Metastable Ion Decay Research Articles

Quantum Chemical Analysis of Molecular and Fragment Ions Produced by Field Ionization of Methyl Stearate.

The formation of molecular and fragment ions observed in the field ionization mass spectrum of methyl stearate has been analyzed on the basis of quantum chemical calculations including time-dependent density functional theory (TDDFT) and natural bond orbital (NBO) analysis. The TDDFT calculations suggest that methyl stearate is ionized via two processes, namely a 7.43 eV excitation and a tunneling effect, while the high electric field of 1010 V/m enables analyte molecules to ionize at an effective 6 eV lower than the 9.26 eV ionization energy. The NBO analysis suggests that the abundances of aliphatic fragment ions [CnH2n+1]+ at m/z 29, 43, and 57 generated in the ionizing cell can be rationalized by hyperconjugation between the sigma (σ)-electrons of sp3 C-H bonds of methyl or methylene groups and the empty p-orbital of the carbocation -CH2+. The C4 periodic methyl ester fragment ions at m/z 115-269 and the complementary McLafferty rearrangement fragment ion at m/z 224 can be explained by metastable ion decay with rearrangement reactions in the ion source.

Fragmentation studies on metastable diethylaniline derivatives using mass-analyzed ion kinetic energy spectrometry.

It is well known that small substituted aromatic amines lose their substituents after electron ionization in a multistep fragmentation mechanism. In this contribution, the fragmentation reactions of N,N-diethyl-aniline and different methyl-substituted N,N-diethyl-methyl-aniline isomers are investigated with respect to the position of the methyl group attached to the aromatic ring system. Metastable ion decay reveals a complicated fragmentation mechanism leading to an interplay of the aromatic methyl group and the ethyl substituents at the amine function. A small change in the substitution leads to a significant change of the observed fragments indicating a participation of o-methyl groups in the fragmentation mechanism of the diethyl amino side chain. Because the underlying mechanisms are not fully understood, the presented investigations deliver through 13C isotopic labeling a deep insight into the hidden rearrangement and fragmentation mechanisms in these compounds.

Study of fragmentation pathways of metastable negative ions in aliphatic dipeptides using the statistical theory

Dipeptide molecules are good model systems for investigation of resonant reactions of low-energy electrons with proteins. The present work is devoted to the study of the processes of formation and fragmentation of negative ions in aliphatic dipeptides glycyl-glycine and glycyl-alanine. The metastable decays of negative ions were detected in these objects, and have been investigated with the aim of clarification of the mechanisms of fragmentation. The effective yield curves for negative ions as functions of electron energy were obtained using a magnetic sector mass spectrometer rebuilt for generation and detection of negative ions. For analysis of the observed metastable decays statistical and thermochemical approaches have been used. The ions structures, the enthalpies of formation of neutral and charged particles, and the rate constants of dissociative reactions have been found. Comparison of the experimental results with theoretical data leads to the conclusion that metastable ion decay proceeds with minimal structural changes avoiding complicated rearrangements and isomerization processes.

Absorption spectrum of isolated tris(2,2′-bipyridine)ruthenium(II) dications in vacuo

Here we report the gas-phase action spectrum of tris(2,2′-bypyridine)ruthenium(II) dications, Ru(bipy) 3 2+, recorded over a broad wavelength region from 210 nm to 650 nm. Ions formed by electrospray ionization were thermalized in collisions with helium buffer gas at room temperature in a 22-pole ion trap. A bunch of ions was then accelerated to keV-kinetic energies and injected into an electrostatic ion storage ring in which they circulated with a revolution time of 69 microseconds. After several milliseconds to allow for the decay of metastable ions, the ions were photoexcited and their decay monitored in time. Dissociation was found to occur on the microsecond time scale. Based on the time spectra, the initial number of photoexcited ions was estimated at each wavelength (relative and not absolute numbers). This approach circumvents the problem of kinetic shifts often encountered when dissociation is sampled within a finite time interval in the case of finite length time-of-flight instruments. The obtained absorption spectrum is found to be similar to the solution phase spectrum (acetonitrile solvent) but with a blueshift of the lowest energy metal-to-ligand charge transfer transition. This finding indicates a localized electronic transition in solution involving only one of the bipyridine ligands. There are small differences in the relative intensities of the bands between the gas-phase and solution-phase spectra. Finally, our gas-phase spectrum serves as a reference to benchmark theoretical calculations of excited states.

