Selective generation of charge-cependent/independent ion energy distributions from a heated capillary electrospray source

Retarding grid and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry variable trap potential measurements are performed to determine factors that contribute to the kinetic energy distribution of ions formed in an electrospray source that uses a heated capillary for desolvation. The control of ion kinetic energies is achieved by manipulating the skimmer position in the postcapillary expansion and by varying the potential appEed to the skimmer. The selective generation of either charge-dependent or charge-independent ion energy distributions is demonstrated. Charge-dependent energy distributions of electro-sprayed ions are created by sampling ions near the Mach disk of the supersonic expansion and by using a larger diameter skimmer orifice; the FTICR spectra acquired under these conditions exhibit mass-to-charge ratio-dependent mass discrimination determined by the potential used to trap the ions. Charge-independent energies of electrosprayed ions are created by positioning the capillary adjacent to the skimmer to sample thermal ions and by using a smaller skimmer orifice to reduce expansion cooling; under these conditions ion kinetic energy is determined primarily by the skimmer potential and no mass-to-charge ratio-dependence is observed in the selection of optimum FTICR trapping conditions. The ability to select between proteins of different conformation on the basis of kinetic energy differences is demonstrated. For example, a 0.4 V difference in trap potential is observed in the selective trapping of open and closed forms of the +10 charge state of lysozyme. Finally, it is demonstrated that by operating the source under conditions which deliver a beam of ions with charge-independent energies to the cell, it is possible to obtain precursor and product ion signal magnitudes in FTTCR spectra without charge-dependent mass discrimina-tion.     

Low-energy mass spectra of some aliphatic ketones

The charge exchange mass spectra of a selection of C₅-C₇ ketones have been measured using [CS₂]⁺˙, [COS]⁺˙ and [N₂O]_+. as reagent ions. The low energy charge exchange with [CS₂]⁺˙ or [COS]⁺˙ provides simple primary ion mass spectra, which readily permit structure elucidation in contrast to metastable ion spectra. In several cases, isomer distinction is easier from the charge exchange mass spectra than from the electron impact mass spectra. The energy transfer from [N₂O]⁺˙ is sufficiently high for complex spectra resembling electron impact mass spectra to be obtained.     

‘Doubly charged ion’ mass spectra of some simple aromatic compounds

Benzene, toluene, phenol, diphenyl ether and the three isomeric dihydroxy-benzenes have been examined using an MS-9 mass spectrometer under conditions that allowed only ions having twice the normal amount of kinetic energy to be detected. These ions are, in fact, singly charged ions arising from charge exchange reactions of doubly charged ions of the same mass, occuring in the first field free region of the Spectrometer. It is argued that the spectra obtained yield essentially the distribution of doubly charged ions in the source region. These ‘doubly charged ion’ mass spectra are compared with the normal singly charged ion spectra of the compounds and the implications of the significant differences that are found, are discussed.     

Charge exchange ionization in collision cells as a method to detect the presence of long-lived excited electronic states of polyatomic ions

Charge exchange ionization in collision cells installed in a double focusing mass spectrometer with reversed geometry has been used to detect the presence of a long-lived excited electronic state of benzene ion. In particular, the first collision cell located between the ion source and the magnetic sector was modified to serve as an ion source for the reagent ion generated by charge exchange with the primary ion. Strong reagent ion signals were observed when the ionization energies of the reagents (1,3-C4H6, CS2, CH3Cl) were lower than the recombination energy (approximately 11.5 eV) of the excited state benzene ion, while the signals were negligible for reagents (CH3F,CH4) with higher ionization energy. The fact that a strong signal is observable only for electronically exoergic charge exchange is useful for detecting the presence of a long-lived electronically excited state.     

Mechanistic studies of multipole storage assisted dissociation

The degree and onset of fragmentation in multipole storage assisted dissociation (MSAD) have been investigated as functions of several hexapole parameters. Strict studies of hexapole charge density (number of ions injected) and hexapole storage time were made possible by placing a pulsed shutter in front of the entrance to the mass spectrometer. The results obtained show that the charge density is the most critical parameter, but also dependencies on storage time, radio-frequency (rf) -amplitude, and pressure are seen. From these data, and from simulations of the ion trajectories inside the hexapole, a mechanism for MSAD, similar to the ones for sustained off-resonance irradiation (SORI), and for low energy collisionally induced dissociation in the collision multipole of a triple quadrupole mass spectrometer, is proposed. It is believed that, at higher charge densities, ions are pushed to larger hexapole radii where the electric potential created by the rf field is higher, forcing the ions to oscillate radially to higher amplitudes and thereby reach higher (but still relatively low) kinetic energies. Multiple collisions with residual gas molecules at these elevated energies then heat up the molecules to their dissociation threshold. Further support for this mechanism is obtained from a comparison of MSAD and SORI spectra which are almost identical in appearance.     

