Mass-analyzed Ion Kinetic Energy Research Articles

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 μs immediately following collision, using mass-analyzed 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 unimoleculr 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.

Metastable ion study of organosilicon compounds, part VII.1 Losses of C2H4 and CH4 from the metastable dimethylsilyl ion [(CH3)2Si+H

AbstractThe spontaneous unimolecular dissociative reactions of dimethylsilyl cations (CH3)2Si+H (1+; m/z 59) have been investigated by mass‐analyzed ion kinetic energy (MIKE) spectrometry and deuterium‐labelling studies. Excepting the losses of H and H2, the ions 1+ mainly eliminate C2H4 and CH4 to produce product ions at m/z 31 and 43, respectively. Almost complete hydrogen scrambling occurs prior to the C2H4 loss, but only a small degree of scrambling occurs in the CH4 loss. Predominantly, the hydrogen atom bound to Si participates in the latter reaction, contrary to the previously reported mechanism.

Leucine and isoleucine in chemical ionization and plasma desorption mass spectrometry: A comparative study

AbstractThe NH3 and CH4 chemical ionization mass spectra of leucine and isoleucine are compared with their plasma desorption mass spectra. Mass‐analysed ion kinetic energy (MIKE) and collision‐induced dissociation (CID)/MIKE experiments are also presented and the mechanisms of the formation and dissociation of immonium ions [M – CO2H2]+ are discussed. A comparison between the various ionization techniques reveals that plasma desorption produces pseudo‐molecular ions whose internal energy lies between zero and 300 kJ mol−1.

Electron impact‐induced fragmentation of 2,1‐benzisothiazoline 2,2‐dioxide

AbstractThe electron impact‐induced fragmentation patterns of 2,1‐benzisothiazoline 2,2‐dioxide nitro derivatives were studied. The rationalizations proposed for the fragmentations are supported by accurate mass measurements, daughter ion (mass analysed ion kinetic energy and B/E linked‐scan), parent ion, and constant neutral loss spectra in metastable and collision‐induced dissociation modes and deuterium labelling.

Fragmentation pathways of isomeric dodecenols studied by electron impact mass spectrometry

AbstractElectron impact‐induced fragmentations of isomeric dodecen‐1‐ols were investigated using mass‐analysed ion kinetic energy (MIKE) spectrometry and high‐resolution data. The principal fragmentation pathways of the positional isomers studied are dominated by the CC double bond and involve migration and radical‐site rearrangements along the carbon chain with the aid of consecutive rearrangements of hydrogen atoms.

Electron ionization mass spectrometry of spiroepoxides generated from E‐3‐arylideneflavanones

AbstractThe electron‐impact induced decomposition of some spiroepoxides prepared from E‐3‐arylideneflavanones has been examined by means of low‐ and high‐resolution mass spectrometry, mass‐analysed ion kinetic energy (MIKE) spectrometry and collision‐induced dissociation (CID) experiments. The compounds studied were found to undergo a rearrangement reaction resulting in a very intense odd‐electron ion. The MIKE and CID‐MIKE spectra of this type of ion generated from the unsubstituted epoxide proved to be identical to those of the molecular ion of trans‐stilbene. The isomerization of epoxides to the corresponding carbonyl compounds prior to fragmentation was not observed.

High-performance liquid chromatographic and mass spectrometric identification of phosphocitrate synthesized by human kidney homogenate

Two high-performance liquid chromatographic techniques for the analysis of phosphocitrate, a metabolite synthesized by the cell that is thought to play a role in a number of physiological as well as pathological events, have been developed. The mass spectra of the isolated compound obtained under fast-atom bombardment and collisionally activated decomposition mass-analysed ion kinetic energy conditions are also reported.

Contributions of fast‐atom bombardment mass spectrometry to studies of cyclic nucleotide biochemistry

AbstractCyclic nucleotides are biochemical second messengers which mediate the actions of many hormones and neurotransmitters. Fast‐atom bombardment (FAB) mass spectrometry and mass‐analysed ion kinetic energy (MIKE) spectra have provided a means of unambiguous identification of these compounds, and in addition to identifying endogenous putative cyclic nueleotides and their synthetic analogues FAB/MIKE analysis has now been applied to good effect to both qualitative and quantitative studies of cyclic nuclcotide‐related enzymes. Here the application of this mass spectrometric technique to cyclic nucleotides is described and its impact upon this field reviewed.

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.

