The importance of charge-separation reactions in tandem mass spectrometry of doubly protonated angiotensin II formed by electrospray ionization: Experimental considerations and structural implications

The occurrence of charge-separation reactions in tandem mass spectrometry of doubly protonated angiotensin II is demonstrated by the use of mass-analyzed ion kinetic energy spectrometry (MIKES) and kinetic energy release distributions (KERDs). Linked scans at a constant B/E severely discriminate against product ions formed by charge-separation reactions. Although the products are significantly more abundant in MIKES experiments, instrumental discrimination still makes quantitation of relative product ion abundances highly inaccurate. The most probable KERs (T _{m. p.}) and the average KERs (T _ave.) of the reactions are determined from the KERDs, and these values are compared to the KERs determined from the peak widths at half-height (T _{0. 5}). The measurement of T _{0. 5} is a poor approximation to T _{m. p.} and T _ave.. The T _{m. p.} is used to calculate a most probable intercharge distance, which is compared to results from molecular dynamics calculations. The results provide evidence with regard to the mechanisms of fragmentation of multiply charged ions and the location of the charge site in relation to the decomposition reactions.     

Gas-phase ion chemistry and organic chemistry-the story of a hybrid six sector mass spectrometer--the "AutoSpec 6F"

The AutoSpec 6F mass spectrometer is a large, floor standing instrument comprising a pair of commercial EBE geometry (AutoSpec) mass spectrometers coupled in series to provide an hybrid EBE-EBE configuration, (E and B being respectively electrostatic and magnetic sectors.) It was designed in close collaboration between Professor R. Flammang and VG Analytical in Manchester, UK. It was equipped with five collision cells and allowed the recording of high energy CID (collision induced dissociation), MIKES (mass analyzed ion kinetic energy spectrometry) and NRMS (neutralization re-ionization mass spectrometry) data as well as consecutive MSn analyses. The field-free regions between sectors allowed the study of unimolecular decomposition products from long-lived metastable ions. The mass spectrometer became even more versatile when an RF-only quadrupole collision cell was installed between the second and the third electric sector. This allowed the study of associative ion/molecule reactions in the low kinetic energy regime. Bimolecular chemical reactions were performed inside the quadrupole cell when a neutral reagent was introduced and the reaction products were analyzed by high energy CID in the downstream sectors. This paper tells the history and summarizes the capabilities of this versatile instrument.     

双电荷离子[C12H12N2O]2+和[C12H12N2S]2+的气相单分子分解反应研究

The unimolecular dissociation reactions of doubly charged ions were reported,which resulted from a tandem mass spectrometer and a reversed geometry double focusing mass spectrometer by electron impact.Mass analyzed ion kinetic energy spectrometry(MIKES) was used to obtain the kinetic energy releases in charge separation reactions of doubly charged ions.The intercharge distances between the two charges at transition states can be calculated from the kinetic energy releases.Transition structures of unimolecular dissociation reactions were infered from MIKES and MS/MS.     

双电荷离子［C_（12）H_（12）N_2O］~（2＋）和［C_（12）H_（12

双电荷离子［Ｃ_（１２）Ｈ_（１２）Ｎ_２Ｏ］~（２＋）和［Ｃ_（１２）Ｈ_（１２）Ｎ_２Ｓ］~（２＋）的气相单分子分解反应研究任达，贾维平，李智立，刘淑莹（中国科学院长春应用化学研究所，长春，１３００２２）关键词双电荷离子，质量分析离子动能谱，串联质谱，４...     

一些双电荷离子的气相碎裂反应

借助质量分析离子动能谱和串联质谱研究了由电子轰击产生的双电荷离子的单分子亚稳碎裂及碰撞诱导分解过程,讨论了两种实验方法导致的差别因素.此外,根据质量分析离子动能谱提供的双电荷离子电荷分离反应的动能释放值计算了两电荷中心间距的最小值,以判别按不同电荷分离方式碎裂的双电荷离子的过渡态结构.     

Sequence Analysis of Polypeptide by Mass Spectrometry

The method described in this work provides a sensitive and fast technique for investigating the primary structure of peptides with molecular weight up to 3340 amu. Usually, the metastable ion kinetic energy spectra (MIKES) and collisional activated decomposition (CAD) spectra provide complementary information for the FAB mass spectra, the MIKES and CAD spectra generally contain high-mass sequence ions.     