Wavelength dependence on the level of post-source metastable ion decay observed in infrared matrix-assisted laser desorption ionization

The levels of post-source metastable ion decay (PSD) observed in several peptides and proteins ionized by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI TOF-MS) are measured utilizing both infrared (IR) and ultraviolet (UV) desorption wavelengths. A gridless deceleration ion optic is employed to temporally separate stable analyte ions from analyte metastable neutral and ion fragments. Comparisons of the extent of PSD that is observed in UV-MALDI at 337 nm and IR-MALDI at multiple wavelengths between 2.8 and 3.0 μm are made using the same matrices and analytes. The amount of PSD observed using IR-MALDI was found to be highly dependent on the specific IR wavelength (2.8–3.0 μm) employed for desorption. IR wavelengths shorter than 2.86 μm tended to produce higher levels of PSD, while longer IR wavelengths typically produced significantly less PSD when using a number of common MALDI matrices. Relative PSD levels are quantified by determining the percentage of the neutral fragment signal intensity to the intensity of the stable singly protonated molecular species observed in decelerated MALDI spectra. These studies suggest that an analyte ion activation pathway leading to significant PSD in IR-MALDI may proceed by way of vibrational excitation of the analyte molecules during the desorption event.

Statistical description of metastable negative ions’ decay

The decay of metastable negative ions resulted from the resonant electron capture by the molecules of acridanoneacetic acid benzyl ester has been studied using the negative ions’ mass spectrometry technique. The simple model of metastable decay has been developed on the basis of the statistical Rice–Ramsperger–Kassel–Markus theory (RRKM). Based on the comparative analysis of the experimental and calculated effective yield curves of ions the shape of the internal energy distribution of molecular negative ions has been estimated.

Storage of negative carbon ions in an electrostatic ring

Negative carbon atomic ions and cluster ions were stored in an electrostatic ion storage ring. The lifetime of the metastable atomic ion is heavily affected by the blackbody radiation from the surrounding environment. By introducing rare gas target into the ring, collision-induced detachment cross sections of the atomic ion C- were obtained. For cluster ions Cn- (n = 2 ∼ 8), decay of metastable ions with sub-millisecond to millisecond lifetime was commonly observed.

Electron-impact ionization of benzoic acid, nicotinic acid and their n-butyl esters: An approach to regioselective proton affinities derived from ionization and appearance energy data

Electron-impact ionization mass spectra, the decay of metastable ions, ionization and appearance energies and bond energies, as dissociation energies, are reported for the title compounds. An ionization energy of 9.47 eV was obtained for benzoic acid, 9.43 eV for benzoic acid n-butyl ester, 9.61 eV for nicotinic acid and 9.97 eV for nicotinic acid n-butyl ester. Molecular ions of both butyl esters show two common main fragmentation pathways: the first process is a McLafferty rearrangement, characterized by the transfer of one H-atom from the aliphatic ester chain, which leads to the ions of either the organic acid or 1-butene. From their appearance energies and known thermodynamic data, gas-phase formation enthalpies ( Δ H f 0 ) of the parent n-butyl esters are calculated. Values of Δ H f 0 = ( − 4.29 ± 0.3 ) eV for benzoic acid n-butyl ester and Δ H f 0 = ( − 3.66 ± 0.3 ) eV for nicotinic acid n-butyl ester were obtained. The second process is characterized by the transfer of two H-atoms from the ester chain leading to a protonated form of the corresponding organic acids and C 4H 7 radicals. Good evidence is provided for the formation of methylallyl radicals. Appearance energies are used to calculate a proton affinity (PA) for benzoic acid. The obtained value of PA = (8.73 ± 0.3) eV, corresponding to a protonation of the carbonyl group, is in close corroboration with published data (PA = 8.51 eV). Activation energies for the intermediate H-transfers were found to be insignificant. This methodic gateway is applied to the system of nicotinic acid and its butyl ester. Adopting the formation of a methylallyl radical, the obtained proton affinity of nicotinic acid, PA = 8.58 eV, is very near to the published data of benzoic acid. An alternative fragmentation mechanism leading to a value of PA ≈ 9.5 eV (typical for a protonation of the pyridine-nitrogen) is very unlikely. It is concluded that this transfer of two H-atoms from the ester chain is controlled by a charge switching between the carboxylic oxygen atoms which leads to a regiospecific protonation site, in this case to the protonated carbonyl group. This is conform with a B3LYP DFT calculation with a corresponding proton affinity of PA = 8.29 eV.

