Formation of M+. ions under matrix fast atom bombardment conditions: Does charge exchange reaction occur?

The formation of molecular ions, M^+., under fast atom bombardment (FAB) conditions using a liquid matrix was examined by using a new type of synthesized compounds in which preferential M^+. peaks appear in their FAB spectra. The FAB spectra were compared with the corresponding mass spectra obtained by the electron impact (EI) ionization, chemical ionization (CI) and charge-exchange ionization (CEI) methods. All of the spectra showed preferential peaks of M^+. ion and a characteristic intense fragment ion peak originating from a β-fission. The FAB spectra were similar in the fragment ions appearing in the EI spectra and were very similar in the fragmentation pattern to the CEI spectra using Ar^+. and Xe^+. as the reagent ions. Further, the FAB spectra did not show any doubly charged ion peaks, while the 70 eV EI spectra showed the peaks of doubly charged molecular and/or fragment ions. The isobutane CI spectra of the synthesized compounds suggested that the formation of M^+. ions occurred through the CE reaction with isobutane ion, C₄H₁₀^+., and the CI spectra showed a marked intense fragment ion peak originating from the β-fission which seemed to occur characteristically in CEI processes. The results obtained suggested that the formation of M^+. ions under matrix FAB conditions occurred mainly by CE reactions between the analytes M and matrix molecular ions B^+. and/or fragment ions b^+..     

Reactive collisions in quadrupole cells. 3: H/D exchange reactions of protonated aromatic amines with ND3

The H/D exchange reactions of a variety of protonated aromatic amines with ND₃ m the collision cell of a hybrid BEqQ tandem mass spectrometer have been studied. The MH⁺ ions were prepared by CH₄, t-C₄H₁₀, and NH₃ chemical ionization (CI) and, for some amines, by fast-atom bombardment (FAB). Evidence is presented that the kinetic energy of the incident ion as well as its internal energy must be dissipated by nonexchanging collisions before exchange occurs, once deactivated the MH⁺ ions exchange efficiently, which leads, in most cases, to [MHJ⁺ d _x ions m which all active hydrogens have been exchanged. The MH⁺ ion of 1,3-phenylenediamine formed by gas-phase CI exchanges only very slightly with ND₃ whereas a significant fraction of the MH⁺ ions formed by FAB exchange efficiently. This difference is rationalized in terms of dominant formation of the ring-protonated species in gas-phase CI reactions and significant formation of the N-protonated species by FAB with only the N-protonated species exchanging efficiently. Similar, although less pronounced, differences are observed for the MH⁺ ion of m-anisidine. In a number of cases apparent exchange of aromatic hydrogens also is observed. Evidence is presented for the interchange of ring and amine hydrogens in protonated aromatic amines and it is suggested that only the N-protonated species undergoes significant exchange with ND₃.     

Injection of reagent ions into the selvedge region in fast-atom bombardment mass spectrometry

A divided probe that incorporates a potassium aluminosilicate glass target and an analyte/glycerol matrix target, spatially separated, was used to inject potassium ions (K⁺) into the high-pressure “selvedge” region formed above the analyte/glycerol matrix target during fast-atom bombardment (FAB); [M+K]⁺ adduct ions that represent the types of gas-phase neutral molecules present in the selvedge region are observed. Computer modeling assisted in designing the divided target and an additional ion optical element for the FAB ion source to optimize interactions between K⁺ ions and the desorbed neutral molecules. The capability of injecting K⁺ ions into the FAB experiment has utility in both mechanistic studies and analyses. Experimental results here are consistent with a model for the desorption/ionization processes in FAB in which some types of neutral analyte molecules are desorbed intact and are subsequently protonated by glycerol chemical ionization. Unstable protonated molecules undergo unimolecular decomposition to yield observed fragment ions. The use of K⁺ cationization of analytes for molecular weight confirmation is demonstrated, as well as its utility in FAB experiments in which mixtures are encountered.     

