Alkali Metal Targets Research Articles

High Yield Organic Superconductors via Solution-Phase Alkali Metal Doping at Room Temperature.

Alkali metal doping is an essential process for developing organic superconductors. The conventional vapor-phase alkali metal doping, however, frequently suffers from low efficiency and poor reproducibility mainly due to the inhomogeneous reaction between alkali metal vapor and target organic molecule powder. To overcome this issue, here we developed a facile and highly reproducible solution-phase alkali metal doping (SPD) and successfully applied it to prepare potassium-doped fullerene (K3C60) superconductors. Different from the conventional vapor-phase alkali metal doping, the SPD method resulted in almost perfect diamagnetism with an unprecedented high shielding fraction (∼99.5%) with high reproducibility (>80%). It works well with popular commercially available solvents, like ammonia solution in THF, methylamine solution in THF, and even pure THF at room temperature. We believe that our highly facile and efficient SPD approach will be a great help for the finding of next-generation organic superconductors, especially searching for high-Tc organic superconductors.

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.

High-energy collision-activated and electron-transfer dissociation of gas-phase complexes of tryptophan with Na+, K+, and Ca2+

The structure and reactivity of gas-phase complexes of tryptophan (Trp) with Na+, K+, and Ca2+ were examined by high-energy collision-activated dissociation (CAD) and electron transfer dissociation (ETD) using alkali metal targets. In the CAD spectra of M+Trp (M = Na and K), neutral Trp loss was the primary dissociation pathway, and the product ion of collision-induced intracomplex electron transfer from the indole π ring of Trp to the alkali metal ion was observed, indicating a charge-solvated structure in which Trp is non-zwitterionic. The NH3 loss observed in the CAD spectrum of Ca2+Trp2 is ascribed to a CZ (mixed charge-solvated/zwitterionic)-type structure, in which one Trp is non-zwitterionic and the other Trp adopts a zwitterionic structure with an NH 3 + moiety. The H atom and NH3 losses observed in the ETD spectrum of Ca2+Trp2 indicate the formation of a hypervalent radical in the complex, R–NH3, via electron transfer from the alkali metal target to the NH 3 + group of the CZ-type structure. Ca2+ attachment to Trp cluster induces the zwitterionic structure of Trp in the gas phase, and an electron transfer to the zwitterionic Trp forms the hypervalent radical as a reaction intermediate.

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.

High-Energy Electron Transfer Dissociation (HE-ETD) Using Alkali Metal Targets for Sequence Analysis of Post-Translational Peptides

Post-translational modifications (PTMs) of proteins are important in the activation, localization, and regulation of protein function in vivo. The usefulness of electron capture dissociation (ECD) and electron-transfer dissociation (ETD) in tandem mass spectrometry (MS/MS) using low-energy (LE) trap type mass spectrometer is associated with no loss of a labile PTM group regarding peptide and protein sequencing. The experimental results of high-energy (HE) collision induced dissociation (CID) using the Xe and Cs targets and LE-ETD were compared for doubly-phosphorylated peptides TGFLT(p)EY(p)VATR (1). Although HE-CID using the Xe target did not provide information on the amino acid sequence, HE-CID using the Cs target provided all the z-type ions without loss of the phosphate groups as a result of HE-ETD process, while LE-ETD using fluoranthene anion gave only z-type ions from z(5) to z(11). The difference in the results of HE-CID between the Xe and Cs targets demonstrated that HE-ETD process with the Cs target took place much more dominantly than collisional activation. The difference between HE-ETD using Cs targets and LE-ETD using the anion demonstrated that mass discrimination was much weaker in the high-energy process. HE-ETD was also applied to three other phosphopeptides YGGMHRQEX(p)VDC (2: X = S, 3: X = T, 4: X = Y). The HE-CID spectra of the doubly-protonated phosphopeptides (= [M + 2H](2+)) of 2, 3, and 4 using the Cs target showed a very similar feature that the c-type ions from c(7) to c(11) and the z-type ions from z(7) to z(11) were formed via N-C alpha bond cleavage without a loss of the phosphate group.

Letter: High-Energy Electron Transfer Dissociation Using a Tandem Time-of-Flight Mass Spectrometer with an Electrospray Ionization Ion Source

A tandem time-of-flight (TOF) mass spectrometer with an electrospray ionization (ESI) ion source (ESI-TOF/quadTOF) was enhanced to perform high-energy electron transfer dissociation (ETD) experiments. Specifically, the collision cell for rare gas targets of ESI-TOF/quadTOF was replaced with one for alkali metal targets. High-energy ETD spectra were obtained of doubly protonated polypeptides and a phosphopeptide using a Cs target. Peaks due to several c- and z-type fragment ions formed by the high-energy ETD process were clearly observed in all cases. These results demonstrate that high-energy ETD can be performed using ESI-TOF/quadTOF.

