Doubly charged ion mass spectra: VI—Amines

Doubly charged ion mass spectra of 22 amines (2–10 carbon atoms) were determined using an Hitachi RMU-7L double focusing mass spectrometer. Molecular ions were not observed in the spectra of aliphatic amines. The most intense product ion peaks in the spectra of lower molecular weight amines resulted from hydrogen elimination from the molecular ion; however, as amine molecular weight increased the largest peaks resulted from both hydrogen and heavy atom elimination from the molecular ion. Dominant ions in the doubly charged ion spectra of lower molecular weight aliphatic amines were from reactions of [C_nH₃N]²⁺ (n:=2, 3, 4) type ions. The spectra of higher molecular weight aliphatic amines spanned a wide mass range. Appearance energies for some of the more prominent ions were measured in the range from 25 to 49 eV. A geometry optimized quantum mechanical self-consistent field molecular orbital treatment was used to compute the energies and structural parameters of prominent ions in the doubly charged ion mass spectra.     

Doubly charged ion mass spectra: 1-Aliphatic nitriles

The doubly charged ion mass spectra for 12 aliphatic nitriles (1–9 carbon atoms) have been obtained using an Hitachi RMU-7L double focusing mass spectrometer. These spectra show some characteristic features such as extensive loss of hydrogen and the grouping of ions in the spectra into n-1 groups where n is the number of carbon atoms in the molecule (n<6). There are no indications of HCN or CN loss in the doubly charged ion spectra of the monosubstituted nitriles. SCF calculations of the energy and structure of doubly charged ions in the propionitrile spectra have been carried out.     

A study of the electron capture induced decompositions and charge separation reactions of the doubly charged isomeric ions of the methylpyridines and aniline

The doubly charged isomeric ions [C₆H₇N]²⁺ formed from 2-, 3- and 4-methylpyridine and aniline were investigated via their unimolecular charge separation reactions and by electron capture induced decompositions (ECID). The ECID spectra were compared with the collision induced decomposition (CID) spectra of the singly charged ions in an attempt to investigate the structure of the doubly charged ions. The four isomers could be unambiguously identified by their unimolecular charge separations. These differences were greater than in the mass spectra, ECID spectra or CID spectra of singly charged ions.     

Comparison of collision‐ versus electron‐induced dissociation of sodium chloride cluster cations

The collision‐induced dissociation (CID) and electron‐induced dissociation (EID) spectra of the [(NaCl)_m(Na)_n]ⁿ⁺ clusters of sodium chloride have been examined in a hybrid linear ion trap Fourier transform ion cyclotron resonance mass spectrometer. For singly charged cluster ions (n = 1), mass spectra for CID and EID of the precursor exhibit clear differences, which become more pronounced for the larger cluster ions. Whereas CID yields fewer product ions, EID produces all possible [(NaCl)_xNa]⁺ product ions. In the case of doubly charged cluster ions, EID again leads to a larger variety of product ions. In addition, doubly charged product ions have been observed due to loss of neutral NaCl unit(s). For example, EID of [(NaCl)₁₁(Na)₂]²⁺ leads to formation of [(NaCl)₁₀(Na)₂]²⁺, which appears to be the smallest doubly charged cluster of sodium chloride observed experimentally to date. The most abundant product ions in EID spectra are predominantly magic number cluster ions. Finally, [(NaCl)_m(Na)₂]^{+ .} radical cations, formed via capture of low‐energy electrons, fragment via the loss of [(NaCl)_n(Na)] ^. radical neutrals. Copyright © 2008 John Wiley & Sons, Ltd.     

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^+..     

‘Doubly charged ion’ mass spectra of some simple aromatic compounds

Benzene, toluene, phenol, diphenyl ether and the three isomeric dihydroxy-benzenes have been examined using an MS-9 mass spectrometer under conditions that allowed only ions having twice the normal amount of kinetic energy to be detected. These ions are, in fact, singly charged ions arising from charge exchange reactions of doubly charged ions of the same mass, occuring in the first field free region of the Spectrometer. It is argued that the spectra obtained yield essentially the distribution of doubly charged ions in the source region. These ‘doubly charged ion’ mass spectra are compared with the normal singly charged ion spectra of the compounds and the implications of the significant differences that are found, are discussed.     