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.     

Scanning a triple quadrupole mass spectrometer for doubly charged ions

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

Investigation into factors affecting precision in ion trap mass spectrometry using different scan directions and axial modulation potential amplitudes

Electrospray ionization mass spectra obtained from different scan directions are observed to be dependent on the axial modulation potential amplitudes used for resonant ejection and on the positive deviation caused by higher even-multipole fields present in most commercial ion traps. The axial modulation voltage influences the dissociation of ions during resonant ejection and the observed mass shifts. The higher even-multipole fields in commercial ion traps are known to influence resonant ejection from the ion trap and can cause a loss in mass resolution for peaks in reverse scan mass spectra compared with that obtained by the forward scan. However, along with the dissociation of ions during resonant ejection causing a loss in resolution, the possibility of resolving an isotopic distribution is also shown to be influenced by the mass shifts caused by the space charge. These mass shifts differ depending on the scan direction employed. A significant loss in resolution can also result from resonant ejection using non-optimal axial modulation voltages. We also present results showing the ejection of ions at betaz = 1/2 using the reverse scan mode without the axial modulation voltage. Ion ejection at betaz = 1/2 is uncommon in commercial (stretched ion traps) with the conventional analytical scan without the use of a frequency of the axial modulation voltage corresponding to this non-linear resonance.     

Charge exchange spectra of simple ketones

Charge exchange spectra of three simple 2-ketones are reported. Comparison of charge exchange data with low voltage electron-impact spectra provides information on the nature of the electron-impact energy distribution. The existence of high energy molecular ions which do not decompose in the mass spectral time scale led us to postulate that isolated states or photon emission reactions are operative in the present system.     

Charge exchange using a double quadrupole mass spectrometer

Charge exchange mass spectra obtained on a double quadrupole (QQ) mass spectrometer are compared with those obtained by other methods. The effects of reagent ion recombination energies and of axial ion translational energy on these spectra are followed.     

High resolution measurements of kinetic energy release distributions of neon, argon, and krypton cluster ions using a three sector field mass spectrometer

Using a newly constructed three sector field mass spectrometer (resulting in a BE1E2 field configuration) we have measured the kinetic energy release distributions of neon, argon, and krypton cluster ions. In the present study we used the first two sectors, B and E1, constituting a high resolution mass spectrometer, to select the parent ions in terms of mass, charge, and energy, and studied the decay of those ions in the third field free region. Due to the improved mass resolution we were able to extend earlier studies carried out with a two sector field machine, where an upper size limit arose from the fact that several isotopomers contribute to a decaying parent ion beam when the cluster size exceeds a certain value. Furthermore we developed a new data analysis. It allows us to model also fragment ion peaks that are a superposition of different decay reactions and thus we can determine the average kinetic energy release for all decay reactions of a given cluster ion. In a further step we used these results to determine the binding energies of cluster ions Rg(n) (n> or =10) by applying finite heat bath theory. The smaller sizes have not been included in this analysis, because the validity of finite heat bath theory becomes questionable below n approximately 10. The present average kinetic energy releases and binding energies are compared with other experiments and various calculations.     

Stereochemical applications of mass spectrometry 4. Energy-resolved study of the fragmentation of esters of maleic and fumaric acids

The fragmentation of the dimethyl and diethyl esters of maleic and fumaric acids have been studied as a function of the internal energy of the molecular ions using charge exchange techniques and metastable ion studies in combination with isotopic labelling. The dimethyl ester molecular ions show distinctive behaviours at both low and high internal energies, indicating that interconversion of the molecular ions does not occur. The fumarate molecular ion fragments by elimination of CH₂O and (CO₂ + CH₃) in the metastable ion time-frame, while the maleate ester fragments primarily by loss of CH₃O. At higher internal energies both molecular ions fragment primarily by loss of CH₃O but the fragment ion from the maleate ester shows a greater stability, presumably because it assumes the cyclic cationated maleic anhydride structure. The diethyl maleate and diethyl fumarate molecular ions show identical metastable ion characteristics; in addition the [COS]⁺· charge exchange mass spectra are very similar. These results indicate that low-energy molecular ions interconvert. At higher internal energies interconversion does not occur, and, although both moiecular ions fragment by loss of C₂H₅O, the resultsint fragment ions show different stabilities and fragmentation reactions.     

