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.     

Chemical synthesis inside the collision cell of a MS/MS system: 1—Formation of adduct ions between protonated esters and ammonia

An evaluation of the TAGA 6000 tandem quadrupole MS/MS capabilities for the study of the formation of adduct ions is presented. It is shown that under chemical ionization conditions the protonated molecular ions of esters react with neutral ammonia inside the collision cell of the instrument to yield adduct ions. Under the same reaction conditions the molecular ions obtained under electron impact conditions do not yield any adduct. Although it is not yet clear whether the adduct structure is of the cluster or covalently bonded type, the experimental results argue mainly in favor of the latter. The center-of-mass energy range usable for the formation of adduct ions is narrow and an order of magnitude lower than the energy required for collision-induced dissociation. However, despite the weak parent ion center-of-mass energies used, the spectra exhibit fragments characteristic of the adduct ion structures. Finally, by controlling the parent ion center-of-mass axial kinetic energy, the number of parent ions and the target thickness, it is possible to measure and control all the usual physicochemical parameters for adduct ion formation.     

Manipulating internal energy of protonated biomolecules in electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

The internal energy of protonated leucine enkephalin has been manipulated in electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry with two newly designed pump-probe experiments. Blackbody infrared radiation was applied to pump an ion population into a well-defined internal energy distribution below the dissociation threshold. Following this pumping stage, the internal energy distribution was probed using on-resonance collisional activation to dissociate the ions. These pump-probe experiments were carried out in two different ways: (a) using on-resonance collisional activation with variable kinetic energies to dissociate the ions at a constant initial ion temperature (determining the precursor ion survival percentage as a function of kinetic energy) and (b) using on-resonance collisional activation with a constant kinetic energy to dissociate the ions at variable initial ion temperatures (to investigate the ion survival yield-initial ion temperature dependence). Using this approach, a detailed study of the effects of the initial ion temperature, the probing kinetic energy and the internal energy loss rate on the effective conversion efficiency of (laboratory-frame) kinetic energy to internal energy was conducted. This conversion efficiency was found to be dependent on the initial ion temperature. Depending on the experimental conditions the conversion efficiency (for collisions with argon) was estimated to be about 4.0 +/- 1.7%, which agrees with that obtained from a theoretical modeling. Finally, the reconstructed curves of the ion survival yield versus the mode of the (final) total internal energy distribution of the activated ion population (after pump and probe events) at different pump-probe conditions reveal the internal energy content of the activated ions.     

Diatomic monocations and dications containing germanium and a rare gas atom.

Diatomic dications and monocations GeY2+ and GeY+, where Y = Ne, Ar, Kr or Xe, have been detected on the microsecond time scale. They are characterized via appearance energies, charge stripping of GeY+ ions, and mass-analysed ion kinetic energy spectroscopy. The formation of GeAr+ is found to involve an excited state of Ar in a Hornbeck-Molnar process.     

Ion/molecule reaction inside the collision cell of a tandem quadrupole mass spectrometric system. 2. Energy dependence of ammonium ion formation

The dependence of the protonation of neutral ammonia on the axial kinetic energy of protonated reactant ions has been studied in the gas phase, using various protonated carbonyl compounds, inside the collision cell of a tandem quadrupole mass spectrometric system. The hypothesis of two different and non-competitive reaction channels has been proposed. The first is characterized by a very low (peaked at ±0.05 eV_cm) and well-defined axial kinetic energy of the reactant ion, while the second is more energy demanding (estimated threshold at ±0.2 eV_cm) and expressed by a collisionally induced dissociation-like energy curve. Fourier transform mass spectrometric experiments have shown that ammonium ion can be generated by direct proton exchange and fragmentation of the adduct ion obtained.     