Metastable ion Characteristics. XXXXIII—separation and comparison of collisional activation spectra and metastable ion spectra

Additional ion acceleration in a separate collision chamber in the field free drift region makes it possible to add extra kinetic energy to those ions undergoing decomposition in the chamber. In this way product ions formed in the collision chamber (the [collisional activation] spectrum) can be distinguished from ions formed from decompositions in the low pressure portion of the field free drift region (the [metastable ion] spectrum). This technique is used to show that often a relatively small proportion of the ions in the [pure] collisional activation spectrum arises from processes which produce the metastable ion spectrum.     

Kinetic energy release in ionic fragmentations: Use as an ion structure probe

The kinetic energy released in unimolecular reactions, as measured from the width of the corresponding metastable peak, shows only a small dependence on such parameters as source temperature, ion-source residence time and ion acceleration voltage. Similarly, fragmenting ions generated from different members of an homologous series of molecular ions have been found to release the same kinetic energy and hence do not exhibit a degrees-of-freedom effect analogous to that for metastable abundances. In general, molecular ions formed by electron-impact have been found to release slightly less kinetic energy on fragmentation than the corresponding ions formed via a fragmentation sequence. These observations suggest that kinetic energy release is a useful method of structural characterization of metastable ions; while increase in the average internal energies of the ions sample lead to larger energy releases, this effect is usually small. The use of a very narrow energy resolving (β) slit and a procedure in which the metastable peak width is extrapolated to zero slit width has been found to improve the accuracy of measurement of the kinetic energy release, particularly when the metastable and main beam peak widths are of comparable magnitude.     

Collision-induced dissociation of the lower alcohols. A thermochemical study

Collision-induced dissociation of the molecular ions and of some of the fragment ions foremed on ionization of methanol, ethanol and n-propnol have been studied at high energy resolution in a mass spectrometer. The translational energy lost upon collision and the kinetic energy releasedupon fragmentation of the collisionally excited species have been measured by the methods of ion kinetic energy spectromety. The results of the translational energy loss measurements compare well with excitation energies predicted for each reaction form breakdown curves showing the relative abundances of the ionsas a fuction of internal energy. This correspondence is evidebce that the ionic reactions following electron-impact excitation and those following collisional excitation with neutral molecules at relative traslational energies in the range of several kilovolts are at least qualitatively independent of the method of excitation. The occurrence of the corresponding spontaneous fragmentations in the alcohols has also been studied and the kinetic energy releases accompanying these reactions have been determined. In a few cases, metastable peaks were observed which did not increase in intensity when collision gas added and this phenomenon is associated with particular features of the reaction thermochemistry. Reactions which generally occur to very small extents in mass spectromety, such as methylene elimination, have been observed in highly excited ions. The methods developed in this study allow the decription of the thermochemistry of the reaction of highly excited ions, indlcuding the experimental determination of the partitioning of the nonfixed energy of the activated complex. This procedure complemets and extends energy partitioning studies made on metastable ions in which the partitioning of the reverse activation energy is the more readily accessible.     

The fragmentation of metastable NH+3 ions and isotopic analogs: an example of tunneling through a rotational barrier

The fragmentation of metastable NH⁺₃ ions and isotopic analogs via the reaction NH⁺₃ → NH⁺₂ + H has been investigated using mass analysed ion kinetic energy spectrometry (MIKES). Kinetic energy release distributions and the metastable intensity were measured as a function of ion source temperature. Both the average kinetic energy release and the metastable intensity increase with ion source temperature. The data are consistent with the metastable reaction arising from tunneling through a rotational barrier. The experimental data are compared with the predictions of a tunneling model.     

The ion kinetic energy spectra of some aromatic hydrocarbons

The technique of ion kinetic energy spectroscopy has been applied to the study of the aromatic hydrocarbons benzene, toluene, naphthalene, 2-methyl naphthalene, biphenyl and anthracene. The method is illustrated by a complete study of naphthalene in which transitions of metastable doubly- and singly-charged ions are listed, including reactions in which singly-charged ions are formed by collision induced charge-exchange reactions of doubly-charged ions and by the double process of charge-exchange and metastable decomposition with loss of one or two hydrogen atoms. Decompositions of doubly-charged ions into two singly-charged ions, together with the kinetic energies released in these decompositions, are also given for all the compounds studied.     