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) (+).     

Direct detection of polyaromatic hydrocarbons, estrogenic compounds and pesticides in water using desorption chemical ionisation membrane inlet mass spectrometry

This paper describes the use of desorption chemical ionisation membrane inlet mass spectrometry (DCI-MIMS) for the detection of a broad range of common contaminants in water. In addition, we discuss the advantages/disadvantages of two different types of chemical ionisation reagent gases, i-butene (a Broensted acid reagent) and argon (a charge exchange reagent). We found that polyaromatic hydrocarbons was detectable at ppt levels, the estrogenic compounds diethyl phthalate and octylphenol at high ppt levels, steroid hormones at ppb levels, hydrophobic pesticides at low ppb levels, whereas hydrophilic pesticides and bisphenol A were not detectable at all. With the exception of the polyaromatic hydrocarbons and pentachlorophenol, none of the reported compounds have to our knowledge been detected previously by other MIMS systems. In most cases the Broensted acid reagent gave characteristic ions at high mass/charge ratio, whereas the charge exchange reagent gave less characteristic ions at low m/z ratio. However, the sensitivity was generally not as good with the Broensted acid reagent as with the charge exchange reagent, since the Broensted acid reagent, i-butene, gave a large chemical background.     

Silylation of an OH-terminated self-assembled monolayer surface through low-energy collisions of ions: a novel route to synthesis and patterning of surfaces

Using a multi-sector ion-surface scattering mass spectrometer, reagent ions of the general form SiR(3) (+) were mass and energy selected and then made to collide with a hydroxy-terminated self-assembled monolayer (HO-SAM) surface at energies of approximately 15 eV. These ion-surface interactions result in covalent transformation of the terminal hydroxy groups at the surface into the corresponding silyl ethers due to Si--O bond formation. The modified surface was characterized in situ by chemical sputtering, a low-energy ion-surface scattering experiment. These data indicate that the ion-surface reactions have high yields (i.e. surface reactants converted to products). Surface reactions with Si(OCH(3))(3) (+), followed by chemical sputtering using CF(3) (+), yielded the reagent ion, Si(OCH(3))(3) (+), and several of its fragments. Other sputtered ions, namely SiH(OCH(3))(2)OH(2) (+) and SiH(2)(OCH(3))OH(2) (+), contain the newly formed Si--O bond and provide direct evidence for the covalent modification reaction. Chemical sputtering of modified surfaces, performed using CF(3) (+), was evaluated over a range of collision energies. The results showed that the energy transferred to the sputtered ions, as measured by their extent of fragmentation in the scattered ion mass spectra, was essentially independent of the collision energy of the projectile, thus pointing to the occurrence of reactive sputtering.A set of silyl cations, including SiBr(3) (+), Si(C(2)H(3))(3) (+) and Si(CH(3))(2)F(+), were similarly used to modify the HO-SAM surface at low collision energies. A reaction mechanism consisting of direct electrophilic attack by the cationic projectiles is supported by evidence of increased reactivity for these reagent ions with increases in the calculated positive charge at the electron-deficient silicon atom of each of these cations. In a sequential set of reactions, 12 eV deuterated trimethylsilyl cations, Si(CD(3))(3) (+), were used first as the reagent ions to modify covalently a HO-SAM surface. Subsequently, 70 eV SiCl(3) (+) ions were used to modify the surface further. In addition to yielding sputtered ions of the modified surface, SiCl(3) (+) reacted with both modified and unmodified groups on the surface, giving rise not only to such scattered product ions as SiCl(2)OH(+) and SiCl(2)H(+), but also to SiCl(2)CD(3) (+) and SiCl(2)D(+). This result demonstrates that selective, multi-step reactions can be performed at a surface through low-energy ionic collisions. Such processes are potentially useful for the construction of novel surfaces from a monolayer substrate and for chemical patterning of surfaces with functional groups.     