Fragmentation of ion–molecule adducts of transition metal ions with aromatic ring-containing nitriles in the gas phase

Gas-phase ion–molecule reactions of transition metal ions, M⁺ (M⁺ = Ni⁺, Co⁺, Fe⁺ and Mn⁺), with six aromatic ring-containing nitriles were investigated in a modified fast atom bombardment (FAB) source. It is shown that the monoadduct, (Ph(CH₂)_nCN)–M⁺, is one of the most abundant ion–molecule reaction products. The main fragments in the FAB source are the [C₇H₇]⁺ and [C₈H₉]⁺ ions, and their formation is shown to involve metal ion insertion into the nitriles rather than direct bond cleavage from the ‘free’ or complexed nitriles after FAB ionization. An intramolecular oxidation–reduction reaction, giving [C₇H₇]⁺, is found in the metastable and collisionally induced dissociations of benzyl nitrile adducts accompanied by neutral MCN formation, but not seen for longer chain samples. An ortho effect is observed in the elimination of HCN from the 2-methylbenzyl nitrile adduct ions. This reaction dominates the metastable ion spectrum of the adduct of Mn⁺, whereas metal detachment is nearly the major process for the other complexes of Mn⁺. The different bond-insertion selectivities of the metal ions are also shown.     

Analysis of long-chain fatty acyl coenzyme a thioesters by negative ion fast-atom bombardment mass spectrometry and tandem mass spectrometry

Long-chain acyl Coenzyme A (CoA) is essentially composed of three major chemical groups, fatty acyl-, phosphopantetheino-, and 3′, 5′,-adenosine diphospho-moieties. The negative ion fast-atom bombardment mass spectrometry spectra of long-chain acyl CoA thioesters were characterized by the formation of abundant [M ? H]^? and two distinct classes of fragment ions, one class which retained the acyl group and another class which is related to CoA that contains the phosphopantethene and adenine. The ions which retained the acyl group in the spectrum of palmitoyl CoA appeared at m/z 675, 657, 595, and 577 and were found to decompose by loss of alkylketene observed at m/z 357 and 339. Those ions which retained the adenine group were observed at m/z 426 and 408. In contrast to these ions observed following fast-atom bombardment ionization, tandem mass spectrometry of the [M ? H]^?, from palmitoyl CoA (m/z 1004), yielded the adenine-containing ions as major products and the acyl-containing ions were of low abundance or not detected. These results suggested that the formation of many characteristic ions observed in direct FAB analysis occurred during the desorption process. The unique relationship between ions which involved the transition from acyl-containing ions to only CoA-containing ions by the loss of alkylketene allowed the development of tandem mass spectrometry protocols for the analysis of acyl CoA mixtures. Precursor scans of either m/z 357 or 339 yielded the identification of each species in a complex mixture. Identification of specific species was obtained with a neutral loss scan of the mass for a specific alkylketene.     

Determination of enantiomeric excess for organic primary amine compounds by chiral recognition fast-atom bombardment mass spectrometry

Enantiomeric excess (ee) of organic primary amine compounds such as phenylglycine methyl ester hydrochloride (2) has been determined by fast-atom bombardment (FAB) mass spectrometry (NBA matrix). Chiral recognition in host–guest complexation systems between crown ethers [H] and amino acid ester ammonium ions [G] has been extended to the ee determination. The method characteristically uses a 1/1 mixture of a pair of enantiomeric hosts whose enantiomer is isotopically labeled [(RRRR)-1 and (SSSS)-1-d₆]. Chiral recognition of a given guest is simply measured with the given host–pair reagent from the relative peak intensities of the two corresponding diastereomeric host–guest complex ions in I[(H_RRRR · G)⁺]/I[(H_SSSS-d6 · G)⁺ = I_R/I_S-d6, so called IRIS value. The IRIS value varies in a linear fashion with the ee quantitiy of 2 and produces a symmetric linear V-shaped plot, indicating that in the case of a primary amine guest (such as 2) with unknown ee, one can determine the ee by this type of chiral recognition FAB mass spectometry. Further, based on the observed concentration effects on the IRIS values, it is suggested that the present IRIS value reflects the concentration ratio of the diastereomeric complex ions formed in the matrix.     

