Charge Inversion Mass Spectrometry Research Articles

Study of Ion Dynamics by Electron Transfer Dissociation: Alkali Metals as Targets.

High energy collision processes for singly charged positive ions using an alkali metal target are confirmed, as a charge inversion mass spectrometry, to occur by electron transfers in successive collisions and the dissociation processes involve the formation of energy-selected neutral species from near-resonant neutralization with alkali metal targets. A doubly charged thermometer molecule was made to collide with alkali metal targets to give singly and doubly charged positive ions. The internal energy resulting from the electron transfer with the alkali metal target was very narrow and centered at a particular energy. This narrow internal energy distribution can be attributed to electron transfer by Landau-Zener potential crossing between the precursor ion and an alkali metal atom, and the coulombic repulsion between singly charged ions in the exit channel. A large cross section of more than 10-14 cm2 was estimated for high-energy electron transfer dissociation (HE-ETD). Doubly protonated phosphorylated peptides obtained by electrospray ionization were collided with Xe and Cs targets to give singly and doubly charged positive ions. Whereas doubly charged fragment ions resulting from CAD were dominant in the case of the Xe target, singly charged fragment ions resulting from ETD were dominant with the Cs target. HE-ETD using the Cs target provided all of the z-type ions by N-Cα bond cleavage without the loss of the phosphate groups. The results demonstrate that HE-ETD with an alkali metal target allowed the position of phosphorylation and the amino acid sequence of peptides with post translational modifications (PTM) to be determined.

High-energy electron transfer dissociation of protonated amino acids

High-energy electron transfer dissociation (HE-ETD) of singly protonated amino acids was investigated using charge inversion mass spectrometry with alkali metal targets in which dissociations of charge-reduced neutrals were induced from the excited neutral intermediates. The dominant trapezoidal peaks of the 17 u loss in the charge inversion spectra of the protonated aliphatic amino acids were assigned to NH3 loss caused by NCα bond cleavage from hypervalent ammonium radical intermediates via a repulsive state; this assignment was based on the peak shape and kinetic energy release (KER) values. The peak of the non-dissociated ion observed in the proline spectra was explained by the same bond cleavage. The triangular peaks of the 2 u loss, which were dominantly observed in all of the spectra, were assigned to H2 elimination, which involved H atom migration in the neutral radical. Neutral intermediates that induced the H2 loss were estimated to have a lower internal energy than that of the NH3 loss by comparing the charge inversion spectra of the different target atoms. Differences in the isobaric ions between leucine and isoleucine were found in the bond dissociation of CβCγ in their side chains following the NH3 loss and in the intensity of the NH3 loss relative to that of the H2 loss. The charge inversion spectra of the protonated amino acids, with side chains that are subject to post-translational modification, provided intense peaks resulting from the consecutive and competitive loss of neutral molecules following the NH3 loss and H2 loss. Along with these losses, side chain losses caused by H atom migration were also observed as sharp peaks with KER values smaller than that of the consecutive loss.

Dissociation mechanisms of excited CH3X (X = Cl, Br, and I) formed via high-energy electron transfer using alkali metal targets

High-energy electron transfer dissociation (HE-ETD) on collisions with alkali metal targets (Cs, K, and Na) was investigated for CH(3)X(+) (X = Cl, Br, and I) ions by a charge inversion mass spectrometry. Relative peak intensities of the negative ions formed via HE-ETD strongly depend on the precursor ions and the target alkali metals. The dependency is explained by the exothermicities of the respective dissociation processes. Peak shapes of the negative ions, especially of the X(-) ions, which comprise a triangle and a trapezoid, also strongly depend on the precursor ions and the target alkali metals. The trapezoidal part of the I(-) peak observed with the Na target is more dominant and much broader than that with the Cs target. This dependence on the targets shows an inverse relation between the peak width and the available energy, which corresponds to the exothermicity assuming formation of fragment pair in their ground internal states. From a comparison of the kinetic energy release value calculated from the trapezoidal shape of I(-) with the available energy of the near-resonant level on the CH(3)I potential energy curve reported by ab initio calculations, the trapezoidal part is attributed to the dissociation to CH(3) + I((2)P(3/2)) via the repulsive (3)Q(1) state of CH(3)I, which is not dominant in the photo-dissociation of CH(3)I. The observation of trapezoid shape of the CH(2)I(-) peak with the Cs target indicates spontaneous dissociation via repulsive potential from the (3)R(2) Rydberg state, although the correlation between the (3)R(2) Rydberg state and relevant repulsive states has not been reported by any theoretical calculation.