Observation of resonant interatomic coulombic decay in Ne clusters

We have measured the electron spectra of Ne clusters after excitation with photon energies around the 2s inner valence threshold. At two photon energies below threshold, a resonantly enhanced surplus of low kinetic-energy electrons is observed. The kinetic energy of the peak does not vary with the photon energy and is slightly larger than the transition energy of Interatomic Coulombic Decay (ICD) above threshold. This leads us to assume that an ICD-like process is present. In analogy to the Auger and the resonant Auger decay this new phenomenon is termed resonant ICD.     

Reliability of estimation of recombination energies of molecular radical cations by charge exchange and a test of the exoergicity rule

Charge exchange occurring in a collision cell of a mass spectrometer was used previously to find the very long-lived excited electronic states of molecular cations generated by electron ionization. The underlying criterion in the method, called the exoergicity rule, was that only electronically exoergic and resonant processes have large cross sections at low collision energy. The validity of this rule has been checked in this work by estimating the recombination energies of some molecular ions based on the presence/absence of charge-exchange signals from gases with different ionization energies, and comparing these estimates with the literature values. It is concluded that a recombination energy can be estimated to within an upper limit uncertainty of 0.1 eV with this technique, which validates the exoergicity rule proposed previously.     

A model for energy transfer in inelastic molecular collisions applicable at steady state or non-steady state and for an arbitrary distribution of collision energies

A new model for energy exchange between translational and internal degrees of freedom in atom-molecule collisions has been developed. It is suitable for both steady state conditions (e.g., a large number of collisions with thermal kinetic energies) and non-steady state conditions with an arbitrary distribution of collision energies (e.g., single high-energy collisions). In particular, it does not require that the collision energies be characterized by a quasi-thermal distribution, but nevertheless it is capable of producing a Boltzmann distribution of internal energies with the correct internal temperature under quasi-thermal conditions. The energy exchange is described by a transfer probability density that depends on the initial relative kinetic energy, the internal energy of the molecule, and the amount of energy transferred. The probability density for collisions that lead to excitation is assumed to decrease exponentially with the amount of transferred energy. The probability density for de-excitation is obtained from microscopic reversibility. The model has been implemented in the ion trap simulation program ITSIM and coupled with an Rice-Rampsberger-Kassel-Marcus (RRKM) algorithm to describe the unimolecular dissociation of populations of ions. Monte Carlo simulations of collisional energy transfer are presented. The model is validated for non-steady state conditions and for steady state conditions, and the effect of the kinetic energy dependence of the collision cross-section on internal temperature is discussed. Applications of the model to the problem of chemical mass shifts in RF ion trap mass spectrometry are shown.     

Intramolecular aromatic substitution reactions in substituted N,N-dimethylthiobenzamide ions

The molecular ions of N,N-dimethylthiobenzamide and its ortho substituted derivatives (substituents CH₃, Cl, Br, I) lose a hydrogen atom and/or the ortho substituent. The mechanism of this process has been studied by measurements of the ionization energies, appearance energies of the product ions m/z 164 and the kinetic energy release during this process. The structure of the product ions m/z 164 and relevant reference ions have been investigated by mass analysed ion kinetic energy spectra, B/E linked scan spectra and collision induced decompositions. The results show clearly the formation of two different kinds of product ions m/z 164 depending on the substituent lost. Type a ions are formed by loss of a H atom or the CH₃ substituent and correspond to protonated 3,4-benzo-N-methylpyroline-2-thione. The formation of these ions occurs by a hydrogen rearrangement followed by an intramolecular substitution via a 5-membered cyclic intermediate and is associated with a large release of kinetic energy. In contrast, the loss of the halogeno substituents to give type b ions probably occurs via a direct displacement reaction by the sulfur atom of the thioamide group giving rise to Gaussian shaped peaks mass analysed ion kinetic energy spectra.     

Interference peaks in mass analysed ion kinetic energy spectra

When mass analysed ion kinetic energy spectroscopy is performed on ions of low abundance and low mass compared with the rest of the ions present in the ion chamber, severe interference may be observed. The major form taken by the interference is of narrow peaks at non-integral mass numbers. Many such peaks can be observed in mass analysed ion kinetic energy spectra and they are mainly attributed to decompositions of metastable ions occurring in the first field free region of the reversed geometry double focusing mass spectrometer.