Observation of doubly charged mercury cluster ions Hg n 2+, n=1-10 using secondary ion mass spectrometry

Mass spectra of doubly charged mercury clusters (m/z=30-1065) were investigated by secondary ion mass spectrometry. Positively charged ions were generated from an amalgam of mercury and silver by bombardment with a xenon ion beam and mass analysis by a grand-scale sector type mass spectrometer. Hg _n ²⁺, n=1-10 and Hg _n +, n =1- 5 were observed. Some doubly charged mercury clusters, (Hg _n ²⁺) survived at least for 0.1 ms.     

Mass spectra of doubly charged ions

The mass spectra of biological molecules, whose molecular mass exceeds 10 kDa, invariably contain multiply charged ions. For example, a survey scan of a small protein will produce singly, doubly and triply protonated molecules, the intensity of the doubly charged species often being greater than that of the singly charged entity. Although the spectra resulting from doubly charged peptides have not previously been studied, collisional activation of such doubly charged species may result in significant additional information pertaining to molecular structure. The techniques employed to study ions originating from multiply charged species were linked scanning of constant B/E and tandem mass spectrometry, namely low collision energy spectra acquired on a BEQQ hybrid instrument. The methodology was applied to model compounds whose tandem mass spectrometry characteristics are well known, e.g. gramicidin S and angiotensin I. The results for the product ions of the [M + 2H]²⁺ species of the models were obtained which highlight the methodology required for high-mass materials.     

Negative and positive secondary ion mass spectra of organic acids

Organic compounds can be ionized by sputtering the solid sample. The resulting negative and positive secondary ions provide mass spectra which characterize both the molecular weights and the structures of the compounds. Ionization occurs either by direct ejection of charged species from the solid into vacuum or by electron or proton transfer. The sputtered secondary ions dissociate unimolecularly to give fragment ions. These reactions are identical to those which occur when the secondary ions are independently generated by chemical ionization, selected by mass and dissociated in a high energy gas phase collision. The negative ion SIMS spectra show molecular ions (M^?.) or (M-H)^? ions as the dominant high mass species together with fragments due to decarboxylation, dehydration and losses of other simple molecules. Stronger acids show larger (M-H)^?/M^?.abundance ratios. The positive ion spectra are complementary and also useful in characterizing molecular structures. Attachment of cations to organic molecules (cationization) occurs much more readily than anion attachment and this makes negative SIMS spectra simpler than these positive ion counterparts.     

Doubly charged ion mass spectra. 7—acetylenes

Doubly charged ion mass spectra of 20 aliphatic and 3 aromatic acetylenic compounds have been measured using a double focusing Hitachi RMU-7L mass spectrometer. Spectra were obtained using 100 eV ionizing electron energy and 3.2 kV ion accelerating voltage. In general, the spectra of aliphatic type acetylenic compounds were dominated by fragment ions formed by extensive H loss from doubly charged molecular ions. Intense molecular ions were observed in the doubly charged ion spectra of phenyl-substituted acetylenes. Total product ion intensities for doubly charged ion spectra of acetylenic compounds were found to be smaller, in general, than the total product ion intensity observed in the benzene doubly charged ion mass spectrum. Measured appearance energies of intense product ions ranged from 24 to 47 eV. A geometry optimized quantum mechanical self-consistent field molecular orbital treatment was employed to compute energies and structural parameters of prominent ions in the doubly charged ion mass spectra of acetylenic compounds.     