Dissociation mechanism of electronically excited CH 2X 2 (X = Cl, Br) formed by near-resonant neutralization using charge-inversion mass spectrometry

The dissociation mechanism of excited CH 2X 2 (X = Cl, Br) was investigated using charge-inversion mass spectrometry, in which positive ions collide with an alkali metal target to generate neutral fragments, and the negative ions formed from the neutral fragments are mass analyzed. Different relative abundances of the negative ions were observed in the charge-inversion spectra for CH 2Cl 2 and CH 2Br 2. The kinetic energy release values calculated from analysis of the peak associated with CH Cl 2 - in the charge-inversion mass spectrum of the parent CH 2 Cl 2 + ion indicate that the excited CH 2Cl 2 formed by neutralization dissociates spontaneously into CHCl 2 + H.

Dissociation channels of silver bromide cluster Ag2Br, silver cluster Ag3 and their ions studied by using alkali metal target

Various dissociation channels of silver bromide cluster ion Ag2Br+ and silver cluster ion Ag3 + were observed in high-energy collisionally-activated dissociation (CAD) using a Cs target. The fragment patterns of the high-energy CAD were compared with those of the metastable dissociation and low-energy CAD. The difference in the fragment patterns between the high-energy CAD and the other dissociation methods was explained in terms of the internal energy distributions. The dissociation mechanisms of neutral silver bromide cluster Ag2Br and silver cluster Ag3 were also investigated by charge inversion mass spectrometry using the Cs target. While the fragment ions AgBr- and Ag2 - were dominantly observed in the charge inversion spectrum of Ag2Br+, the undissociated ion Ag3 - was observed as a predominant peak in the case of Ag3 +. The dissociation behavior of Ag2Br* can be explained on the basis of the calculated thermochemical data. Contrary to this, the predominant existence of the undissociated Ag3 - cannot be explained by the reported thermochemical data. The existence of undissociated Ag3 - suggests that the dissociation barrier is higher than the internal energy of Ag3 * (theoretical: 1.03 eV, experimental: 2.31 eV) estimated from the ionization potentials of Ag3 and Cs.

Definitive evidence for the existence of isomeric chlorophenyl radicals (C 6H 4Cl [rad]) from charge inversion mass spectrometry and DFT calculations

Free radical species are much more reactive than stable molecules, and so usually exist only as transient intermediates in chemical reactions. Charge inversion mass spectrometry using alkali metal targets is an effective method for determining the structure and dissociation processes of radicals, and can also enable differentiation between isomeric forms of compounds whose parent ions have similar mass spectra and similar collisionally activated dissociation spectra, such as the isomers of dichlorobenzene and chlorophenol. The charge inversion process using alkali metal targets proceeds via near-resonant neutralization, followed by spontaneous dissociation of the excited neutrals, and then endothermic negative ion formation. In the normalized charge inversion spectra of ortho-, meta-, and para-dichlorobenzene (C 6H 4Cl 2) measured in this work, the intensities of the peaks associated with chlorine anions (Cl −) are almost same for each of the isomers, whereas the intensities of the peaks associated with the chlorophenyl anions (C 6H 4Cl −) display a strong dependence on the isomeric structure of the parent compound. The similarities of the Cl − ion peak intensities indicate that neutralization cross-sections and branching ratios to produce Cl radicals are the same for each of the isomeric precursor C 6H 4Cl 2 + ions. The strong isomer-dependence of the peak intensities of C 6H 4Cl − anions suggests that the chlorophenyl radicals (C 6H 4Cl ) formed from C 6H 4Cl 2 by loss of Cl do not undergo isomerization, and that the electron transfer cross-sections to form the negative ions are strongly isomer-dependent. Density functional theory (DFT) calculations on the o-, m-, and p-C 6H 4Cl radicals show that the barriers to isomerization are in excess of 2.8 eV, and these high isomerization barriers are believed to be the reason for the absence of isomerization among the C 6H 4Cl radicals during the charge inversion process. Calculated adiabatic electron affinities and vertical electron affinities both show differences for the o-, m-, and p-isomers, but the trend in the magnitude of the electron affinities has an inverse relation to the peak intensities of the isomeric C 6H 4Cl − ions in the charge inversion spectra. It is, therefore, assumed that the cross-sections for negative ion formation are influenced by many factors other than the electron affinity, such as the relatively large geometrical distortion in the chlorophenyl radicals predicted in this work. The good agreement between the experimental results and the theoretical predictions obtained in this work provides compelling evidence for the existence of isomeric C 6H 4Cl radicals that do not undergo isomerization during charge inversion mass spectrometry.