Fragmentation Reactions of Adduct Ions [M CH_3CO]~ of IsomericPhenylenediamines

Acetonehasbeenusedasachemicalionization(CI)reagentgasinaconventionalionsourceandfoundtoproducenumerousreagentions,whichreactwithdifferentclassesofcompoundstoformavarietyofadductions,mostlyusedtogainmolecularmasses,stereochemicalstructureinformation,toassistinthedifferentiationofvariousisomersandalsotogivecharacteristicreactionwithdifferentfunctionalgroups'w'.Intramolecularfunctionalgroupinteractionsplayimportantrolesinthedissociationreactionsofgas-phaseions6".Theadductions[M CH,CO] formedbyi…     

Atmospheric pressure chemical ionization of fluorinated phenols in atmospheric pressure chemical ionization mass spectrometry, tandem mass spectrometry, and ion mobility spectrometry

Atmospheric pressure chemical ionization (APCI)-mass spectrometry (MS) for fluorinated phenols (C6H5-xFxOH Where x = 0-5) in nitrogen with Cl- as the reagent ion yielded product ions of M Cl- through ion associations or (M-H)- through proton abstractions. Proton abstraction was controllable by potentials on the orifice and first lens, suggesting that some proton abstraction occurs through collision induced dissociation (CID) in the interface region. This was proven using CID of adduct ions (M Cl-) with Q2 studies where adduct ions were dissociated to Cl- or proton abstracted to (M-H)-. The extent of proton abstraction depended upon ion energy and structure in order of calculated acidities: pentafluorophenol > tetrafluorophenol > trifluorophenol > difluorophenol. Little or no proton abstraction occurred for fluorophenol, phenol, or benzyl alcohol analogs. Ion mobility spectrometry was used to determine if proton abstraction reactions passed through an adduct intermediate with thermalized ions and mobility spectra for all chemicals were obtained from 25 to 200 degrees C. Proton abstraction from M Cl- was not observed at any temperature for phenol, monofluorophenol, or difluorophenol. Mobility spectra for trifluorophenol revealed the kinetic transformations to (M-H)- either from M Cl- or from M2 Cl- directly. Proton abstraction was the predominant reaction for tetra- and penta-fluorophenols. Consequently, the evidence suggests that proton abstraction occurs from an adduct ion where the reaction barrier is reduced with increasing acidity of the O-H bond in C6H5-xFxOH.     

Characterization of a high-pressure quadrupole collision cell for low-energy collision-indneed dissociation

A RF-only quadrupole collision cell of new design has been evaluated for use in tandem mass spectrometry experiments as a component of a triple quadrupole mass spectrometer. The new design permits operation at values of collision gas thickness higher by 1 order of magnitude than those used in most cells of this type. When operated at sufficiently high collision gas pressures, the transmission efficiency for precursor ions increases with increasing pressure, often to values greater than those observed in the absence of collision gas. Simultaneously, the attainable resolving power for fragment ions across the entire mass-to-charge ratio range, even for multiply charged precursors, also increases to the point where isomers of a quadruply charged fragment are resolved. The performance of the cell, judged in terms of yields and resolution of fragment ions, has been investigated as a function of the nature and pressure of collision gas, the kinetic energy of the precursor ions that enter the cell, and of the size and charge state of the precursors. The enhanced performance is explicable in terms of a marked deceleration of all ions that emerge from the cell to very low energies, probably a few tens of millielectronvolts, so that the cell effectively acts as an ion source for the second mass filter (fragment ion analyzer) to provide a spectrum of ions of fixed axial energy. The high transmission efficiency appears to arise from a collisional focusing effect analogous to that exploited in three-dimensional RF ion traps. The low axial energies imply that ion transit times through the cell are sufficiently long (several milliseconds) that, in precursor ion experiments where the first mass filter is scanned, a hysteresis effect is observed. This implies that in this operating mode compromises must be sought between scan speed and quality of peak shape. Examples are given of spectra obtained under realistic operating conditions that employ flow injection of samples.     

