Collisional activation of ions formed by [Li]⁺ ion attachment

Abstract

Since no unimolecular fragmentation is observed with [M+Li]+ ions under normal operating conditions the collisional activation method was used to study the fragmentation behaviour of these ions. it was found that the liberation of the The ionization of organic molecules by alkali ion attachment leads to quasimolecular ions which are very stable with respect to monomolecular decomposition^.'-^ Methods by which an alkali ion attachment can be achieved are of particular interest in organic mass spectrometry for molecular weight determination and the analysis of mixtures. The stability of these ions has been attributed to the localization of the positive charge at the alkali atom and to a hi h Since the alkali ion affinities of organic molecules are often smaller (<2 eV4) than the bond energies in neutral molecules it has been suggested that the most probable decomposition reaction of a cationized molecule is the elimination of the alkali ion and not a fragmentation of the molecule. However, so far no information on the decomposition behaviour of these ions has been obtained because the mass spectra produced by [Li]+ surface ionization (s.i.) and field desorption (f.d.) do not show any unimolecular fragmentation.Only bimolecular reactions of halogenated compounds with alkali ions are known, leading predominantly to the elimination of a neutral salt m o l e c~l e .~ A similar type of reaction is assumed to occur on the surface of the salt layer under conditions of [Li]+ s.i. of halogenated compounds.6The unimolecular decomposition behaviour of ions formed by alkali ion attachment can be studied by collisional activation (~.a.).~*' For this purpose a [Li]' s.i. source has been connected with a c.a. mass spectrometer. Collisions of the [M+Li]' ions with a neutral target gas, e.g. helium, leads to an excitation of these ions which is sufficient to induce a decomposition.activation energy for a charge shift within the ion. Table 1 showing only the more intense fragment ions in the order of their relative intensities. In addition to these fragments a variety of smaller peaks were observed in the c.a. spectra. As an unambiguous mass assignment of these weak signals was difficult due to

Effect of lithium cationization on oxygen atom transfer from the nitro group to the double bond during collision-induced decomposition in certain substituted β-nitrostyrenes

“…LiNO 2 loss has been reported earlier in the CID spectra of lithiated benzenes. 9 The elimination of LiNO 2 is favoured only in compounds where R 3 is not equal to H. It is further favoured when the aryl group contains an NMe 2 group at the p-position. Elimination of HNO 2 occurs from [M + Li] + ions when R 3 is a cycloolefin group.…”

Differentiation of α- or β-Aspartic Isomers in the Heptapeptides by the Fragments of [M + Na]+Using Ion Trap Tandem Mass Spectrometry

“…In fact, Lee et al reported the highly selective cleavage of sodium-associated peptides from Asp residues; the sequential loss of amino acids from the C-terminus under low-energy CID were also observed when the peptide lacked acidic residues [32]. Rollgen et al first reported that the sodium ion interacts selectively with the polar functional groups of the peptides, initiating the backbone cleavage through a "charge remote" mechanism resulting in significant differences of the product ions from those formed by protonated peptides [38]. Various fragmentation mechanisms of the sodium-associated peptides have been proposed Table 2.…”

“…The presence of a fixed positive charge or a sodium ion in the peptide promoted this selective reaction [31][32][33]. Various mechanisms had been proposed to explain the formation of product ions formed via CID of [M+Na] + of peptides [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. For example, a sodium ion associated with the C-terminal carboxylic group has been proposed to initiate a rearrangement reaction of hydroxyl group leading to sequential loss of the C-terminal amino acid during multiple stages of MS/MS process [34][35][36].…”

Ionization of Unpolar Molecules by Alkali Ion Attachment on the Surface of Electrolytic Solutions Exposed to High Electric Fields

“…The new ion source is of particular interest for producing high (M +alkali) + ion currents with unpolar or weakly polar molecules in order to study the decomposition behaviour of these ions following collisional activation [6,7]. In addition the mechanism of ion formation represents an attractive new field of research.…”