Dissociation mechanisms of neutral methyl stearate and its hydrogen atom adduct formed from the respective positive ions by electron transfer

The mechanism of dissociation of neutral methyl stearate and its hydrogen atom adduct was investigated by charge inversion mass spectrometry using an alkali metal target. Migrations of functional groups in fatty acid ester ions are often observed during the dissociation of the cations in collisionally activated dissociation (CAD). In the charge inversion spectrum, the main dissociation channels of methyl stearate molecule are the loss of a CH3 radical or a H atom. To identify the source of the CH3 radical and the H atom, the charge inversion spectra of partially deuterated methyl stearate (C17H35COOCD3) were measured. The loss of CH3 occurred through elimination from the methoxy methyl group and that of H occurred through elimination from the hydrocarbon chain of the fatty acid group. In the protonated ester, a simultaneous loss of CH3 (from the methoxy methyl group) and a H atom or a H2 molecule was observed. The charge inversion process gave the dissociation fragments with almost no migration of atoms. Only a few peaks that were structure sensitive were observed in the higher mass region in the charge inversion spectra; these peaks were associated with dissociations of energy-selected neutral species, unlike the case of CAD spectra in which they result from dissociation of ions. Charge inversion mass spectrometry with alkali metal targets provided direct information on the dissociation mechanism of methyl stearate and its hydrogen atom adduct without any migration of functional groups.

Dissociation mechanism of electronically excited CHn (n = 3–5) neutrals formed by near-resonant neutralization using charge inversion mass spectrometry

Charge inversion mass spectrometry was used to produce the electronically excited species CHn (n=3–5) from their corresponding positive ions by neutralization with an alkali metal target, and then to subsequently detect and mass-analyze the negative ions formed from the neutral fragments produced from the dissociation of the excited neutrals. The trapezoidal shape and the intensity of the peak associated with CH2 - ions in the charge inversion spectrum of CH3 + ions indicated that the CH3 neutrals dissociated mainly into CH2 + H without a large activation barrier. The most intense peak in the spectrum of CH4 + ions was that associated with CH2 - ions, and this peak comprised a combination of both trapezoidal and triangular shaped peaks. The trapezoidal shaped peak was attributed to CH2 - ions resulting from direct dissociation of CH4 into CH2 + H2. The concurrent dissociation of CH4 into CH3 + H was followed by the further subsequent dissociation of the deformed CH3 fragments into CH2 + H, and this was proposed to be the origin of the triangular shaped component of the CH2 - peak. In the spectrum of CH5 + ions, the CH3 - peak was much less intense than the CH2 - peak, which was proposed to be the result of the geometry of the CH3, formed from the dissociation of CH5 into CH3 + H2,being substantially distorted from the D3h symmetry, leading to its further subsequent dissociation.

