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: 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: V—Oxygen containing hydrocarbons

Doubly charged ion mass spectra were obtained for 46 low molecular weight oxygen containing compounds. A double focusing Hitachi RMU-7L mass spectrometer, operated at 3.2 kV accelerating voltage, was used to measure spectra for aliphatic alcohol, ketone, ether, aldehyde, ester and acid molecules, as well as several aromatic oxygen containing compounds. In general, the spectra were dominated by fragment ions which resulted from extensive H loss and C? C bond rupture as well as O elimination from the doubly charged molecular ions. Total product ion intensities from doubly charged ion spectra of aliphatic oxygen containing compounds were approximately 1% of the corresponding total ion intensity in the benzene doubly charged ion spectrum. Appearance energies for forming prominent doubly charged molecular and fragment ions were determined. Measured values ranged from 26 to 45 eV. A geometry optimized quantum mechanical SCF treatment was used to compute the energies, charge densities and structures for several of these oxygen containing doubly charged ions.     

Doubly charged ion mass spectra. 2—aromatic hydrocarbons

Doubly charged ion mass spectra for 15 aromatic hydrocarbons have been obtained using a Nier-Johnson geometry, Hitachi RMU-7L mass spectrometer operating at 1.6 kV accelerating voltage. The doubly charged ion spectra have features that are characteristic of the individual compounds. Unsaturated aromatic molecules show intense molecular ions in contrast to saturated, substituted or heteroatom compounds which undergo extensive fragmentation. Ionization energies for forming doubly charged molecular ions and appearance energies for the prominent doubly charged fragment ions have been measured. Calculations of the SCF energies and structures of various doubly charged ions have been carried out. Measured and calculated ionization/appearance energies are in reasonable accord and lend support to the suggested ion structures.     

Doubly charged ion mass spectra 8—alkenes

Doubly charged ion mass spectra of 23 alkenes have been measured using a double focusing Hitachi RMU-7L mass spectrometer. Ion mass spectra were obtained using 100 eV electron energy and 3.2 kV ion accelerating voltage. Each 2E spectrum was determined using the olefinic compound under investigation as the target gas. In general, spectra are dominated by fragment ions which result from extensive hydrogen loss from the doubly charged molecular ion. Appearnce energies have been measured for intense fragment ions in each spectrum.     

Doubly charged ion mass spectra of organophosphorus compounds

Doubly charged ion mass spectra have been obtained for 11 organophosphorus compounds. Methane has been used as a target gas to increase the probability of single electron transfer collisions in the first field-free region of an Hitachi RMU-7L mass spectrometer. In general, the spectra of organophosphorus compounds do not exhibit molecular ions but are dominated by fragment ions, many of which must be formed by rearrangement processes. A geometry-optimized self-consistent field molecular orbital method has been employed to compute energies and structural parameters for prominent ions. In addition, a diabatic curve crossing model has been used to examine the single electron transfer reactions responsible for intense ions in the doubly charged ion mass spectra. Appearance energies measured for ions prominent in the 2E spectra of organophosphorus compounds have ranged from 23 to 38 eV.     

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.     

Capillary electrophoresis-electrospray mass spectra of the herbicides paraquat and diquat

The positive ion electrospray mass spectra of the quaternary ammonium salt herbicides paraquat and diquat are examined by on-line separation with capillary electrophoresis (CE) and by direct infusion of the analytes. The analytes are separated by CE in 7–10 min at pH 3.9 in 50% methanol-water by using several different separation buffer electrolytes. The capillary electrophoresis-electrospray ionization (CE-ES) mass spectra of paraquat and diquat consist primarily of doubly charged molecular ions, singly charged molecular ions, and singly charged deprotonated ions. The direct infusion spectra consist primarily of doubly charged molecular ions and singly charged deprotonated ions. The relative abundances of the doubly charged and deprotonated ions depend strongly on the presence or absence of ammonium ion in the CE separation buffer or the direct infusion solution. A deprotonation mechanism is proposed in which the free base ammonia is the deprotonating agent in the desolvating charged droplets or in the gas phase. The analytical potential of the CE-ES electrospray approach for environmental analyses is evaluated in terms of the precision of replicate injections, linear concentration range, and estimated detection limit.     

