Jeff Brown

Applications in gaseous ion and neutral chemistry using a six-sector mass spectrometer

Abstract A tandem mass spectrometer of EBEEBE geometry (B = magnetic sector; E = electric sector), designed for fundamental studies of gaseous ions and neutral reactive molecules, is described. The production of neutrals in the instrument can be performed by neutralization of fast ion beams during the flight or by flash-vacuum pyrolysis within the ion source. Examples involving ionization of selected precursor ions and pyrolysis are illustrated.

The Application of Matrix‐assisted Laser Desorption/Ionization Combined with Collision‐induced Dissociation to the Analysis of Synthetic Polymers

Matrix-assisted laser desorption/ionization combined with collision-induced dissociation (CID) has been applied to the structural determination of synthetic polymers. Post-source decay/CID experiments on a time-of-flight (TOF) instrument have been compared with CID data from a hybrid sector-TOF mass spectrometer. Fragment-ation spectra of polymers, with molecular weights of up to 4500u2009Da, have been shown to aid structural and end-group determination. The polymers studied were poly(methyl methacrylate), poly(ethylene glycol) and poly(ethylene terephthalate).

Time-lag focusing and cation attachment in the analysis of synthetic polymers by matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry

Ultraviolet matrix-assisted laser desorption/ionization-mass spectrometry has been employed with time-lag focusing to explore its utility for the characterization of synthetic polymers with broad distributions. Mixtures of five polymer standards with narrow molecular weight distributions were analyzed. The spectra were found to be broadly those expected for three different types of polymer systems—poly(styrene), poly(methyl methacrylate), and poly(ethylene glycol)—when equimass mixtures were used. Large changes in the apparent molecular weight distribution of poly(ethylene terephthalate) were observed when the cation was varied. The shift in the envelope was found to be related to the size and the ability of the oligomers to solvate the cation.

The effect of the variation of cation in the matrix-assisted laser desorption/ionisation-collision induced dissociation (MALDI-CID) spectra of oligomeric systems

Abstract The effect of the variation of the cation, on the matrix-assisted laser desorption/ionisation collision induced dissociation (MALDI-CID) spectra obtained, has been studied in a hybrid sector-orthogonal acceleration-time of flight tandem mass spectrometer for some synthetic oligomeric systems. Oligomers of poly(methyl methacrylate) and poly(styrene) in the molecular weight range of approximately 1000–3000 u were analysed. They were ionised by attachment of various alkali metal and copper and silver ions, respectively. The MALDI-CID spectra are shown to vary with different cations more from precursor ions of lower than those of higher mass-to-charge ratio.

Relationship between in-source and post-source fragment ions in the matrix-assisted laser desorption (ionization) mass spectra of carbohydrates recorded with reflectron time-of-flight mass spectrometers

Abstract The reflectron matrix-assisted laser desorption (ionization) [MALD(I)] mass spectra of many complex carbohydrates (and other compounds) recorded with time-of-flight (TOF) instruments fitted with time-lag focusing (delayed extraction) contain focused fragment ions resulting from fragmentation within the ion source, together with unfocused (metastable) fragments derived from post-source decay. The latter ions are particularly abundant from carbohydrates containing sialic acids as the result of loss of the sialic acid moieties. The relative abundance of these metastable ions was found to decrease with ion-source pulse delay whereas the mass difference between the focused and unfocused ions increased. The mass difference was also found to vary with the instrument used to record the spectra. An equation has been derived that links the masses of the precursor ion with the two fragment ions (focused and unfocused). It contains a term reflecting the geometry of the mass spectrometer and can, thus, be used with any TOF instrument. The formula can be rearranged such that the mass of an absent molecular ion can be predicted from the masses of the two fragment ion peaks. The metastable ions can also be used to confirm fragmentation pathways in the manner similar to that used for spectra recorded with magnetic sector instruments. Metastable ions were also found in spectra recorded with an orthogonal-TOF mass spectrometer using field ionization. The formulae relating the masses of the fragment and precursor ions were also found to apply in this case, further demonstrating their instrument independence.

Fragmentation of complex carbohydrates following ionization by matrix‐assisted laser desorption with an instrument fitted with time‐lag focusing

Matrix-assisted laser desorption/ionization time-of-flight spectra of carbohydrates recorded with an instrument using time-lag focusing (delayed extraction) were found to exhibit fragment ions, whereas, in the non time-lag-focusing mode, fragment ions were not observed. Fragment ions consisted both of glycosidic and cross-ring cleavage products whose absolute, but not relative abundance increased with the delay time and indicated that they were formed within the ion source. Only the glycosidic cleavage ions were present in post-source decay spectra and their abundance was inversely correlated with the delay time. The results indicated that the cross-ring cleavages were predominately the products of fast reactions whereas the glycosidic cleavages occurred over a longer time frame.