Doubly charged ion mass spectra: III—N-alkanes

Doubly charged ion mass spectra have been obtained for 15 n-alkane hydrocarbons. Spectra were measured using a Nier-Johnson geometry Hitachi RMU-7L mass spectrometer operated at 1.6kV accelerating voltage. Fragment ions, which resulted from C? C bond rupture and extensive H loss, dominated the spectra. Molecular ions have not been observed. The most intense ions in the doubly charged ion mass spectra of n-alkanes were [C₂H₄]²⁺, [C₃H₂]²⁺, [C₄H₃]²⁺, [C₅H₂]²⁺, [C₆H₆]²⁺, [C₆H₈]²⁺, [C₇H₆]²⁺, [C₇H₈]²⁺, [C₈H₆]²⁺ and [C₈H₈]²⁺. Appearance energies for forming the prominent doubly charged fragment ions have been measured and range from 27.5 eV to energies greater than 60eV. A geometry optimized SCF approach has been used to compute the energies and structures of prominent ions in the doubly charged mass spectra.     

Investigation of cationic organic dyes used as molecular probes by liquid secondary ion mass spectrometry

The positive-ion mass spectra of twelve organic dyes used as molecular probes were measured using liquid secondary ion mass spectrometry (LSIMS). Nine of the twelve dyes were singly charged cations and the other three were doubly charged cations. The mass spectra of each of the dyes in m-nitrobenzyl alcohol contain abundant signals for the intact cation, C⁺ (singly charged cation dyes), or for singly-charged forms of the doubly charged cation formed by proton loss, [C²⁺? H⁺]⁺, or halogen counter ion attachment, [C²⁺ + X^?]⁺. Fragmentation is usually minimal under the conditions used. However, the cations of five of the singly charged compounds appear to undergo charge-remote fragmentation. Collision-induced dissociation experiments on a hybrid mass spectrometer of EBqQ geometry at collision energies up to 300 eV failed to access this fragmentation pathway. In contrast to the LSIMS of many other doubly charged organic compounds, two of the dicationic dyes produced a doubly charged ion of reasonable abundance (2–20%) in the mass spectrum. When glycerol was used as a matrix solvent, the addition of the matrix modifier trifluoroacetic acid increased the abundance of C²⁺.     

Mass spectrometry of aryl substituted di- and trisiloxanes

The mass spectra of arylpentamethyldisiloxanes, sym-diaryltetramethyldisiloxanes and 1,5-diaryl-1,1,3,3,5,5-hexamethyltrisiloxanes were examined. Isotopic labeling and peak matching were used to substantiate the proposed fragmentation mechanisms. Siliconium ions dominate the spectra. Loss of neutral fragments from the [M-15]⁺ ions is important. Phenylpentamethyldisiloxane, sym-tetramethyldiphenyldisiloxane and 1,1,3,3,5,5-hexamethyl-1,5-diphenyltrisiloxane are representative examples of the three classes of compounds discussed. The [M-15]⁺ ion of phenylpentamethyldisiloxane loses methane, dimethylsilanone [(CH₃)₂Si?O] and phenylmethylsilanone [PhCH₃Si?O] to yield daughter siliconium ions. The [M-15]⁺ ion of sym-tetramethyldiphenyldisiloxane loses benzene, methane, dimethylsilanone and phenylmethylsilanone to yield daughter siliconium ions. The [M-15]⁺ ion of 1,1,3,3,5,5-hexamethyl-1,5-diphenyltrisiloxane loses benzene, tetramethylcyclodisiloxane and phenyltrimethylcyclodisiloxane to yield daughter siliconium ions. Finally, doubly charged ions are important in the mass spectra of the three series of aryl substituted di- and trisiloxanes discussed.     

Reactions of poly(ethylene glycol) cations with iodide and perfluorocarbon anions