電荷逆転質量分析法の基礎 Ｉ：歴史的展望と４種の電荷逆転の違い

The most valuable technique for the structure analysis in mass spectrometry, tandem mass spectrometry (MS/MS), involves the excitation of mass-selected species followed by their dissociation into products, the mass-to-charge ratios of which are also measured. Collisionally Activated Dissociation (CAD) has been used as a most common technique to dissociate the mass-selected precursor ions. Charge inversion mass spectrometry is a special branch of MS/MS method, in which the electric charge of the precursor ions is opposite to that of the secondary product ions. Charge inversion mass spectrometry is classified into four types depending on the electric charge and time scales of collisions. Electric charges, which are either positive ions or negative ions, provide the characteristic difference between reactants and products. Either successive single electron transfer in two collisions or double electron transfer in one collision depend time scales of collisions. Charge inversion mass spectrometry using collisions with gaseous targets in the keV collision energy range have provided insights into the structures and reactions of ions and neutral molecules. The characteristics of these four types of charge inversion experiments are presented in terms of the reaction endothermicities, the cross sections and their dependence on the target species. In the case of rare gas or simple molecular targets, double electron transfer in one collision is effective to form positive ions from negative ions, while in the case of alkali metal targets successive single electron transfers in two collisions is effective to form negative ions from positive ions. While CAD is due to dissociation of activated ions with broad internal energy distributions, the charge inversion processes using alkali metal target is due to dissociation of energy-selected neutral species with narrow internal energy distributions.

Double- and single-electron transfer inH++Kcollisions from0.3to4keV: Separation of direct double transfer and two-step successive single-electron transfer

Double-electron transfer and two-step single-electron transfer in collisions of proton with potassium-metal target are measured in the collision energy from 0.3 to 4 keV by using a charge-inversion mass spectrometry. Two prominent H{sup -} ion peaks are observed with different values of the energy loss and show different target density dependences. The peak with larger energy loss is identified as double-electron transfer and the other as two-step successive single-electron transfer from the analysis of the target density dependence. The two-step single-electron transfer is considered to occur as the process accompanying spontaneous Ly-{alpha} emission, followed by negative H{sup -} formation. A theoretical analysis is also carried out, and the single-electron transfer cross section obtained is found to be in excellent agreement with the present measurement, while the present measurement for double-electron transfer is found to be much smaller than those evaluated earlier and the present theory.

Charge Inversion Mass Spectrometry: Dissociation of Resonantly Neutralized Molecules

AbstractFor Abstract see ChemInform Abstract in Full Text.

Charge inversion mass spectrometry: dissociation of resonantly neutralized molecules.

Charge inversion mass spectrometry is an MS/MS method in which the electric charge of the precursor ions is opposite to that of the secondary product ions. Charge inversion mass spectrometry is classified into four types depending on the electric charge and time scale of collisions. Charge inversion mass spectrometry using collisions with gaseous targets in the keV energy collision range has provided insights into the structures and reactions of ions and neutral molecules. The characteristics of charge inversion experiments are presented in terms of the reaction endothermicities and the cross sections and their dependence on the target species. In the case of rare-gas or simple molecular targets, double-electron transfer in one collision is effective to form positive ions from negative ions, while, in the case of alkali metal targets, successive single-electron transfers in two collisions is effective to form negative ions from positive ions. On the basis of the observed target-density dependence of the product ion intensity and thermochemical considerations for internal energy distribution using thermometer molecules, the charge inversion processes using alkali metal targets have been confirmed to occur by electron transfers in successive collisions and the dissociation processes are found to occur in energy-selected neutral species formed from near-resonant neutralization with alkali metal targets. While collisionally activated dissociation (CAD) is due to dissociation of activated ions with broad internal energy distributions, the charge inversion process using alkali metal targets is due to dissociation of energy-selected neutral species with narrow internal energy distributions. The charge inversion/alkali metal spectra provide clear differentiation of the isomeric cations of C(2)H(2), C(3)H(4) and dichlorobenzenes. The CAD spectra of these isomeric cations are similar.

Evidence for a difference in the dissociation mechanisms of acetylene (HCCH) and vinylidene (H2CC:) from charge inversion mass spectrometry

Vinylidene and acetylene are the simplest hydrocarbon isomers, and vinylidene is the simplest unsaturated carbene. The charge inversion mass spectra of C2H2+ cations derived from acetylene using Na, K, Rb and Cs targets were found to be clearly different from those derived from vinylidenechloride (1,1-dichloroethylene). The process of formation of the negative ions in charge inversion mass spectrometry is via near-resonant neutralization followed by spontaneous dissociation, and then endothermic negative ion formation. The intensity of the C2− peak relative to the C2H− peak in these spectra increased with decreasing ionization potential of the targets for both of the isomeric C2H2+ cations. The formation of the C2− anion is proposed to result from the dissociation of excited C2H2 neutrals into C2 and H2. The dependence on target species of the intensities of the C2− peak relative to the C2H− peak for HCCH and H2CC: cannot be rationalized by the internal energy of the excited C2H2 neutrals. The differences indicate that the isomeric C2H2 neutrals dissociate into C2H and H prior to 1,2-hydrogen atom migration.

