Salvador J. Pastor

Fourier transform mass spectrometry—advancing years (1992–mid. 1996)

This article is one of a series of Fourier transform mass spectrometry (FTMS) reviews that has appeared in this journal at ca. 3-4 year intervals. A comprehensive review of the recent theoretical developments, instrumental developments, electrospray ionization (ESI), and MALDI is given. Ion dissociation techniques are also discussed because of their contributions to gaining insight into chemical structure. Special sections have been devoted to discussing the emerging fields of surface analysis, polymer analysis, Buckminsterfullerenes (buckyballs), and hydrogen/deuterium exchange studies. This review, although not all-inclusive, is intended to be a starting point for those wishing to learn more about the current status of FTMS, and also as a representative cross-section of the literature for those familiar with the technique.

Acetylenic Cyclophanes as Fullerene Precursors: Formation of C60H6 and C60 by Laser Desorption Mass Spectrometry of C60H6(CO)12

A logical precursor of macrocycle C60 H6 , cyclophane C60 H6 (CO)12 (1) represents a building block in a possible total synthesis of C60 . In Fourier transform ion cyclotron resonance laser desorption mass spectroscopic experiments in the negative-ion mode, 1 fragments to C60 H6 (2) under successive loss of CO. Further loss of six H atoms and rearrangement gives C60 ions with a fullerenic structure.

Sustained off-resonance irradiation and collision-induced dissociation for structural analysis of polymers by MALDI-FTMS

Abstract A method is presented for collision-induced structure analysis of polymer samples using matrix-assisted laser desorption/ionization Fourier transform mass spectrometry. Sustained off-resonance irradiation collision-induced dissociation (SORICID) can impart the energy required to initiate fragmentation of intractable low mass polymers with masses below 3000 Da. The method has been applied to both polar poly(ethylene glycol) polymers and nonpolar hydrocarbon polymers such as polyisoprene and polystyrene. For low-mass polymers, single oligomers are selected for SORI dissociation and the remainder of the polymer distribution is left intact, allowing the other oligomer ions to be used for internal calibration. Ions resulting from SORI-CID consist of lower mass intact oligomer species and, equally important, distributions of the oligomer fragments which have lost an end group through chain cleavage. This latter feature facilitates identification of the end groups. The upper mass limit of this technique is determined by cyclotron frequency spacing as one moves to higher masses. Decreased frequency spacing translates to SORI ‘spill-over’ to neighboring oligomers causing retention of some while ejecting others, rendering data interpretation more ambiguous.

Analysis of hydrocarbon polymers by matrix-assisted laser desorption / ionization-fourier transform mass spectrometry

A quick and effective sample preparation is demonstrated for matrix-assisted laser desorption/ionization (MALDI) analysis of nonpolar polymers. Polyisoprene, polystyrene, and polybutadiene polymers were investigated by using as matrix a 2,5-dihydroxybenzoic acid and silver nitrate combination. Silver cationized oligomers produce useful spectra that can be signal averaged to characterize polymer distributions extending up to 6000 u by using a 3-T Fourier transform mass spectrometer. Because an electrostatic ion deceleration protocol was used to extend the mass range, trapping discrimination is shown to exist for molecular weight distributions broader than about 2500 u. However, an integral procedure can be used to reconstruct the true polymer profiles through co-addition of signal transients obtained by using various gated deceleration times. For polymers with narrower mass distributions, silver cationization along with signal averaging provides rapid and accurate polymer characterization for nonpolar polymer systems by using standard MALDI Fourier transform mass spectrometry instrumentation.

Poly(ethylene glycol) limits of detection using internal matrix-assisted laser desorption/ionization fourier transform mass spectrometry

Detection limits of poly(ethylene glycol) were examined in the mass range 2000-6000 Da. Using an aerospray sample deposition technique, highly uniform sample surfaces were produced. This method allows signal averaging of spectra from up to 400 shots on the same sample spot. It is found that, as the material available for desorption is decreased, the overall average sample consumption per shot is decreased. Experimentally determined detection limits of 40 and 280 fmol (based on the average molecular masses of 2000 and 6000) were found for PEG 2000 and PEG 6000, respectively. The sample spectra show oligomer distributions in agreement with their higher concentration counterparts. However, at the lowest signal-to-noise levels, oligomers at the extremes of the distribution are no longer detected, making the polymer distribution appear to be narrower in mass range.

Investigation of low-voltage on-resonance ion selection for Fourier transform mass spectrometry

Low-voltage on-resonance ion selection (LOIS) was recently introduced as an alternative technique for ion selection and storage. Under high pressure conditions and similar to the technique of quadrupolar axialization, unwanted (unselected) trapped ions are eliminated from the analysis cell through collisions with cell plates following orbital expansion. The ions remaining after tens of seconds of mass selection can be detected with better coherence, leading to improvements in ion detection and sensitivity. Here, experiments designed to test ion remeasurement and ion transfer capabilities are presented. Simulations of ion motion give insight into the possible mechanism of ion cooling, which does not appear to be the same as that of the axialization process. Because of its ease of use, lack of need for additional hardware devices, and comparable ion selection results, LOIS is an attractive alternative for trapped ion experiments.