Mass-spectrometric study of the formation of silver nanoclusters in polyether media: 2. Fast atom bombardment and modeling

Within the problem of the synthesis of silver nanoclusters and nanoparticles in polyether media, systems containing silver nitrate AgNO₃ and low-molecular-weight polyethers, poly(ethylene glycol) PEG-400 or oxyethylated glycerol OEG-5, were studied by fast atom bombardment (FAB) mass spectrometry. The formation of stable clusters of polyether oligomers (M _m ) with silver cations M _m · Ag⁺ was shown, in agreement with the previous data of laser desorption/ionization. Quantum-chemical DFT calculations have shown that the M _m · Ag⁺ clusters are stabilized by wrapping of the polyether chain around the silver cation with the cation coordinating ether oxygen atoms. Silver nanoclusters were not found in the FAB mass spectra of liquid systems, but Ag _n ⁺ clusters were detected for silver nanoparticles separated from the reaction medium. No products of chemical transformations of PEG-400 or OEG-5 were observed by FAB. A plausible mechanism of the reduction of silver cations involving nitrate anions is discussed.     

Matrix effects on the fast atom bombardment mass spectra of palladium complexes

In an attempt to identify and characterize the intermediates involved in multi-step palladium catalyzed coupling reactions, under fast atom bombardment (FAB) conditions, the selection of a matrix to enhance ion formation without degrading the palladium complex is critical. In general, it is observed that the analysis of tetrakis(triphenylphosphine)palladiurn(0) using glycerol, m-nitrobenzyl alcohol or o-nitrophenyl octyl ether as the FAB matrix compound produces several ions which correspond to complexes of the type Pd_xL_yO_z⁺ (L = PPh₃; x = 1-4; y = 1-4; z = 0-2), in addition to clusters containing one or more matrix molecules. FAB mass spectra generated using triethanolamine- or tetramethylene sulfone (sulfolane) are observed to contain ions related to the palladium(0) complex with little or no interference from ions related to the oxidation or reduction processes.     

Benzene as a selective chemical ionization reagent gas

Dilute mixtures of C₆H₆ or C₆D₆ in He provide abundant [C₆H₆]^+· or [C₆D₆]^+· ions and small amounts of [C₆H₇]⁺ or [C₆D₇]⁺ ions as chemical ionization (CI) reagent ions. The C₆H₆ or C₆D₆ CI spectra of alkylbenzenes and alkylanilines contain predominantly M^+· ions from reactions of [C₆H₆]^+· or [C₆D₆]^+· and small amounts of MH⁺ or MD⁺ ions from reactions of [C₆H₇]⁺ or [C₆D₇]⁺. Benzene CI spectra of aliphatic amines contain M^+·, fragment ions and sample-size-dependent MH⁺ ions from sample ion-sample molecules reactions. The C₆D₆ CI spectra of substituted pyridines contain M^+· and MD⁺ ions in different ratios depending on the substituent (which alters the ionization energy of the substituted pyridine), as well as sample-size-dependent MH⁺ ions from sample ion-sample molecule reactions. Two mechanisms are observed for the formation of MD⁺ ions: proton transfer from [C₆D₆]^+· or charge transfer from [C₆D₆]^+· to give M^+·, followed by deuteron transfer from C₆D₆ to M^+·. The mechanisms of reactions were established by ion cyclotron resonance (ICR) experiments. Proton transfer from [C₆H₆]^+· or [C₆D₆]^+· is rapid only for compounds for which proton transfer is exothermic and charge transfer is endothermic. For compounds for which both charge transfer and proton transfer are exothermic, charge transfer is the almost exclusive reaction.     