Discrimination of Isomers of Dichlorobenzene Using Charge Inversion Mass Spectrometry.

Chlorinated aromatic compounds are well-known environmental pollutants and their toxicity often depends dramatically on the chlorine substitution pattern within a homological group. Analysis of extremely toxic environmental pollutants such as dioxins requires high sensitivity, typically in the parts-per-trillion (ppt) range. Mass spectrometry (MS), being the most sensitive of analytical methods, has been used for pollutant analysis, but conventional mass spectrometry alone cannot be used for isomer-specific analysis of these toxic chemicals. In this work, we investigated the possibility of discriminating among the isomeric precursors of chlorinated aromatic compounds by charge inversion mass spectrometry using an MS/MS instrument. Charge inversion mass spectra using Cs, K, and Na targets were measured for C6H4Cl2+ ions produced from the ortho-, meta-, and para-isomers of dichlorobenzene (C6H4Cl2). The charge inversion mass spectra were found to display a clear dependence on the nature of the isomeric precursors for each of the targets used. The clear discrimination among the isomers of dichlorobenzene achieved using charge inversion mass spectrometry in the present work indicates that most of the C6H4Cl2+ ions obtained by electron impact retain the structure of the parent molecules, demonstrating the potential utility of this technique for isomer-selective microanalysis of various pollutants.

電荷逆転質量分析法におけるピーク形状と運動エネルギー放出

A trapezoidal peak shape of dissociative ions is observed in the charge inversion mass spectra. The relation between the kinetic energy release (KER) and the peak shape was simulated. Dissociation with a specific KER value having very narrow distribution for both ionic and neutral species affords the trapezoidal peak shape in a mass spectrum. The KER value was evaluated from the full width at half maximum (FWHM) of the trapezoidal peak associated with the CH3O- ions formed from CH3OH+ ions in the charge inversion mass spectrum. The KER value showed the good agreement with that evaluated from the velocity distribution of H atom formed from the photo-dissociation of neutral methanol. This agreement demonstrated that the excited neutral methanol dissociated predominantly into the methoxy radical and the hydrogen atom, and that charge inversion mass spectrometry gave the dissociation of energy-selected neutral species.

Study of the dissociation of neutral intermediates using charge inversion mass spectrometry

In charge inversion experiments using a tandem mass spectrometer, mass-selected positive ions are made to collide with an alkali metal target, and the resulting negative ions formed upon two-electron transfer are mass-analyzed. The internal energy depositions are measured for the so-called thermometer molecule W(CO)6. The difference between the centered value of the internal energy of W(CO)6 and the energy level of the precursor ion is in good agreement with the ionization energy of the Cs target. This correlation indicates that dissociation occurs from energy-selected neutrals formed via near-resonant neutralization.

Dissociation mechanism of electronically excited C 3H 4 isomers by charge inversion mass spectrometry

The mechanism for dissociation of C 3H 4 isomers to form neutral radicals was investigated by charge inversion mass spectrometry using alkali metal targets. Electronically excited allene and propyne were formed from the corresponding positive ions by neutralization with alkali metal, and the neutral fragments resulting from dissociation of the excited molecules were converted into negative ions which were mass-analyzed and detected. From the comparison of peak intensities and peak widths among allene, propyne and 3,3,3-trideutero-propyne (CD 3CCH), the dissociation mechanism of electronically excited C 3H 4 was elucidated. The principal feature of the dissociation process involves the loss of two H atoms from the electronically excited hydrocarbons, which is attributed not to two independent CH bond cleavages, but rather to loss of a H 2 molecule. The loss of a H 2 molecule has a higher activation energy than loss of a H atom, despite the final state of the former having a lower heat of formation than that of the latter. In a CH bond cleavage, the weaker the bond, the more easily it is cleaved. Cleavage of a CC bond is substantially less favorable than the loss of hydrogen atoms, even though the former has a lower energy than the later. This may be attributed to the lower frequency of the CC bond vibration compared with that of the CH bond.