Electron-Impact Ionization of Mandelic Acid and Mandelic Acid Methyl Ester as Prototypes for the C6H5CH(OH)-X System: Ionization and Appearance Energies, Activation Energies and Enthalpies of Formation

Electron-impact ionization mass spectra, the decay of metastable ions, ionization and appearance energies and bond energies, as dissociation energies, are reported for mandelic acid (MA) and mandelic acid methyl ester (MAME) . An ionization energy of 8.55 eV was obtained for both compounds. For the formation of C6H5+ ions, appearance energies of 14.25 eV or 14.40 eV were determined for MA or MAME, respectively. On the main fragmentation route from molecular ions to these C6H5+ ions, an activation energy of (1.1±0.1)eV in excess of the calculated total reaction enthalpy was found for the elimination of CO from the prominent C7H7O+ ions to C6H7+ ions, involving the transfer of two H-atoms. The second (minor) fragmentation route via C7H6O+ions resembles the benzaldehyde decomposition pathway and the individual steps were found to be near to their calculated thermochemical reaction enthalpies. Gas-phase enthalpies of formation of (–4.652±0.3 eV) [≡ (–448.8±30) kJ mol−1] for MA and (–4.243±0.3) eV [≡ (–409.4±30) kJ mol−1] for MAME are derived, based on the appearance energy of their C7H7O+ ions at m/z 107 and the formation enthalpy adopted from protonated benzaldehyde. The results are compared with data for benzylalcohol.

The combination of an electrospray ion source and an electrostatic storage ring for lifetime and spectroscopy experiments on biomolecules

An electrospray ion source has been coupled to an accelerator that injects ions into an electrostatic heavy-ion storage ring. Since the dc ion current produced by electrospray ionization is low (∼106 ions/s), ions are accumulated in a cylindrical ion trap filled with a helium buffer gas. The ions are collisionally damped in the buffer gas and confined to the central trap region by a rf field. Extraction from the trap occurs within a few microseconds and after acceleration through 22 kV, the ions of interest are selected by a magnet according to their mass to charge ratio. The ion bunch is subsequently injected into the ring. Both positive and negative ions have been stored, with masses ranging over 3 orders of magnitude (∼102–104 Da). From a pickup signal in the ring, the number of ions in a bunch is estimated to be of the order of 103–104 when the accumulation time is 0.1 s. Our first measurements show that we can store a sufficient number of ions to study the decay of metastable ions and to determine relative destruction cross sections. The technique could be useful to probe conformers differing only in size. Furthermore, our setup can be used for spectroscopic measurements of the ion-photon interaction such as the excitation of [Cytochrome c+17H]17+ protein ions with 532 nm photons.

Ion chemistry of anti-o,o dibenzene.