Infrared Spectra of Protonated Coronene and Its Neutral Counterpart in Solid Parahydrogen: Implications for Unidentified Interstellar Infrared Emission Bands

Large protonated polycyclic aromatic hydrocarbons (H⁺PAHs) are possible carriers of unidentified infrared (UIR) emission bands from interstellar objects, but the characterization of infrared (IR) spectra of large H⁺PAHs in the laboratory is challenging. IR absorption spectra of protonated coronene (1‐C₂₄H₁₃⁺) and mono‐hydrogenated coronene (1‐C₂₄H₁₃^.), which were produced upon electron bombardment of parahydrogen containing a small proportion of coronene (C₂₄H₁₂) during matrix deposition, were recorded. The spectra are of a much higher resolution than those obtained by IR multiphoton dissociation by Dopfer and co‐workers. The IR spectra of protonated pyrene and coronene collectively appear to have the required chromophores for features of the UIR bands, and the spectral shifts on an increase in the number of benzenoid rings point in the correct direction towards the positions of the UIR bands. Larger protonated peri‐condensed PAHs might thus be key species among the carriers of UIR bands.     

Competitive formation of M+˙ and [M + H]+ ions under fast atom bombardment conditions

The competitive formation of molecular ions M^+˙ and protonated molecules [M + H]⁺ under fast atom bombardment (FAB) conditions was examined using various kinds of organic compounds. The use of protic/hydrophilic matrices such as thioglycerol and glycerol resulted in relatively large values of the peak intensity ratio I([M + H]⁺)/I(M^+˙) compared with the use of relatively aprotic/hydrophobic matrices such as m-nitrobenzyl alcohol and o-nitrophenyl octyl ether. The change of matrix from thiol-containing such as thioglycerol and dithiothreitol to alcoholic such as glycerol and pentamethylene glycol increased the I([M + H]⁺)/I(M^+˙) ratio. Furthermore, the change of matrix increased the peak intensity ratio of the doubly charged ion [M + 2H]²⁺ to [M + H]⁺ in the FAB mass spectra of angiotensin I and gramicidin S. The addition of acids to the matrix solution increased the I([M + H]⁺)/I(M^+˙) ratio, although such an effect did not always occur. The acetylation of simple aniline compounds markedly increased the I([M + H]⁺)/I(M^+˙) ratio. It was concluded from these results that the hydrogen bonding interaction between hydroxyl groups(s) of the matrix and basic site(s) of analyte molecules in solution acts advantageously as a quasi-preformed state for [M + H]⁺ formation, and that the presence of significant proton acceptor(s) such as carbonyl group in analytes hinder the M^+˙ formation which may generally occur under FAB conditions. The formation of M^+˙ and [M + H]⁺ ions seemed to occur competitively, reflecting or according to the interaction or solvation states between the analyte and matrix molecules in solution and the structural characteristics of the analytes.     

Study of Host-Guest Complexation of Alkaline Earth Metal Ion,Aluminum Ion and Transition Metal Ions with Crown Ethers by Fast Atom Bombardment Mass Spectrometry

Complexations of crown ethers with alkali metal ions have been investigated extensively by FAB mass spectrometry over the past decade, but very little attention has been paid to reactions of crown ethers with other classes of metal ions such as alkaline earth metal ions, transition metal ions and aluminum ions. Although fast atom bombardment ionization mass spectrometry has proven to be a rapid and convenient method to determine the binding interactions of crown ethers with metal ions, problems in reliabilities for quantitative measurements of” binding strength for the host-guest complexes have been described in the literature. Thus, in this paper, applications of FAB/MS for investigating the complexation of crown ethers with various classes of metal ions is discussed. Extensive fragmentations for neutral losses such as C₂H₄O or C₂H₄ molecules from the host-guest complexes could be observed. The reason is attributed to the energetic bombardment processes of FAB occuring in the formation of these complexes. Complexes of cyclen with metal ions also show neutral losses of C₂H₄NH molecules leading to fragment ions. Transition metal ions usually form (Crown + MCl)⁺ type of ions, alkaline earth metal ions can form both (Crown + MCl)⁺ and (Crown + MOH)⁺ type of ions. But for aluminum ions, only (Crown + Al(OH)₂)⁺ type of ions could he observed.     