Discrimination of C 3H 4+ isomeric ions by charge inversion mass spectrometry using an alkali metal target

Charge inversion mass spectrometry was performed using an MS/MS instrument in which mass-selected positive ions were made to collide with alkali metal targets, and the resulting negative ions formed upon two-electron transfer were mass analyzed. Charge inversion spectra using Cs targets were measured for C 3H 4 + ions produced from allene (CH 2CCH 2) and propyne (CH 3CCH) by electron impact. The peak associated with C 3H 4 - in the charge inversion spectra is twice as intense for propyne as for allene, whereas the profile of the peaks associated with C 3H n − ( n = 0–2) is similar for both isomeric precursors. The kinetic energy release values at FWHM of the peak associated with C 3H 3 − formed from allene ions and propyne ions are 0.28 ± 0.04 eV and 0.64 ± 0.06 eV, respectively. A clear differentiation between the isomeric precursors can be made on the basis of these differences in the charge inversion spectra. The differences observed are attributed to the formation of excited C 3H 4 states by near-resonant neutralizations of allene and propyne ions. Collisionally activated dissociation (CAD) spectra using He targets were measured for the same C 3H 4 + isomeric precursor ions. The CAD spectra are similar for both isomeric precursor ions, although slight differences were detected in the weak peaks associated with the species formed upon CC bond cleavage. Charge inversion mass spectrometry using alkali metal targets was found to provide a much clearer differentiation between the isomeric ions of unsaturated hydrocarbons than CAD.

Energy Loss Measurement in Collisions of H+ and D+ Ions with Alkali Metals Using Charge Inversion Mass Spectrometry.

Charge inversion mass spectrometry measures negative ions formed both by double-electron-transfers in one collision and by successive single-electron-transfers in two collisions. Mechanism for H- ion formation in collisions of H+ ions with alkali metals has been studied by developing calibration methods of translational energy losses along with measurements of target density dependences in the charge inversion mass spectrometry. Two peaks with different translational energies were observed in magnified spectra of the peak associated with H- ions with Cs and K targets. One of the two peaks with the higher translational energy had higher intensity and showed a quadratic dependence on target density, while the other with the lower translational energy had lower intensity and showed a linear dependence on the target density. The translational energy difference was calibrated both by monitoring the magnetic flux density and by changing the accelerating voltage. The translational energy differences between the two peaks were evaluated as 10.7±0.4 eV and 17.4±0.5 eV with Cs and K targets, respectively. The translational energy differences of H- ions coincided with those of D- ions for both Cs and K targets. For Na target, only one peak with the quadratic dependence on the target density was observed. From the energy difference and the linear dependence on target density, the peak with the lower translational energy was ascribed to double electron transfer forming H-(1s2) ions in one collision. Existence of a double electron transfer process forming H-(1s2) and excited Cs2+(5s, 5p5) was suggested for a weak peak with the energy loss of about 34 eV with Cs target. The peak with the higher translational energy and the quadratic dependence was ascribed to three-step process which was composed of neutralization forming H(2p), spontaneous Ly-α emission from H(2p) and anionization forming H-(1s2) ions from H(1s).

Fragmentation of the enolate ion of 2,3‐butanedione. Characterization of the acetyl anion by charge inversion mass spectrometry

AbstractThe unimolecular and collision‐induced fragmentation reactions of the enolate ion of 2,3‐butanedione, [CH3COCOCH2]−, have been studied, Unimolecular fragmentation on the metastable ion time‐scale forms [HCCO]−, [C2H3O]−, [C3H5O]− and [CH3CO2]−. Charge inversion mass spectrometry shows that the [C2H3O]− ion is the acetyl anion while the [C3H5O]− product is the acetone enolate ion; formation of the latter product involves a large release of kinetic energy (T 1/2 = 0.99 eV). The fragmentation reactions occurring following collisional activation have been determined for 8 keV collisions and over the range 1.5–30 eV center‐of‐mass collision energy. Formation of [HCCO]− and [CH3CO]− are of the most important reactions following collisional activation and it is concluded that the two reactions have similar critical reaction energies even though formation of [HCCO]− is favored thermochemically.