高エネルギー電子移動解離が可能なタンデム質量分析装置の開発

A JEOL JMS-HX110 double-focusing mass spectrometer was combined with the collision cell and spherical electric sector of a HITACHI M80-B instrument to perform tandem mass spectrometry experiments. A quadrupole lens doublet was mounted between the mass spectrometer and the collision cell to improve ion transmission into the collision cell. The detection system was also improved to expand the mass range of product ions. These improvements provided reliable spectra of high-energy electron transfer dissociation (HE-ETD) of peptides using an alkali metal target. The present results demonstrated that HE-ETD by a single collision with an alkali metal target could determine the amino acid sequence and phosphorylation site of peptides.

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.

Differences between collisionally activated and electron‐transfer dissociations found for CH2X2(X = Cl, Br, and I) by using alkali‐metal targets

High-energy collisionally activated dissociation (HE-CAD) and high-energy electron- transfer dissociation (HE-ETD) on collisions with alkali-metal targets (Cs, K, and Na) were investigated for CH(2)X(2) (+) (X = Cl, Br, and I) ions by tandem mass spectrometry (MS/MS). In the HE-CAD spectra observed, peaks associated with CH(2)X(+) ions formed by a loss of a halogen atom are always predominant regardless of precursor ions and target metals. The observation of the predominant CH(2)X(+) ions is explained by the lowest energy levels of the fragments of CH(2)X(+) + X among the possible fragment energy levels and internal-energy distribution in HE-CAD. In the charge-inversion spectra, relative peak intensities of the negative ions formed by HE-ETD strongly depend on the precursor ions and the target metals. While the CHCl(2) (-) ion was predominant in the spectra of CH(2)Cl(2) (+) regardless of target species, the most intense peaks in those of CH(2)Br(2) (+) and CH(2)I(2) (+) were ascribed to either Br(-) or CH(2)Br(-) and either I(-) or I(2) (-), respectively, depending on the target metals. The dependence of the relative intensities of the fragment ions by HE-ETD on the precursor ions and target species are discussed on the basis of the energy levels of the neutral fragments and the narrow internal-energy distribution resulting from the near-resonant neutralization. It was demonstrated that HE-ETD using the alkali-metal targets provided rich information on the dissociation of the neutral species.

Study of the dissociation of a charge‐reduced phosphopeptide formed by electron transfer from an alkali metal target

Doubly protonated phosphopeptide (YGGMHRQET(p)VDC) ions obtained by electrospray ionization were collided with Xe and Cs targets to give singly and doubly charged positive ions via collision-induced dissociation (CID). The resulting ions were analyzed and detected by using an electrostatic analyzer (ESA). Whereas doubly charged fragment ions resulting from collisionally activated dissociation (CAD) were dominant in the CID spectrum with the Xe target, singly charged fragment ions resulting from electron transfer dissociation (ETD) were dominant in the CID spectrum with the Cs target. The most intense peak resulting from ETD was estimated to be associated with the charge-reduced ion with H2 lost from the precursor. Five c-type fragment ions with amino acid residues detached consecutively from the C-terminal were clearly observed without a loss of the phosphate group. These ions must be formed by N--Calpha bond cleavage, in a manner similar to the cases of electron capture dissociation (ECD) and ETD from negative ions. Although the accuracy in m/z of the CID spectra was about +/-1 Th because of the mass analysis using the ESA, it is supposed from the m/z values of the c-type ions that these ions were accompanied by the loss of a hydrogen atom. Four z-type (or y--NH3, or y--H2O) ions analogously detached consecutively from the N-terminal were also observed. The fragmentation processes took place within the time scale of 4.5 micros in the high-energy collision. The present results demonstrated that high-energy ETD with the alkali metal target allowed determination of the position of phosphorylation and the amino acid sequence of post-translational peptides.

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.

Potential crossing position in electron transfer of a doubly charged ion and an alkali metal target measured using thermometer molecule W(CO) 6

Doubly charged tungsten hexacarbonyl W(CO) 6 2+ ions were made to collide with K and Cs targets to give singly and doubly charged positive ions by collision-induced dissociation (CID). The internal energy depositions resulting from the electron transfer were evaluated from the relative abundances of the singly charged fragment W(CO) n + ions. The internal energy deposition resulting from the electron transfer with the Cs target was very narrow and centered at a particular energy, 8.2 eV above the energy level of the charge reduced W(CO) 6 + ion, which was higher than 7.2 eV with the K target. These internal energies correspond to energy differences of 2.9 and 3.5 eV between the entrance channel of W(CO) 6 2+ + Cs (or K) and the exit channel of W(CO) 6 +* + Cs + (or K +), respectively. The potential energy between W(CO) 6 2+ ions and the alkali metal atoms was found to decrease markedly in the entrance channel of the electron transfer as a result of the large polarizability of the alkali metal targets. The internuclear separation of the Landau–Zener potential crossing of the electron transfer between a W(CO) 6 2+ ion and the Cs target was evaluated as 6.8 × 10 −8 cm, which was larger than the value for the K target (5.9 × 10 −8 cm). Large cross sections for the electron transfer of the order of 10 −14 cm 2, estimated from the internuclear distances, indicate that the electron capture of the doubly charged ion on collisions with an alkali metal targets is a very effective process for producing charge-reduced ions. The difference in the cross sections between K and Cs target indicates that a target having lower ionization energy is more effective for electron transfer.