Production of doubly charged clusters (H2O)n · Ba2+ and (H2O)n · Ca2+ under low temperature fast atom bombardment conditions

Production of doubly charged ions of alkaline earth metals Ba²⁺ and Ca²⁺ and their doubly charged clusters with water molecules (H₂O)_n · Ba²⁺, (H₂O)_n · Ca²⁺ (n = 1, 2, 3) by means of low temperature fast atom bombardment technique is observed in the case of crystalline hydrates of BaCl₂ and CaCl₂ salts, formed during freezing of water-salt solutions. Reasons for a possibility of production of the doubly charged species in the case of the two indicated salts and their absence in the case of chlorides of some other divalent metals (Mg, Mn, Co, Cu, Zn) are discussed. As to singly charged secondary ions Me⁺, MeCl⁺, MeOH⁺, [(H₂O)_n · MeCl]⁺, [(H₂O)_n · MeOH]⁺ (where Me is metal), high efficiency of their production from crystalline hydrates was observed and possible explanation of the phenomenon is suggested.     

Odd-electron molecular ion and loss of toluene in fast atom bombardment mass spectra of some carotenoids

Positive-ion fast atom bombardment (FAB) and B/E linked scan FAB mass spectra of seven carotenoids are reported. In all cases the M ^+· ions are observed, and the [M + H]⁺ ions are absent in the hydrocarbons and weak in the oxygenated compounds. The usefulness of B/E linked scan FAB mass spectra to distinguish isomers and to attribute the loss of toluene from the M ^+· to an ionic fragmentation and not to a thermal process is discussed.     

Structures of decomposing and non-decomposing gas-phase [C4H6O]+· radical cations,and their [C2H2O]+· and [C3H6]+· product ions

The structures of gas-phase [C₄H₆O]^+· radical cations and their daughter ions of composition [C₂H₂O]^+· and [C₃H₆]^+· were investigated by using collisionally activated dissociation, metastable ion measurement, kinetic energy release and collisional ionization tandem mass spectrometric techniques. Electron ionization (70 eV) of ethoxyacetylene, methyl vinyl ketone, crotonaldehyde and 1-methoxyallene yields stable [C₄H₆O]^+· ions, whereas the cyclic C₄H₆O compounds undergo ring opening to stable distonic ions. The structures of [C₂H₃O]^+· ions produced by 70-eV ionization of several C₄H₆O compounds are identical with that of the ketene radical cation. The [C₃H₆]^+· ions generated from crotonaldehyde, methacrylaldehyde, and cyclopropanecarboxaldehyde have structures similar to that of the propene radical cations, whereas those ions generated from the remainder of the [C₄H₆O]^+· ions studied here produced a mixed population of cyclopropane and propene radical cations.     

Fast-atom bombardment tandem mass spectrometry of [13C]arachidonic acid labeled phospholipid molecular species

A method employing stable isotope labeling and fast-atom bombardment (FAB) tandem mass spectrometry has been developed to directly assess events of biosynthesis and metabolism of arachidonic acid containing phospholipid molecular species by cells carried in culture. Mast cells, cultured with [¹³C]linoleic acid, converted this precursor into arachidonic acid which was then incorporated into cellular phospholipids. Over a 24 hour period, the extent of label enrichment in each arachidonate-containing phospholipid molecular species was monitored by using negative FAB ionization with selected reaction monitoring. Specific incorporation of [¹³C₁₇] labeled arachidonate was determined from the ratio of the carboxylate anions at m/z 320 and 303, which correspond to [¹³C₁₇]arachidonate and unlabeled arachidonate, respectively, produced by collision-induced dissociation of each specific molecular anion. The use of [¹³C]linoleic acid as a precursor of arachidonic acid avoids the problem of changing the endogenous pool size by directly adding labeled arachidonic acid. Measurement of the [¹³C₁₇]label also avoids interferences from endogenous isobaric fatty acids that are naturally present at low levels.     

