra of maleic and fumaric acids. Isomeric phthalic acids are readily differentiated.     

A mechanism for the loss of 60 u from peptides containing an arginine residue at the C-terminus

The loss of 60 u from protonated peptide ions containing an arginine residue at the C-terminus has been investigated by means of low energy tandem mass spectrometry. The lowest energy conformation of singly charged bradykinin is thought to involve a salt-bridge structure, which may lead to the formation of two isomeric forms. It is thought that one isomer retains the ionizing proton at the C-terminal end of the peptide, leading to the formation of the [b _n?1 + H + OH]⁺ fragment ion, and the other isomer retains the charge at the N-terminus, leading to the formation of the [M + H ? 60]⁺ fragment ion. It was found that the formation of the [M + H ? 60]⁺ ion occurs only from singly charged precursor ions. In addition, the loss of 60 u occurs from peptides in which the charge is localized at the N-terminus. These results indicate that the mechanism of formation of the [M + H ? 60]⁺ ion may be driven by a charge-remote process.