The ion chemistry of anti-o,o'-dibenzene (1) was examined in the gaseous and the condensed phase. From a series of comparative ion cyclotron resonance (ICR) mass spectrometry experiments which involved the interaction of Cu+ with 1, benzene, or mixtures of both, it was demonstrated that 1 can be brought into the gas phase as an intact molecule under the experimental conditions employed. The molecular ions, formally 1*+ and 1*- , were investigated with a four-sector mass spectrometer in metastable-ion decay, collisional activation, charge reversal, and neutralization-reionization experiments. Surprisingly, the expected retrocyclization to yield two benzene molecules was not dominant for the long-lived molecular ions; however, other fragmentations, such as methyl and hydrogen losses, prevailed. In contrast, matrix ionization of 1 in freon (77 K) by gamma-radiation or in argon (12 K) by X-irradiation leads to quantitative retrocyclization to the cationic dimer of benzene, 2*+. Theoretical modeling of the potential-energy surface for the retrocyclization shows that only a small, if any, activation barrier is to be expected for this process. In another series of experiments, metal complexes of 1 were investigated. 1/Cr+ was formed in the ion source and examined by metastable ion decay and collisional activation experiments, which revealed predominant losses of neutral benzene. Nevertheless, comparison with the bis-ligated [(C6H6)2Cr]+ complex provided evidence for the existence of an intact 1/Cr+ under these experimental conditions. No evidence for the existence of 1/Fe+ was obtained, which suggests that iron mediates the rapid retrocyclization of 1/Fe+ into the bis-ligated benzene complex [(C6H6)2Fe]+.

Static SIMS: metastable decay and peak intensities

The decay of a metastable ion to a daughter ion along the flight path of a time-of-flight (ToF) mass spectrometer leads to well-defined peak structure in the mass spectrum. Through interference, these daughter ion peaks can reduce the detection limits in static SIMS and lead to uncertainties in both the true peak area intensity and the peak position. The area of the peak for the metastable parent ion is reduced to an extent which depends on its half life, the analyser design and the chosen instrument settings. These intensity changes directly affect the quality and reproducibility of spectra. The decay of metastable ions is analysed for the reference material, PTFE, used in a recent inter-laboratory study. A method is developed, for a ToF reflectron analyser, to characterise the decay process of the parent ion so that both parent and daughter are accurately identified. This method involves measuring the transit time shift of the metastable peak as a function of the reflectron voltage. To illustrate the method, the mass of the C 4F 4 + parent ion is determined to an uncertainty of 0.6 amu. This parent ion decays by emission of CF 2 0 to C 3F 2 +. The choice of instrument parameters to avoid interference from metastable peaks and to improve data transferability is discussed.

Identifying proteins using matrix-assisted laser desorption/ionization in-source fragmentation data combined with database searching.

Metastable ion decay in matrix-assisted laser desorption/ionization (MALDI) has become a routine method for obtaining primary structures of peptides. Significant fragmentation occurs in the MALDI ion source and can be observed via delayed ion extraction TOF-MS. In-source decay (ISD) can provide C- and N-terminal primary sequence data for even moderate-sized peptides (< 5000 Da). The unique cn series fragmentation that occurs in ISD has been exploited to obtain partial C-terminal sequences for proteins as large as human apotransferrin (75 kDa). Two approaches for combining this ISD MALDI-generated partial sequence information with protein database searching techniques are presented. In one approach, cyanogen bromide is used to cleave relatively large peptide fragments from a sample of human apotransferrin. One of the larger cleavage products (6034.84 Da) was isolated by HPLC and subjected to ISD MALDI analysis. An easily identified cn fragment ion series allowed two noncontiguous segments of the peptide's sequence to be determined (about 55% of the total sequence). This partial sequence information was used to search protein and oligonucleotide sequence databases. In addition to uniquely identifying human apotransferrin in a protein sequence database, an example of the use of this ISD MALDI-determined partial sequence information to search expressed sequence tag databases is presented. Such searches have the potential for rapidly identifying new genes that code for target proteins. An alternate approach for obtaining partial sequence information on proteins is also demonstrated that utilizes ISD MALDI fragmentation of the intact protein to generate partial sequence information. This approach is shown to generate about 5-7% of a protein's sequence, usually near the C-terminus of the protein. Examples of the ISD MALDI fragmentation data obtained from intact (reduced) human apotransferrin and intact (nonreduced) bovine serum albumin (66 kDa) proteins are presented.