Applications of collision dynamics in quadrupole mass spectrometry

Some applications of collision dynamics in the field of quadrupole mass spectrometry are presented. Previous data on the collision induced dissociation of ions in triple quadrupole mass spectrometers is reviewed. A new method to calculate the internal energy distribution of activated ions directly from the increase in the cross section for dissociation with center of mass energy is presented. This method, although approximate, demonstrates explicitly the high efficiency of transfer of translational to internal energy of organic ions. It is argued that at eV center of mass energies, collisions between protein ions and neutrals such as Ar are expected to be highly inelastic. The discovery and application of collisional cooling in radio frequency quadrupoles is reviewed. Some previously unpresented data on fragment ion energies in triple quadrupole tandem mass spectrometry are shown that demonstrate directly the loss of kinetic energy of fragment ions in the cooling process. The development of the energy loss method to measure collision cross sections of protein ions in triple quadrupole instruments is reviewed along with a new discussion of the effects of inelastic collisions in these experiments and related ion mobility experiments.     

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.     

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.     

Charge exchange mass spectrometry at high energy

It is shown that charge exchange mass spectra using various reagent ions can be determined on a commercial double focusing mass spectrometer. The experiment relies upon the presence of a collision region held at a potential above ground with the product ions being selected by their unique kinetic energies. Cyclohexene has been studied in detail and the results are in agreement with previous thermochemical and field ionization kinetics data. Agreement with reported charge exchange spectra of n-propanol, taken at lower energies, is good. The present results, including both the variation of spectra with reagent gas and the small range of kinetic energies present in the charge exchange beam, indicate that a resonant or near resonant process is involved. Thus, the internal energy of the nascent ion is defined within quite narrow limits, just as is the case for charge exchange at lower energy.     

Time-resolved photodissociation of singly protonated peptides with an arginine at the N-terminus: A statistical interpretation

Time-evolution of product ion signals in ultraviolet photodissociation (UV-PD) of singly protonated peptides with an arginine at the N-terminus was investigated by using a tandem time-of-flight mass spectrometer equipped with a cell floated at high voltage. Observation of different time-evolution patterns for different product ion types--an apparently nonstatistical behavior--could be explained within the statistical framework by invoking consecutive formation of some product ions and broad internal energy distributions for precursor ions. a(n) + 1 and b(n) ions were taken as the primary product ions from this type of peptide ions. Spectral characteristics in post-source decay, UV-PD, and collisionally activated dissociation at low and high kinetic energies could be explained via rough statistical calculation of rate constants. Specifically, the striking characteristics in high-energy CAD and UV-PD--dominance of a(n) and d(n) formed via a(n) + 1--were not due to the peculiarity of the excitation processes themselves, but due to quenching of the b(n) channels caused by the presence of arginine.     

Cross sections and ion kinetic energy analysis for the electron impact ionization of acetylene

Using a Nier-type electron impact ion source in combination with a double focusing two sector field mass spectrometer, partial cross sections for electron impact ionization of acetylene are measured for electron energies up to 1000 eV. Discrimination factors for ions are determined using the deflection field method in combination with a three-dimensional ion trajectory simulation of ions produced in the ion source. Analysis of the ion yield curves obtained by scanning the deflectors allows the assignment of ions with the same mass-to-charge ratio to specific production channels on the basis of their different kinetic energy distributions. This analysis also allows to determine, besides kinetic energy distributions of fragment ions, partial cross sections differential in kinetic energy. Moreover a charge separation reaction, the Coulomb explosion of the doubly charged parent ions C2H2++ into the fragment ions C2H+ and H+, is investigated and its mean kinetic energy release (KER=3.88 eV) is deduced.     

