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Notes: Collision-induced dissociation of peptide ions yields „sequence“ ions arising from dissociations of the peptide backbone. Recently, Biemann and his collaborators have elucidated fragment ions involving cleavage of all or part of the side-chains, and have characterized them as remote-site fragmentations of the type investigated for other molecular species by Gross et al. The present work reports results of experiments conducted using a tandem, hybrid mass spectrometer, and devoted to investigating whether remote-site fragmentations (including side-chain cleavages for peptide ions) can be observed for collision energies substantially lower than the keV range used previously. It was found that (laboratory-frame) collision energies of at least 200 eV, and preferably greater, are required for the formation of such fragments. At collision energies in this range the transmission efficiency of the qQ assembly is much lower than for the more usual range of a few tens of eV; this drop in transmission efficiency becomes increasingly severe with increasing mass of the precursor.

Notes: The collision-induced dissociation (CID) of Cs4I3+ to form Cs3I2+ has been investigated by mass-analysed ion kinetic energy spectroscopy, When helium was used as collision gas the CID process was accompanied by sizable loss of translational energy in the collisional-activation step. However, use of krypton or xenon as collision gas yielded product ions with translational energies higher than that observed for unimolecular reaction in the absence of collision gas. Neon and argon provided intermediate behaviour. Extensive experiments designed to investigate possible explanations for these observations left superelastic collisions of an excited state as the only possibility. Since nothing is known about the structures and electronic states of species like Cs4I3+, the necessarily speculative discussion is based upon general considerations of coupling of translational and electronic energies via crossings between states of the transitory collision complex. These observations emphasise the complexity of the possible processes which can occur as a result of collisions in the keV range.

Notes: Collision induced dissociation (CID) experiments using RF-only quadrupole collision cells normally employ laboratory-frame collision energies Elab of a few tens of eV. Difficulties in inducing CID reactions for larger precursors (〉 600-800 Da) can be attributed in part to collision energies, in the appropriate centre-of-mass reference frame, which are too low. Attempts to overcome this problem by using values of Elab in the range of a few hundreds of eV have been largely unsuccessful. The present work shows that a major reason for this lack of success has been an inability of the instruments used to fulfil the focusing requirements of an RF-only quadrupole cell for such precursors, and their higher mass fragments, in this range of values for Elab. A simple modification permitting use of much higher values of the RF amplitude has been shown to permit the collision cell to be operated so that the ion-beam profile at the entrance aperture is imaged at the exit; improvements in precursor transmission efficiency by up to two orders of magnitude are thus obtainable. Examples of corresponding improvements in fragment-ion spectra of a test octapeptide are also demonstrated. The principal drawback of this approach concerns the poor transmission for low-mass fragments near the stability cut-off (Mathieu parameter qu 〉 0.91); reasons for the non-zero transmission efficiency for fragments with qu 〉 0.91 are discussed, together with methods for alleviating the low-mass transmission problem.

Notes: A method is described for the investigation of the structure of neutral products from the unimolecular (metastable) dissociative ionizations of mass selected ions, by means of the collisionally induced dissociative ionization of the neutral species themselves. The neutral species, with kilovolt translational energies, enter a positively charged collision cell situated in the second field free region of a standard ZAB-2F mass spectrometer. Dissociative ionization of the neutrals results therein from their collisions with He target gas. The resulting ions are analysed by means of the electric sector and the relative ion abundances are shown to be structure characteristic. For such experiments the neutral flux should be c. ≥ 0.5% of the selected precursor ion flux; the collision gas pressure must be insufficient to cause significant precursor ion fragmentation in the field free region preceding the collision cell. It was shown that HNC is generated in the fragmentation of aniline molecular ions, whereas HCN is the neutral product in the dissociative ionizations of pyridine, benzonitrile and benzyl cyanide. The neutral radical [C, H3, O·] formed together with [CH3CO]+ from ionized methyl acetate has the structure ·CH2OH, but that from the analogous fragmentation of the methyl propanoate molecular ion has the structure CH3O·. Allyl radicals were shown to be generated from [(CH3)2CHCH2OH]+· together with [CH3OH2]+ ions.

Notes: The ion retardation method (whereby an ion beam is prevented from entering a collision gas cell by means of a voltage applied thereto) for permitting the examination of the neutral products of unimolecular ion fragmentations has been extended to include observations of neutral products generated by collisions before the gas cell and their related phenomenology. Observations obtained using an ion beam deflection electrode, an alternative method of preventing the ion beam from entering the collision cell, are also reported. When low collision gas pressures are employed (〈2×10-7 Torr He), this latter method provides collisionally induced dissociative ionization (CIDI) mass spectra of unimolecularly generated neutral fragmentation products, free of complications arising from events induced by collisions occurring outside the collision cell. The CIDI mass spectra of CH3·, C2H4, CH3ĊO, CH3OH and C2H6 generated from positive ion precursors and CH3·, CH3O· and C6H5NO2 generated by electron loss from negative ions are described.

Notes: Methods are described for the unequivocal identification of the acetyl, [CH3—\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document} =O] (a), 1-hydroxyvinyl, [CH2=\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}—OH] (b), and oxiranyl, (d), cations. They involve the careful examination of metastable peak intensities and shapes and collision induced processes at very low, high and intermediate collision gas pressures. It will be shown that each [C2H3O]+ ion produces a unique metastable peak for the fragmentation [C2H3O]+ → [CH3]++CO, each appropriately relating to different [C2H3O]+ structures. [CH3—\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}=O] ions do not interconvert with any of the other [C2H3O]+ ions prior to loss of CO, but deuterium and 13C labelling experiments established that [CH2=\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}—OH] (b) rearranges via a 1,2-H shift into energy-rich leading to the loss of positional identity of the carbon atoms in ions (b). Fragmentation of b to [CH3]++CO has a high activation energy, c. 400 kJ mol-1. On the other hand, , generated at its threshold from a suitable precursor molecule, does not rearrange into [CH2=\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}—OH], but undergoes a slow isomerization into [CH3—\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}=O] via [CH2\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}HO]. Interpretation of results rests in part upon recent ab initio calculations.The methods described in this paper permit the identification of reactions that have hitherto lain unsuspected: for example, many of the ionized molecules of type CH3COR examined in this work produce [CH2=\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}—OH] ions in addition to [CH3—\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}=O] showing that some enolization takes place prior to fragmentation. Furthermore, ionized ethanol generates a, b and d ions. We have also applied the methods for identification of daughter ions in systems of current interest. The loss of OH· from [CH3COOD]+· generates only [CH2=\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}—OD]. Elimination of CH3· from the enol of acetone radical cation most probably generates only [CH3—\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}=O] ions, confirming the earlier proposal for non-ergodic behaviour of this system. We stress, however, that until all stable isomeric species (such as [CH3—\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm O}\limits^{\rm + } $\end{document}=C:]) have been experimentally identified, the hypothesis of incompletely randomized energy should be used with reserve.