A Terminal Product Ion in the Fragmentation of Methyl Stearate under Electron Ionization Conditions.

Fragmentations of the molecular and fragment ions of methyl stearate, under 70 eV electron ionization (EI) conditions, have been studied by using collision-induced dissociation (CID) and metastable ion decay (MID) techniques. The CID and MID spectra of aged molecular ions Mi+· quite differed from the EI mass spectrum resulted from the in-source fragmentation of initially produced molecular ions M0+· in pattern. The CID spectra of the Mi+· ion mainly showed peaks of m/z 255, 199, and 143 ions, while CID spectra of these 56-Da periodical fragments showed only a preferential product ion at m/z 101. CID spectra of other fragment ions also indicated that the m/z 101 ion is a terminal product. The structure of the ion at m/z 101 has been discussed.

Liquid secondary ion mass spectrometry of methyl glycosides of oligosaccharides using matrices containing carboxamides.

Intense cluster ions corresponding to proton-bound hetero-dimers of an amide molecule and an oligosaccharide molecule are observed in the liquid secondary ion mass spectra of methyl glycosides of oligoxylans if a solution of an aliphatic carboxamide in glycerol is used as the liquid matrix. These cluster ions are particularly abundant and persist for a long period if urea (U) or thiourea (TU) is used as the matrix additive. In these cases, cluster ions containing more than one molecule of U or TU and two oligosaccharide molecules are also observed. The intense signal due to the proton-bound hetero-dimer between U or TU and the oligosaccharide can be used with advantage for a molecular weight determination. The bonding interactions between a protonated saccharide molecule and a molecule U or TU in the proton-bound hetero-dimers are so strong that the urea molecules remain attached to the fragment ions during the decay of metastable cluster ions and even during collision-induced dissociation. Thus, the mass-analysed ion kinetic energy spectra of these proton-bound hetero-dimers are dominated by abundant cluster ions [Bn+U] and [Ym+U] arising from cleavage of the glycosidic bonds within the oligosaccharides. The collisionally-activated mass spectra of the proton-bound hetero-dimers additionally contain peaks of the free ions Bn and Ym. Therefore, these spectra clearly reflect the arrangement of the monosaccharide residues in the oligosaccharide and can be used conveniently for structural analysis.

Factors that influence the observed fast fragmentation of peptides in matrix-assisted laser desorption

Fragmentation processes that occur very early during matrix-assisted laser desorption ionization (MALDI) of peptides are examined by utilization of delayed pulsed ion extraction with a linear time-of-flight mass spectrometer. The oxidized B chain of bovine insulin (MW=3495. 95 u), which produces a wide range of fragment ions, is utilized as a probe to examine the effects of several experimental parameters on this process. Experimental evidence suggests that this MALDI process is not prompt fragmentation and involves metastable ion decay that is quite different from that which is observed with postsource decay experiments. This conclusion is based upon the significant differences observed in the fragmentation products produced by the two techniques. This metastable ion decay process also appears to be over within the minimum pulse delay period (320 ns) that is possible with the current pulsed ion extraction hardware. These two observations suggest that either different activation processes are involved in the two techniques or that the much different time frame of the methods influences the observed ion decay pathways. This fast MALDI metastable ion fragmentation also is shown to be influenced by both the MALDI matrix and the laser fluence.

Time-of-flight mass spectrometry of underivatized single-stranded DNA oligomers by matrix-assisted laser desorption.