Ion-molecule reactions observed for nitroaromatic compounds in negative ion fast atom bombardment mass spectrometry

Analyses of a series of nitroaromatic compounds using fast atom bombardment (FAB) mass spectrometry are discussed. An interesting ion-molecule reaction leading to [M + O ? H]^? ions is observed in the negative ion FAB spectra. Evidence from linked-scan and collision-induced dissociation spectra proved that [M + O ? H]^? ions are produced by the following reaction: M + NO₂^? → [M + NO₂]^? → [M + O ? H]^?. These experiments also showed that M^?˙ ions are produced in part by the exchange of an electron between M and NO₂^? species. All samples showed M^?˙, [M ? H]^? or both ions in their negative ion FAB spectra. Not all analytes studied showed either [M + H]⁺ and/or M⁺˙ in the positive ion FAB spectra. No M⁺˙ ions were observed for ions having ionization energies above ～9 eV.     

Electron ionization induced metastable decompositions of isomeric trimethylsilylmethanol, (CH3)3SiCH2OH, and methoxytrimethylsilane, (CH3)3SiOCH3

The metastable decompositions of trimethylsilylmethanol, (CH₃)₃SiCH₂OH (MW: 104, 1) and methoxytrimethylsilane, (CH₃)₃SiOCH₃ (MW: 104, 2) upon electron ionization have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectroscopy and D labeling. The metastable ions of 1 ^·+ decompose to give the fragment ions m/z 89 (CH ₃ ^· loss) and 73 (^·CH₂OH loss), whereas those of 2 ^·+ only yield the fragment ion m/z 89 (CH ₃ ^· loss). The latter fragment ion is generated by loss of a methyl radical from the trimethylsilyl group via a simple cleavage reaction as shown by D labeling. However, the fragment ions m/z 89 and 73 from 1 ^·+ are generated following an almost statistical exchange of the original methyl and methylene hydrogen atoms in the molecular ion as shown also by D labeling. This exchange indicates a complex rearrangement of the molecular ion of 1 ^·+ prior to metastable decomposition for which as key step a 1,2-trimethylsilyl group migration from carbon to oxygen is suggested. A different behavior is also found between the source-generated m/z 89 ions from 1 ^·+ which decompose in the metastable time region to give ions m/z 61 by loss of ethylene and those from 2 ^·+ which decompose in the metastable region to yield ions m/z 59 by elimination of formaldehyde.     

Complexes of iron(II) with cysteine-containing peptides in the gas phase

Gas-phase interactions of peptides that contain cysteine with iron(II) atoms were examined by using fast-atom bombardment and tandem mass spectrometry. Specific and strong interactions of iron and sulfur from the thiol group of the cysteine side chain occur in the gas phase and are the basis for highly specific fragmentation to give abundant [a _n ?⁺ ions. For peptides that contain two cysteines, an internal ion, which results from the interaction of Fe and both thiol groups, is formed upon collisional activation. The mechanism for the formation of [a _n ?2H+Fe]⁺ fragment ions requires the metal to be coordinated at sulfur in close proximity to the site of reaction. Iron-bis(pentapeptide) complexes, which form under the same conditions, decompose predominantly to lose a pentapeptide molecule and, to a lesser extent, to give [a _a ?2H+Fe]⁺ ions.     

Electron impact ionization and dissociation of neutral and charged fullerenes

Three different types of electron impact ionization experiments have been performed, involving neutral and charged C₆₀ and C₇₀. 1) We have determined absolute partial ionization cross sections for formation of parent ions C ₆₀ ^z+ and C ₇₀ ^z+ in charge states up to z = 4, and of singly and multiply charged fragments of size n ≥ 44 and n ≥ 50 from C₆₀ and C₇₀ neutral precursors, respectively. 2) Previous appearance energy measurements of C₇₀ have been improved and extended to z = 5; ionization energies are found to depend linearly on the charge state of the precursor, in agreement with theoretical predictions. 3) A beam of mass selected C ₆₀ ²⁺ has been crossed with an intense electron beam; the induced reactions (fragmentation, post- ionization, and dissociative post-ionization) have been analyzed.