Gas-phase halonium metathesis and its competitors. Skeletal rearrangements of cationic adducts of saturated ketones

The methyl cation and CF(3)(+) attack saturated, acyclic ketones to make vibrationally excited adduct ions. Despite their high internal energies and short lifetimes, these adducts undergo deep-seated rearrangements that parallel slower processes in solution. Observed pathways include alkene and alkane expulsions, in addition to (in the case of CF(3)(+)) the precedented loss of CF(2)O + HF. For the vast majority of ketones, the principal charged products are the CF(3)(+) adducts of lighter carbonyl compounds, ions that are not easily prepared by other avenues. Evidence for ion structures comes from collisionally activated unimolecular decomposition and bimolecular ion-molecule reactions. Typical examples are di-n-propyl and diisopropyl ketones (both of which produce CH(3)CH=OCF(3)(+) as the principal ion-molecule reaction product) and pentamethylacetone (which produces (CH(3))(2)C=OCF(3)(+) as virtually the sole ion-molecule reaction product). Isotopic labeling experiments account for mechanisms, and DFT calculations provide a qualitative explanation for the relative abundances of products from unimolecular decompositions of the chemically activated CF(3)(+) adduct ions that are initially formed.     

Gas phase substitution under ammonia chemical ionization

Evidence for the isomerization of the [M + NH₄]⁺ adducts of nitrobenzene and triphenylnitrocyclohexenes to the isonitroso derivatives in the first field free region has been obtained. The isomerized adducts undergo successive loss of ?O and ?H or NH₃ to give ions corresponding to the substitution products. Mass analysed ion kinetic energy and collisionally activated dissociation/mass analysed ion kinetic energy data reveal that the adduct decomposes by similar pathways in the second field free region. In the ion source an addition elimination reaction and nucleophilic substitution contribute to the formation of the substituted product ion.     

Probing trapped ion energies via ion-molecule reaction kinetics: Fourier transform ion cyclotron resonance mass spectrometry

The kinetic energy-dependent Ar⁺+ N₂ ion-molecule reaction has been used as a chemical “thermometer” to determine the kinetic energy of ions produced by electron ionization and trapped by using a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. The rate constant for this reaction obtained on the FTICR mass spectrometer was compared to previous work, which allowed a kinetic energy estimate to be made. In addition, the effects of varying parameters such as trapping voltage and pressure on ion kinetic energy were investigated. No evidence of the differing reactivity of higher energy electronic states of Ar⁺, such as ²P_1/2, was observed and the results of a model of this system are presented that support this observation. Pressure studies revealed that with an average of as few as 13 ion-molecule collisions, Ar⁺ ions are collisionally relaxed to an extent unaffected by additional collisions. Based on recent variable temperature selected ion flow drift tube measurements, FTICR ion energies are estimated to be slightly above thermal.     

Delayed dissociation spectra of survivor ions from high-energy collisional activation

Collisional activation (CA) of large ions at kiloelectronvolt energies is accompanied by unexpectedly large losses of translational energy, which vary with the nature of the collision gas. Previous investigations have concentrated upon subsequent fragmentations occurring within a time window covering a few fis immediately following collision, using massanalyzed ion kinetic energy spectrometry. In the present work, survivor ions were selected for specified values of translational energy loss, and their internal energy contents assessed via their subsequent unimolecular fragmentation reactions within a later time window. Beam collimation was also applied when circumstances permitted to impose angular selection, thus minimizing cross talk between effects of collisional scattering and energy dispersion. It was shown that internal excitation of the reactant ion can account for only a small fraction of the observed loss of translational energy. The recoil energy of the target is thus the principal sink for the translational energy loss, since the latter was always chosen to be less than the lowest excitation energy of the target. This conclusion is shown to be consistent with theoretical models of the CA process. The practical implications of these conclusions for CA of large ions at kiloelectronvolt energies are discussed.     

Energy deposition in [Fe(CO)5]+˙ upon collision with a metal surface

Collision of the title ion upon a stainless steel surface at near-normal incidence leads to deposition of internal energy in a well-defined narrow distribution. The energy deposited increases with laboratory collision energy and exceeds 7 eV (average) for 100 eV collisions. The translational-to-vibrational energy transfer efficiency is 15% (assuming an infinitely massive target) at 25 eV collision energy. Comparison is made with the internal energy distributions associated with gas-phase collisional activation using both low and high ion kinetic energies. The narrowness of the distribution of internal energies, the easy access to ions excited to different extents, and the high internal energies accessible, make the ion/surface collision process superior to gas-phase collisional activation for this system.     

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) (+).