Matrix-assisted laser desorption with concomitant ionization (MALDI) in conjunction with time-of-flight mass spectrometry (TOF-MS) has been used to analyze underivatized random-base single-stranded DNA (ssDNA) oligomers ranging from 10 to 89 nucleotides in length by embedding them in a solid matrix of 3-hydroxypicolinic acid. At 355-nm desorption wavelength, mass spectra of positive and negative ions measured by reflecting and linear time-of-flight mass spectrometers are compared. Results from the linear system show the ionization yield is approximately equal for each polarity. Metastable ion decay is significant for the larger ssDNA oligomer ions, which results in a decrease in signal intensity and the broadening of mass peaks. In order to obtain an acceptable signal-to-noise ratio on a reflecting TOF system, a higher laser irradiance is needed, which consequently causes further degradation of mass resolution. With the apparent advantages of better sensitivity and mass resolution, it is concluded that a linear TOF system is better suited for the mass spectrometric analysis of ssDNA oligomers larger than about a 25-mer. The current system permits one-base resolution up to about a 40-mer. Mass accuracy for a 20-mer or smaller is within +/- 0.05%. Comparison of mass spectra from 5-ns and 35-ps pulse widths at the same energy density shows no significant differences. Mechanisms for oligonucleotide ion production in these experiments are discussed.

Multiphoton excitation of ferrocene and vanadocene at 351 nm in comparison with 248 and 193 nm. Wavelength dependent competition between ionization and dissociation

AbstractMultiphoton ionization of ferrocene and vanadocene upon nanosecond laser excitation at 351 nm is reported in comparison with results at 248 and 193 nm. A similar behaviour was found for both compounds at each wavelength. The excitation and fragmentation mechanism was elucidated by the determination of the number of absorbed photons for the ion formation and the analysis of the metastable ion decay. On biphotonic excitation at 248 nm extensive fragmentation of the neutral parent molecule to metal atoms is observed, followed by the ionization of these metal atoms after absorption of two further photons. In contrast, at 193 nm the molecular ion is formed after absorption of two photons and dominates the fragmentation pattern up to moderate laser intensities. At high laser intensities the metal ion, formed after resonant absorption of three photons, becomes prominent. At 351 nm the exclusive formation of molecular ions is observed only near the threshold of the laser intensity for ion detection. Metal ion formation prevails at higher laser intensities involving the absorption of five photons. Most likely this absorption mechanism is resonant.

Multiphoton ionization of vanadocene and ferrocene at 248 and 193 nm. Wavelength-dependent competition between dissociation and ionization

Multiphoton ionization upon nanosecond laser excitation at 248 and 193 nm of vanadocene and ferrocene in comparison with electron impact ionization is reported. A similar behaviour was found for both compounds. Upon laser excitation at 248 nm extensive fragmentation is observed. The ionic decay pattern is only slightly dependent on the laser intensity and is dominated by the metal ion, which is formed after absorption of four photons. At 193 nm the fragmentation is strongly dependent on the laser intensity. Up to moderate laser intensity the molecular ion is dominant and no metal ion is detected. Increasing the laser intensity a higher degree of fragmentation is observed and the metal ion, formed after absorption of three photons, becomes more prominent. These results are consistent with a fast dissociation to metal atoms after biphotonic excitation at 248 nm on an excited repulsive surface, followed by ionization of these metal atoms. The slower dissociation at 193 nm points to a tendency to a statistical redistribution of the excitation energy prior to dissociation. The absorbed photon energy is corroborated by ionization and appearance energies measured by calibrated electron impact ionization. The absorption pathway and the ionic fragmentation mechanism are investigated by analysis of the metastable ion decay.

Multiphoton and electron impact ionization of azirines: 3-methyl-2-phenyl-1-azirine at λ = 248 nm and 193 nm

Multiphoton ionization of 3-methyl-2-phenyl-1-azirine is reported upon nanosecond laser excitation with λ = 248 nm and 193 nm in comparison with electron impact ionization. The number of absorbed photons per molecule for the molecular ion and prominent fragment ions is obtained from the dependence of the ion intensity on the laser power. At λ = 248 nm and 193 nm ions are formed after absorption of two or three photons. The absorbed photon energy is compared with ionization and appearance energies upon electron impact ionization. The ionic decay mechanism is elucidated by means of the pressure dependence of the photoion formation and the analysis of the decay of metastable ions produced upon photoionization as well as upon electron impact ionization.