Low-energy Tandem Mass Spectrometry Research Articles

Ion dissociation reactions induced in a high‐pressure quadrupole collision cell

This work concerns a new high-pressure quadrupole collision cell, designed for triple-quadrupole mass spectrometers. This new collision cell operates at pressures up to 10 mTorr, an order of magnitude higher than conventional cells of this type. Previous investigations have concentrated upon the significant increases in transmission efficiency and in resolving power for fragment ions which result from the use of this new design. The present work reports an investigation into the nature of the dissociation reactions which can be induced by collisions in this high-pressure cell. Charge-site-remote fragmentations of a simple precursor ion were chosen as a test case, and were found to be observable at laboratory collision energies lower by a factor of 4-5 than those found previously to be necessary when using conventional low-pressure quadrupole collision cells. It was also shown that the charge-site-remote reactions were accompanied by the mixed-site-fragmentation reactions described by Tuinman and Cook (J. Am. Soc. Mass Spectrom. Vol. 1, p. 85 (1989)). Ionization of collision gas was observed in the case of xenon. Efforts to observe charge-site-remote fragmentations of peptide ions were marginally successful. Highly basic peptides, which have been problematic for sequencing by low-energy tandem mass spectrometry, did not yield useful fragment-ion spectra in the new cell. The fragmentation behaviour of protonated Leu-enkephalin, for which fragmentation pathways have been thoroughly studied previously, suggested that the observed spectra reflected integration of the fragmentation kinetics over a considerably longer time, thus involving many more reaction steps. These combined observations are considered in terms of a qualitative model based on a rapid decrease of ion kinetic energy during passage through the cell, with much longer residence times than for conventional quadrupole cells.

Differentiation of Regioisomeric Esters of Sucrose by Ionspray Tandem Mass Spectrometry

Abstract Structural characterization and differentiation of distinct regioisomenc esters of sucrose were obtained using lonspray ionization and low energy tandem mass spectrometry. Low energy CAD MS/MS analyses of the protonated molecules [M + H]+ provided characteristic fingerprint patterns, and permitted differentiation of the various regioisomers. MS/MS analyses of selected intermediate fragment ions formed during the ionization process provided additional structural data, and established the fragmentation routes of their [M + H]+ precursors.

Low-energy tandem mass spectrometry of the molecular ion derived from fatty acid methyl esters: A novel method for analysis of branched-chain fatty acids

Low-energy collision-induced dissociation (CID) of the molecular ions of fatty acid methyl esters obtained by electron ionization (70 eV) decompose in the tandem quadrupole mass spectrometer to yield a regular homologous series of carbomethoxy ions. Even at energies up to 200 eV ( E lab), primarily carbomethoxy ions are present, with the most abundant found at m/z 101 at higher energies. The lack of any other CID ions, including m/z 74 (McLafferty rearrangement) or m/z 87, suggest a rearranged molecular ion structure on leaving the first quadrupole mass analyzer. Analyses of various stable isotope variants support the hypothesis of alkyl radical migration to the carboxy carbonyl oxygen atom, with subsequent radical site directed cleavage eiher with or without a cyclization event. Decomposition of the molecular ions (70 eV) of several methyl branched fatty acid methyl esters, including phytanic acid, iso-methyl and anteiso-methyl branched acids, and tuberculostearic acid, reveals enhanced radical site cleavage at the alkyl branching positions. This method can be used to readily determine methyl (or alkyl) branching positions in a saturated fatty acid methyl esters.

Tandem mass spectrometry at low kinetic energy

Recent progress in mass spectrometry, as applied to molecular analysis, is reviewed with emphasis on tandem mass spectrometry. Tandem instruments use multiple analyzers (sector magnets, quadrupole mass filters and time-of-flight devices) to select particular molecules in ionic form, react them in the gas-phase and then record the mass, momenta or kinetic energies of their products. The capabilities of tandem mass spectrometry for identification of individual molecules or particular classes of compounds in complex mixtures are illustrated. Several different types of experiments can be run using a tandem mass spectrometer; all share the feature of sifting the molecular mixture being analyzed on the basis of chemical properties expressed in terms of ionic mass, kinetic energy or charge state. Applications of mass spectrometry to biological problems often depend upon desorption methods of ionization in which samples are bombarded with particle beams. Evaporation of preformed charged species from the condensed phase into the vacuum is a particularly effective method of ionization. It is suggested that the use of accelerator mass spectrometers be extended to include problems of molecular analysis. In such experiments, low energy tandem mass spectrometry conducted in the eV or keV range of energies, would be followed by further characterization of the product ion beam using highly selective MeV collision processes.