Determination of high-energy fragmentation of protonated peptides using a beqq hybrid mass spectrometer

A hybrid tandem instrument of BEqQ geometry was used to determine high-energy decomposition of protonated peptides, such as side-chain fragmentation yielding d _n and w _n ions. The transmission through both E and Q of such product ions, formed in the second field-free region, permits improved mass resolution and confident mass assignment. The experimental technique may involve synchronous scanning of E and Q, or, for the purpose of identification of specific products, limited-range scanning of either E or Q with the other analyzer fixed. These techniques are not equivalent, with respect to product ion transmission, to the double focusing of product ions achieved with four-sector instruments but nevertheless represent a critical improvement over conventional mass-analyzed ion kinetic energy spectrometry analyses. Fragmentation of protonated peptides occurring in the second field-free region inside and outside the collision cell were distinguished by floating the collision cell above ground potential. Mass filtering using Q confirmed the mass assignments. The data indicate that product ions resulting from spontaneous decomposition are in some instances quantitatively more significant than those resulting from high-energy collisional activation. Furthermore, the differentiation of the products of low- and high-energy processes should facilitate spectral interpretation.     

Origin of the tailing signal on the low-energy side of the main beam in mass-analyzed ion kinetic energy spectra

The tailing signal on the low-energy side of the precursor ion signal observed during fast atom bombardment (FAB) mass-analyzed ion kinetic energy spectrometric (MIKES) analyses is due largely to ions of higher m/z value than the chosen precursor. The majority of these ions are independent, unfragmented species that emerge from the ion source with less than the full amount of kinetic energy predicted by the source potential. The tailing precursor ion signal observed under helium collision-activated decomposition conditions is too short to account for the protracted MIKES tail (as judged from mass-to-charge ratio-deconvoluted MIKES analyses performed on a BEqQ hybrid instrument), and a tailing precursor signal is not observed under unimolecular decomposition conditions. Measurements of the mass-to-charge ratios of the ionic species comprising the MIKES tail demonstrated that ions higher in mass-to-charge ratio than the chosen precursor are present throughout the tail, with the mass-to-charge ratio increasing as kinetic energy decreases. These ions possess the same momentum as the chosen precursor, and thus were formed prior to the magnetic field. The existence of intact, source-formed [M + H]⁺ ions with reduced kinetic energy was demonstrated through several types of tandem mass spectrometric experiments. These [M + H]⁺ ions with reduced kinetic energy do not appear to have undergone collisional deceleration, because they do not possess increased internal energy (as judged by observation of their fragmentation patterns). The kinetic energy profiles of unfragmented FAB-desorbed ions were determined and found to exhibit a tailing character similar in appearance to that of the MIKES tail. The population of ions emerging from the source under FAB conditions thus incorporates the characteristics necessary to account for the MIKES tail, namely, the presence of ions of a mass-to-charge ratio higher than the chosen precursor (due to matrix and other background ions), which possess reduced kinetic energy such that their momentum is identical to that of the selected precursor. These ions may arise via desolvation and declustering processes in the acceleration region of the ion source, or via FAB or chemical ionization processes in regions removed from the FAB target.     

Consecutive reactions in the vitamin D series. A New insight into the mechanism of vitamin D and provitamin D isomerizations in the mass spectrometer

The individual steps of the consecutive reactions arising from metastable molecular ions, derived from vitamin D₃, vitamin D₂ and their respective provitamins (7-dehydrocholesterol, ergosterol), were examined in different field-free regions of a triple-sector mass spectrometer of B/E/E geometry. The comparison of the translational energy release (T) and the metastable peak shapes corresponding to these reactions, as well as unimolecular and collision-induced dissociation mass-analysed ion kinetic energy spectra, showed that there are probably two structures of the [M – H₂O]⁺˙ and [M – CH₃˙]⁺ ions depending upon whether the respective ions are formed in the ion source through high-energy reactions, or from the fragmentation of metastable molecular ions through slow, low-energy processes which occur in the first field-free region.     

Parent ion resolution in linked scans for dissociations occurring in the first field-free region of sector mass spectrometers

A frequently employed means of studying reactions of ions under high energy collisional activation conditions involves linked scanning a double-focusing mass spectrometer to detect product ions formed in the first field-free region. Experiments have demonstrated that the effective resolution of the parent ion varies during the course of a product ion scan, and this variation is attributed to the kinetic energy released during dissociation. Carefully generated Lacey-Macdonald ion intensity diagrams accurately reflect the dissociation events occurring in various regions of sector mass spectrometers. Consideration of such diagrams leads to an understanding of the noted phenomena and allows the derivation of an expression for the effective parent ion resolution. Furthermore, the approach predicts that parent ion resolution varies during constant neutral loss scans and that the relatively poor resolution (and its variation) can provide particularly misleading data. Results from product ion scans with stable isotope-labeled compounds and constant neutral loss scans with rubidium bromide and stable isotope-labeled peptide derivatives were consistent with the theoretical predictions     