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.

Differences between the internal energy depositions induced by collisional activation and by electron transfer of W(CO)62+ ions on collision with Ar and K targets

Doubly charged tungsten hexacarbonyl W(CO)(6) (2+) ions were made to collide with Ar and K targets to give singly and doubly charged positive ions by collision-induced dissociation (CID). The resulting ions were analyzed and detected by using a spherical electrostatic analyzer. Whereas the doubly charged fragment ions resulting from collisional activation (CA) were dominant with the Ar target, singly charged fragment ions resulting from electron transfer were dominant with the K target. The internal energy deposition in collisionally activated dissociation (CAD) evaluated with the Ar target was broad and decreased with increasing internal energy. The predominant peaks observed with the K target were associated with singly charged W(CO)(2) (+) and W(CO)(3) (+) ions: these ions were not the result of CA, but arose from dissociation induced by electron transfer (DIET). The internal energy deposition resulting from the electron transfer was very narrow and centered at a particular energy, 7.8 eV below the energy level of the W(CO)(6) (2+) ion. This narrow internal energy distribution was explained in terms of electron transfer by Landau-Zener potential crossing at a separation of 5.9 x 10(-8) cm between a W(CO)(6) (2+) ion and a K atom, and the coulombic repulsion between singly charged ions in the exit channel. A large cross section of 1.1 x 10(-14) cm(2) was estimated for electron capture of the doubly charged W(CO)(6) (2+) ion from the alkali metal target, whose ionization energy is very low. The term "collision-induced dissociation," taken literally, includes all dissociation processes induced by collision, and therefore encompasses both CAD and DIET processes in the present work. Although the terms CID and CAD have been defined similarly, we would like to propose that they should not be used interchangeably, on the basis that there are differences in the observed ions and in their intensities with Ar and K targets.

S-wave elastic scattering of antihydrogen off atomic alkali-metal targets

We have investigated the $s$-wave elastic scattering of antihydrogen atoms off atomic alkali-metal targets (Li, Na, K, and Rb) at thermal energies $({10}^{\ensuremath{-}16}--{10}^{\ensuremath{-}4}\phantom{\rule{0.3em}{0ex}}\mathrm{a.u.})$ using an atomic orbital expansion technique. The elastic cross sections of these systems at thermal energies are found to be very high compared to $\overline{\mathrm{H}}\text{\ensuremath{-}}\mathrm{H}$ and $\overline{\mathrm{H}}\text{\ensuremath{-}}\mathrm{He}$ systems. The theoretical models employed in this study are so chosen to consider long-range forces dynamically in the calculation. The mechanism of cooling suggests that Li may be considered to be a good candidate as a buffer gas for enhanced cooling of antihydrogen atoms to ultracold temperature.

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.

Comparison of dissociation mechanism between collisionally activated dissociation and charge inversion using alkali metal targets for chlorophenol isomers

Chlorinated aromatic compounds are well-known environmental pollutants whose toxicities depend dramatically on the chlorine substitution pattern, making differentiation of chlorophenol isomers important for environmental analysis. Collisionally activated dissociation (CAD) spectra and charge inversion spectra of ortho-, meta-, and para-chlorophenols (ClC 6H 4OH) and their partially deuterated forms (ClC 6H 4OD) were measured using alkali metal targets. The peaks associated with C 6H 4O + and C 5H 5Cl + ions observed in the CAD spectra result from the loss of HCl and CO fragments, respectively, after the re-arrangement of the hydroxyl hydrogen atom. The peaks associated with C 6H 4OH − and ClC 6H 4O − ions observed in the charge inversion spectra result from Cl loss and from hydroxyl bond dissociation, respectively. Isomeric differentiation is possible based on the clear differences observed in the relative intensities of these pairs of peaks. Although the intensities of the peaks associated with C 6H 4O + relative to those of C 5H 5Cl + in the CAD spectra are independent of the target species, the intensities of the peaks associated with C 6H 4OH − relative to those of ClC 6H 4O − in the charge inversion spectra are target dependent. The isomeric dependence of the positive ion distribution patterns in the CAD spectra is proposed to be due to the differences in the rate of the hydrogen atom re-arrangement process. In contrast, the isomeric dependence of the negative ion distribution patterns in the charge inversion spectra is attributed to differences in the bond strength involved in the direct dissociation process in the neutral intermediate species.

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.