Multiply charged poly(ethylene glycol) ions of the form (M+nNa) ⁿ⁺ derived from electrospray ionization have been subjected to reactions with negative ions in the quadrupole ion trap. Mixtures of multiply charged positive ions ranging in average mass from about 2000 to about 14,000 Da were observed to react with perfluorocarbon anions by either proton transfer or fluoride transfer. Iodide anions reacted with the same positive ions by attachment. In no case was fragmentation of the polymer ion observed. In all cases, the multiply charged positive ion charge states could be readily reduced to +1, thereby eliminating the charge state overlap observed in the normal electrospray mass spectrum. With all three reaction mechanisms, however, the +1 product ions were comprised of mixtures of products with varying numbers of sodium ions, and in the case of iodide attachment and fluoride transfer, varying numbers of halogen anions. These reactions shift the mass distributions to higher masses and broaden the distributions. The extents to which these effects occur are functions of the magnitudes of the initial charges and the width of the initial charge state distributions. Care must be taken in deriving information about the polymer molecular weight distribution from the singly charged product ions arising from these ion/ion reactions. The cluster ions containing iodide were shown to be intermediates in sodium ion transfer. Dissociation of the adduct ions can therefore lead to a +1 product ion population that is comprised predominantly of M+Na⁺ ions. However, a strategy based on the dissociation of the iodide cluster ions is limited by difficulties in dissociating high mass-to-charge ions in the quadrupole ion trap.     

Fragmentation of dihydro- and tetrahydro-1,5-benzothiazepines under electron impact

The fragmentation mechanisms of dihydro- and tetrahydro-1,5-benzothiazepines under electron impact have been studied in detail using high resolution mass spectrometry, metastable decompositions and deuterium labelling techniques. Both kinds of the benzothiazepines possess high stability. The [M? SH]⁺ and the cyclic benzothiazole ions derived from the fragmentation and severe skeletal rearrangement of the molecular ion comprised the main features of the spectra. Some doubly charged ions were noticed in the low resolution electron impact mass spectra.     

Are liquid chromatography/electrospray tandem quadrupole fragmentation ratios unequivocal confirmation criteria?

Multiple reaction monitoring (MRM) ratios as provided by tandem mass spectrometers are used to confirm positive residue findings (e.g. veterinary drugs or pesticides). The Commission Decision 2002/657/EEC defines tolerance levels for MRM ratios, which are intended to prevent the reporting of false positives. This paper reports findings where blank sample extracts have been spiked by a drug (difloxacin) and the corresponding measured MRM ratios significantly deviated from MRM ratios observed in matrix‐free solution. The observation was explained by the formation of two different [M+H]⁺ analyte ions within the electrospray ionization (ESI) interface. These two ions vary only by the site of analyte protonation. Since they are isobaric, they are equally transmitted through the first quadrupole, but are differently fragmented in the collision chamber. The existence of two isobaric ions was deduced by statistical data and the observation of a doubly charged analyte ion. It was hypothesized that the combined presence of [M+H]⁺ and [M+2H]²⁺ implies the existence of two different singly charged ion species differing only by the site of protonation. Low‐ and high‐energy interface‐induced fragmentation was performed on the samples. The surviving precursor ion population was mass selected and again fragmented in the collision chamber. Equal product ion spectra would be expected. However, very different product ion spectra were observed for the two interface regimes. This is consistent with the assumption that the two postulated isobaric precursor ions show different stability in the interface. Hence the abundance ratio among the two types of surviving precursor ions will shift and change the resulting product ion spectra. The existence of the postulated singly charged ions with multiple chargeable sites was finally confirmed by successful ion mobility separation. Copyright © 2009 John Wiley & Sons, Ltd.     

Gas-Phase Synthesis of Singly and Multiply Charged Polyoxovanadate Anions Employing Electrospray Ionization and Collision Induced Dissociation