The modern mass spectrometer—a complete chemical laboratory

Using nitrobenzene as an example, various ways in which a contemporary mass spectrometer can be utilized to yield a wealth of information about the compound studied are reviewed. Applying a variety of different techniques and procedures, in addition to the conventional low resolution mass spectrum, the following nitrobenzene spectra have been obtained: collision induced dissociation mass spectrum, mass analysed ion kinetic energy spectra, collision induced dissociation mass analysed ion kinetic energy spectra, spectra obtained at constant B/E, spectra obtained at constant B²/E, high voltage scans of metastable ion fragmentation processes, consecutive fragmentations in different field free regions, charge exchange mass spectra, charge stripping mass spectra, doubly charged ion mass spectra, chemical ionization mass spectra, negative ion mass spectra, negative ion mass analysed ion kinetic energy spectra, negative ion mass analysed ion kinetic energy collision induced dissociation spectra, charge inversion spectra, etc. The complementary types of information available from the above studies are discussed to show the unique versatility of mass spectrometry as a technique for the examination of organic compounds.     

气相二茂铁双电荷离子的动能谱研究

The unimolecular charge separation reactions of the doubly charged ions FeC_(10)H_(10)~(2+)、FeC_(10)H_9~(2+)、FeC_(10)H_8~(2+) produced in the ion source by electron impact from ferrocene have been studied using Mass analyzed Ion Kinetic Energy Spectrometry (MIKES) technique. From the values of the kinetic energy releases (T), the intercharge distances (R) of the exploding doubly charged ions in their transition structures have been estimated and some structural informations about the transition states can be obtained. The collision induced reactions of the FeC_(10)H_(10)~(2+) ion with Ar have been studied using MIKES, we postulate a new type of continuing reaction which may be "collisional charge separation induced dissociation".     

Collision Energetics in a Tandem Time-of-Flight (TOF/TOF) Mass Spectrometer with a Curved-Field Reflectron

Collisions of fullerene ions (C(60) (+)) with helium and neon were carried out over a range of laboratory energies (3-20 keV) on a unique tandem time-of-flight (TOF/TOF) mass spectrometer equipped with a curved-field reflectron (CFR). The CFR enables focusing of product ions over a wide kinetic energy range. Thus, ions extracted from a laser desorption/ionization (LDI) source are not decelerated prior to collision, and collision energies in the laboratory frame are determined by the source extraction voltages. Comparison of product ion mass spectra obtained following collisions with inert gases show a time (and apparent mass) shift for product ions relative to those observed in spectra obtained by metastable dissociation (unimolecular decay), consistent with impulse collision models, in which interactions of helium with fullerene in the high energy range are primarily with a single carbon atom. In addition, within a narrow range of kinetic energies an additional peak corresponding to the capture of helium is observed for fragment ions C(50) (+), C(52) (+), C(54) (+), C(56) (+) and C(58) (+).     

Deconvolution of composite metastable peaks: A new method for the determination of metastable transitions occurring in the first field free region

The masses of daughter ions resulting from metastable transitions in the first field free region of a “reversed geometry” mass spectrometer can be accurately determined through a mass analysed ion kinetic energy scan at the apparent mass at which these daughter ions travel through the magnetic sector. A combination of these mass analysed ion kinetic scans with scanning the magnetic flux allows the resolution of overlapping metastable diffuse peaks.     

The scope of metastable peak observations

This short review focuses attention upon the present status of metastable ion studies with emphasis upon the relationship between metastable peak shapes, ion structur and fragmentation mechanisms. Some recommendations are made concerning nomenclature and the reporting of observations on Gaussian-type metastable peaks. Experimental methods for recording relative abundances of metastable peaks are critically appraised. The relationship between metastable ion phenomena and isomerization of gaseous ions is reviewed with particular attention drawn to the effect of rate-determining isomerizations. The shapes of Gaussian-type metastable peaks are discussed in some detail and selected examples from recent studies are used to show that such peaks may, by appropriate experiments, be separated into two Gaussian-type components thus revealing new features of the fragmentation reaction. The magnitude and significance of released kinetic energies, T, are considered and it is stated that few conclusions can be drawn from the evaluation of T alone; the importance of accurate thermochemical data as an aid to understanding and interpreting kinetic energy release data is emphasized. Other topics discussed include composite metastable peaks, metastable peaks produced in chemical ionization and field ionization and the partitioning of internal energy of the fragmenting ion into translational degrees of freedom of the products, for reactions with and without a reverse energy barrier.