Metastable ion study of fluorinated organic compounds. Part 3. α, α, α-trifluoroanisole and α, α, α-trifluorocresols

The spontaneous unimolecular dissociation reactions of the molecular ions of the C 7H 5F 3O positional isomers α, α, α-trifluoroanisole ( 1), o-α, α, α,-trifluorocresol ( 2) and m-α, α, α-trilfluorocresol ( 3), have been investigated by mass-analyzed ion kinetic energy (MIKE) spectrometry and deuterium-labellin. The results are compared with those of the non-fluorinated analogues anisole ( 4), o-cresol ( 5) and m-cresol ( 6). Ions 1 .+ and 4 .+ fragment in an analogue fashion yielding MIKE spectra which are characteristically different from those isaomers of the isomers 2 .+, 3 .+, 5 .+ and 6 .+ whose MIKE spectra are closely similar, the fluorinated analogues 2 .+ and 2 .+ have different characteristics. This is because a pronounced ortho-effect is operative in 2 .+.

Metastable ion study of organosilicon compounds VI—triethoxymethylsilane and tetraethoxysilane

AbstractThe fragmentations of triethoxymethylsilane ((C2H5O)3SiCH3 (1)) and tetraethoxysilane ((C2H5O)4Si (3)) induced by electron impact were investigated by mass‐analysed ion kinetic energy (MIKE) spectrometry and a deuterium‐labelling study. These molecular ions begin to fragment by the loss of methyl, ethyl and/or ethoxy radicals. Almost complete scrambling of methylene hydrogen takes place in these resultant intermediate ions, prior to the elimination of acetaldehyde molecule. The fragmentations of [1]+˙ and [3]+˙ were compared with those of the corresponding carbon analogues, 1,1,1‐triethoxyethane ((C2H5O)3CCH3 (2)) and tetraethoxymethane ((C2H5O)4C (3)), respectively.

Dependence on scattering angle of the internal energy distribution of products of charge‐changing collisions

AbstractThe internal energy distributions arising from charge‐changing collisions were measured as a function of scattering angle, θ, for the ‘thermometer’ molecule W(CO)6. The experiments were performed by modifying a reverse‐geometry mass‐analyzed ion kinetic energy (MIKE) spectrometer by adding angle‐resolving slits which allow measurement of the scattering angle in the non‐focusing plane of the instrument. Charge exchange of W(CO) with benzene to give W(CO), and charge stripping of W(CO) on collision with O2 to give W(CO), were studied at 6 keV with the product ions being collected over laboratory scattering angles selected in the range 0–0.60°. The results show that charge‐changing collisions accompanied by scattering have the potential for depositing extremely large internal energies. The observation of the W(CO)2+ ion formed in dissociative charge stripping of W(CO) shows that it is possible to deposit at least 27 eV into the colliding W(CO) ion; of this energy, 15 eV is used for the charge‐stripping process, leaving 12 eV of internal energy in the nascent W(CO). Even greater internal energies (more than 15 eV) can be deposited into scattered W(CO) produced by charge exchange of the doubly charged ion. The availability of such high internal energies has potential use in causing dissociation of refractory ions such as those of biomolecules. The average internal energy, εAVE, deposited increases with the scattering angle at a rate of 10 eV degree−1 for charge exchange, and at approximately 5 eV degree−1 for charge stripping and for simple collision‐induced dissociation (CID). This observation suggests that non‐zero angle charge stripping and CID may occur via similar mechanisms in which direct vibrational activation occurs in small impact parameter collisions which also lead to angular scattering. The higher internal energies and larger εAVE vs. θ dependence observed for charge exchange are consistent with the formation of the products upon a highly repulsive surface associated with coulombic repulsion between the separating products.

Influence of instrumental parameters and sample preparation on mass‐analysed ion kinetic energy spectra with fast atom bombardment exemplified with the oxonium ion of peracetylated ribopyranose

AbstractWhen mass‐analysed ion kinetic energy (MIKE) spectra are required to discriminate between isomeric ions formed under conditons of fast atom bombardment (FAB) in the ion source, severe interference may be observed. The interfering peaks in MIKE spectra obtained with a reversed‐geometry instrument can arise from different sources. Moreover, the intensity distribution of the true ions from the selected precursor ion may depend strongly on the instrument being used. This means that the FAB–MIKE or collisionally induced dissociation (CID) spectrum is not an absolute characteristic of a particular ion. The [M + H − HOAc]+ ion in the spectrum of peracetylated ribopyranose is used as an example to illustrate this and to trace and discuss the origin of the phenomena observed.

Differentiation of anomeric glycosyl azides using mass spectrometric methods

AbstractElectron ionization (EI), chemical ionization and mass‐analysed ion kinetic energy (MIKE) spectra have been applied with the aim of obtaining an unambiguous differentiation between the anomeric 2,3‐O‐isopropylidene α‐and β‐D‐ribofuranosyl azide derivatives. The MIKE spectra of their metastable [M  CH3]+ ions, originating from EI, showed a decomposition pattern allowing distinction between the anomers. The origin of the ions in the EI mass spectra, supported by examination of the deuterium‐labelled derivatives, makes it possible to identify the actual azido group orientation in a particular anomer.