Electrospray ionization mass spectrometry (ESI-MS) combined with in-source fragmentation and tandem mass spectrometry (MS/MS) experiments were used to generate a wide range of singly and multiply charged vanadium oxide cluster anions including V_xO_y ^n– and V_xO_yCl^n– ions (x = 1–14, y = 2–36, n = 1–3), protonated clusters, and ligand-bound polyoxovanadate anions. The cluster anions were produced by electrospraying a solution of tetradecavanadate, V₁₄O₃₆Cl(L)₅ (L = Et₄N⁺, tetraethylammonium), in acetonitrile. Under mild source conditions, ESI-MS generates a distribution of doubly and triply charged V_xO_yCl^n– and V_xO_yCl(L)^(n–1)– clusters predominantly containing 14 vanadium atoms as well as their protonated analogs. Accurate mass measurement using a high-resolution LTQ/Orbitrap mass spectrometer (m/Δm = 60,000 at m/z 410) enabled unambiguous assignment of the elemental composition of the majority of peaks in the ESI-MS spectrum. In addition, high-sensitivity mass spectrometry allowed the charge state of the cluster ions to be assigned based on the separation of the major from the much less abundant minor isotope of vanadium. In-source fragmentation resulted in facile formation of smaller V_xO_yCl^(1–2)– and V_xO_y ^(1–2)– anions. Collision-induced dissociation (CID) experiments enabled systematic study of the gas-phase fragmentation pathways of the cluster anions originating from solution and from in-source CID. Surprisingly simple fragmentation patterns were obtained for all singly and doubly charged V_xO_yCl and V_xO_y species generated through multiple MS/MS experiments. In contrast, cluster anions originating directly from solution produced comparatively complex CID spectra. These results are consistent with the formation of more stable structures of V_xO_yCl and V_xO_y anions through low-energy CID. Furthermore, our results demonstrate that solution-phase synthesis of one precursor cluster anion combined with gas-phase CID is an efficient approach for the top-down synthesis of a wide range of singly and multiply charged gas-phase metal oxide cluster anions for subsequent investigations of structure and reactivity using mass spectrometry and ion spectroscopy techniques.      

Formation of doubly charged ions in field ionization mass spectra of para substituted acetophenones

The doubly charged [M]²⁺, [M+1]²⁺ and [M-O]²⁺ ions are observed in the field ionization mass spectra of para substituted acetophenones. The effect of the type of the substituent on the formation of the doubly charged ions is described.     

Multiply charged cluster ions of Ar,Kr, Xe,N2, O2, CO2, SO2 and NH3: Production mechanism,appearance size and appearance energy

Clusters of Ar, Kr, Xe, N₂, O₂, CO₂, SO₂ and NH₃ formed by supersonic nozzle expansion have been studied by electron impact ionization mass spectrometry (up to 15000 amu). Besides mass spectra of singly charged ions showing the characteristic anomalous distributions, we have in particular investigated the properties of multiply charged cluster ions. Critical appearance sizes of doubly and triply charged cluster ions, n₂ and n₃ respectively, found in the present study confirm recent theoretical predictions about n₃/n₂ and their dependence on the properties of the cluster constituents. The appearance energies of multiply charged cluster ions determined are shifted way below the appearance energies of the respective monomer ions. These huge red shifts together with the observed linear threshold laws and large maximum ionization efficiencies indicate that multiply charged cluster ions are produced by sequential single ionization events of one incoming electron at different cluster sites. Furthermore, we have also obtained for the first time clear evidence that (for electron energies above the appearance energy of doubly charged ions) an appreciable amount of singly charged (also fragment) ions is produced via Coulomb explosion of unstable doubly charged ions in the ion source.     

Experimental evidence for the formation of doubly charged oxide and hydroxide ions in inductively coupled plasma mass spectrometry

The formation of doubly charged polyatomic ions in inductively coupled plasma mass spectrometers was investigated using commercially available instruments. The species observed were ThO²⁺ and ThOH²⁺, which were found in similar amounts with the different instruments used in this study, when operated under routine analytical conditions. The signal ratios for ThO²⁺ were between 1.8 × 10^–4 and 4.2 × 10^–4 relative to the singly charged elemental ion and between 1.4 × 10^–2 and 2.2 × 10^–2 relative to the doubly charged elemental ion. The formation of ThOH²⁺ was between 1.1 × 10^–4 and 2.8 × 10^–4 relative to the singly charged elemental ion and between 0.72 × 10^–2 and 1.3 × 10^–2 relative to the doubly charged elemental ion. A mechanism is proposed for the formation of the doubly charged oxide and hydroxide ions that is based on the condensation of the doubly charged elemental ion with water or oxygen molecules in the interface region of the mass spectrometer. Received: 20 December 2000 / Revised: 19 March 2001 / Accepted: 22 March 2001