‘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.     

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.     

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.     

Influence of collision gas on doubly charged ion mass spectra

The influence of the collision gas on doubly charged ion mass spectra of a selected number of hydrocarbons has been examined. Relative abundances of various product ions in 2E spectra, resulting from charge exchange collisions of doubly charged hydrocarbon ions, do not vary drastically as the collision gas is varied. However, the absolute intensities of these doubly charged ion mass spectra increase significantly when the collision gas is changed from argon to methane to isobutane. Tbese observations are rationalized in terms of a multichannel diabatic curve crossing model.     

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^+..     

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

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

Scanning a triple quadrupole mass spectrometer for doubly charged ions

Charge exchange reactions within a triple quadrupole mass spectrometer characterize doubly charged ions formed in the ion source. Two methods have been developed for identifying the singly charged ions formed from doubly charged ions by charge exchange in the collision quadrupole. The first is based on the characteristically high kinetic energy-to-charge ratios of the products of charge exchange; this property can be used to separate these ions from all other singly charged ions. This retarding potential method is analogous to procedures for recording doubly charged ion mass spectra using sector instruments. The second method is based on the fact that, although mass remains constant in the charge exchange reaction, the change in mass-to-charge ratio can be followed. A charge exchange linked scan, predicated on changes in charge rather than mass, but otherwise analogous to neutral loss/gain scans, is described. Information on the structure of doubly charged ions can be obtained by recording the fragmentation products of dissociative charge exchange. The utility of the charge exchange linked scan for the selective identification of polynuclear aromatic compounds in a complex mixture is described. The methods given can be generalized to cover other charge permutation reactions.     

Effects of the position of internal histidine residues on the collision-induced fragmentation of triply protonated tryptic peptides

The collision-induced dissociation spectra of a series of synthetic, tryptic peptides that differed by the position of an internal histidine residue were studied. Electrospray ionization of these peptides produced both doubly and triply protonated molecular ions. Collision-induced fragmentation of the triply protonated peptide ions had better efficiency than that of the doubly protonated ions, producing a higher abundance of product ions at lower collision energies. The product ion spectra of these triply protonated ions were dominated by a series of doubly charged y-ions and the amount of sequence information was dependent on the position of the histidine residue. In the peptides where the histidine was located towards the C-terminus of the peptide, a more extensive series of sequence specific product ions was observed. As the position of the histidine residue was moved towards the N-terminus of the peptide, systematically less sequence information was observed. The peptides were subsequently modified with diethylpyrocarbonate to manipulate the product ion spectra. Addition of the ethoxyformyl group to the N-terminus and histidine residue shifted the predominant charge state of the modified peptide to the doubly protonated form. These peptide ions fragmented efficiently, producing product ion spectra that contained more sequence information than could be obtained from the corresponding unmodified peptide.     

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⁺.     

Collision induced decomposition of peptides. Choice of collision parameters

Collision-induced dissociation product ion spectra of a series of doubly charged tryptic peptide ions produced by electrospray ionization were obtained by triple-quadrupole tandem mass spectrometry. The sequence information content of the product ion spectra was explored as a function of collision energy and collision-cell gas pressure for parent ions with molecular masses ranging from 300 to 2000 u. The energy range (at a given pressure) in which the degree of fragmentation is acceptable was found to be narrow for parent ions of a given mass, and the optimal collision energy was observed to exhibit a strong linear correlation with parent ion mass. This observed correlation opens the way for on-line software-controled selection of optimal mass spectrometric conditions in the enzymatic digestion-liquid chromatography-tandem mass spectrometric strategy of amino acid sequencing of proteins.     

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.     

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.