Photodissociation breakdown pattern and the internal energy of n-heptane ion generated by charge exchange ionization

Photodissociation breakdown pattern of n-heptane molecular ion generated by charge exchange ionization was obtained with the photodissociation—mass-analyzed ion kinetic energy spectroscopy method. Symmetric charge exchange and CS 2/N 2 charge exchange were used to generate molecular ions with well-defined internal energy. The breakdown pattern has been found to be in good agreement with the photodissociation results for the same ion with Fourier transform ion cyclotron resonance. The above results indicate that symmetric charge exchange ionization is useful to produce molecular ions with very little internal energy.

Anisotropic carbon-fluorine bond cleavage in collisionally activated dissociation of a hexafluorobenzene molecular ion beam

C-F bond cleavage in collisionally activated dissociation of C 6 F 6 .+ has been investigated using mass-analyzed ion kinetic energy spectrometry. With He and Ne collision gases the kinetic'energy profiles of the daughter ion, C 6 F 5 + , displayed regular patterns with the peak maxima downshifted from the maximum for the corresponding unimolecular dissociation. With Ar and Xe, however, unusual upshifts were observed. It was found that the C-F bond cleavage occurred via two channels, one statistical and theother direct or nonstatistical

Characterization of mutagenic compounds in heated orange juice by UV and mass spectra

Gel filtration and high-performance liquid chromatography were used to obtain mutagenic fractions from heated orange juice (93°C, 2 min). Each fraction was submitted to the authors' modified Salmonella mutagenicity assay (pH adjustment to 7.4 and 4 h preincubation at 37°C) and examined for UV and mass spectra. Using fast atom bombardment-mass spectrometry and mass analyzed ion kinetic energy techniques, an attempt was made to elucidate the molecular weight and chemical structure of the mutagenic compounds. They consisted of several compounds with molecular weight as follows: 162, 180, 254, 288, 342, 360 and 540 daltons.

Analysis of mercapturic acid conjugates of xenobiotic compounds using negative ionization and tandem mass spectrometry

Mass spectra of mercapturic acid conjugates of two xenobiotic products of lipid peroxidation (trans-4-hydroxy-2-hexenal and trans-4-hydroxy-2-nonenal) as well as conjugates of 1,3-dichloropropene, styrene oxide, 1,2-naphthalene oxide and alpha-chlorotoluene were obtained using fast atom bombardment or negative chemical ionization. Fragmentation pathways were investigated using linked scan and mass-analyzed ion kinetic energy spectrometric techniques. Characteristics of the spectra obtained using different ionization and sample introduction techniques are compared. Deprotonated molecular ions of mercapturic acids gave simple daughter ion spectra, with the dominant mode of decomposition involving cleavage of C-S bonds giving a characteristic neutral loss of 129 Da. Screening for mercapturates in urine samples was performed using neutral loss scanning and yielded limits of detection in the low nanogram per milliliter range. Quantitative analysis of the S-benzyl mercapturic acid at 1 p.p.b. in urine has been demonstrated using combined gas chromatography/electron capture mass spectrometry with d3-S-benzyl mercapturic acid as internal standard.

Rearrangements in the Molecular Ions of Some ortho-Substituted Schiff Bases

Under electron ionization conditions, the ortho-substituted Schiff bases N-benzylidene-o-toluidine (1a), N-(o-methylbenzylidene)aniline (1b), N-salicylideneaniline (1c) and N-(o-methoxybenzylidene)aniline (1d) give fragment ions which have been shown by collision-activated mass-analysed ion kinetic energy spectra to have the structure of the protonated molecular ions of indole (2), benzofuran (3), and 1,2-benzisoxazole (4). The molecular ion of N-(o-methylbenzylidene)-o-toluidine (1f) gives as fragment ions not only the protonated molecular ion (2) of indole and the tropylium ion but also the molecular ion of anthracene. Attempts to find supporting evidence for a mechanism for this rearrangement by deuterium labelling of a methyl group in (1b), such as (1g), have been unsuccessful.

Mass spectrometric studies on some macrocyclic, macrobicyclic and macroacyclic Schiff bases

The mass spectrometric behaviour of four macrocyclic, one macrobicyclic and one macroacyclic Schiff bases, obtained by one-step multiple condensation reactions of 2,6-diformyl-4-methylpropane, 1,3-diamino- 2,2-dimethylpropene, 1,5-diamino-3-oxapentane or tris-(2-aminoethyl) amine under fast-atom bombardment conditions, was studied in detail with the aid of mass-analyzed ion kinetic energy spectrometry. The mass spectrometric behaviour of a mononuclear macrobicy La(III) complex is also described.