Joseph E. Campana

High-performance secondary ion mass spectrometer

A high‐performance research‐oriented secondary ion mass spectrometer (SIMS), based on a double‐focusing mass spectrometer, has been designed, constructed, and evaluated. This instrument is relatively free of some of the instrumental limitations associated with conventional molecular SIMS instrumentation such as energy and mass discrimination. Theoretical design considerations and its construction are discussed. Its performance has been evaluated in various operational modes using a variety of samples and some important instrumental parameters are reported. Finally, the novel and outstanding high‐mass capability of this SIMS instrument is demonstrated.

Secondary ion mass spectrometry of metal salts: Polyatomic ion emission

The mass and intensity distribution of cluster ions emitted from several alkali halide (MX) salts bombarded with Ar+ and Xe+ ions are reported. Cluster ions of the type [M(MX)n]+ and [X(MX)n]− are seen where 1≤ n < 100. We present evidence supporting the formation of cluster ions with specific geometric structures. In addition, we find that the stability of the cluster ions is dependent on the surface energy of the cluster, on the size of the constituent atoms, and on the effective lifetimes of the ions.

Secondary ion mass spectrometry (SIMS) of metal halides. IV. The envelopes of secondary cluster ion distributions

The relative secondary ion intensities of cluster ions of the type [M(MI)n]+ and [I(MI)n]− from the fast atom bombardment mass spectrometry (FABMS) of alkali iodides (MI) for n<100 are presented, and mass-resolved mass spectra of cesium iodide (CsI) clusters exceeding m/z 25 000 are reported. n nThe FABMS spectra are qualitatively consistent with previously reported secondary ion mass spectrometry (SIMS) spectra. The envelopes of the extended mass spectra are fitted to the distributions predicted by a model based on random bond breaking. The bond-breaking model (BBM) distributions fit well on the overall envelope of both the positive and negative ion sodium iodide (NaI, simple cubic) and CsI (body-centered cubic) cluster ion distributions, when Bethe lattices of the appropriate coordination number are used to approximate the true lattices. Ion abundance enhancements at certain “magic numbers” in the envelopes can be predicted by the BBM; however, these enhancements and the reduced ion abundance immediately following the “magic number” are consistent with experimentally observed unimolecular, or more appropriately, unicluster decompositions. Finally, we contrast secondary cluster ion distributions with those obtained using other cluster sources and we discuss in detail possible origins of the large cluster ions observed in these studies.

Fast atom bombardment: Evidence of disproportionation and recombination of a synthetic porphyrin in the matrix☆

In addition to dimerization and polymerization of samples as previously suggested, it appears that during FAB-MS, reactions in the sample matrix can occur to yield new compounds that are recombinations of molecular fragments. This type of reaction may be especially critical to the integrity of peptide sequencing using FAB, since the reactions cited in this report involve the formation and rupture of amides or peptide bonds.

Direct fatty acid profiling of complex lipids in intact algae by fast-atom-bombardment mass spectrometry

Abstract Fast-atom-bombardment mass spectrometry (FABMS) is used for the semiquantitative determination of the fatty acids of complex lipids directly from intact algal cells, crude algal lipid extracts, and vegetable oils. Carboxylate ions, RCOO − , corresponding to the fatty acid moieties of the complex lipids are detected. The relative abundances of the carboxylate fatty acid ions in the FAB mass spectra agreed with the relative percentages found by gas chromatography of the fatty acid methyl esters derived from the extracted lipids. Chemical ionization/fast-atom-bombardment mass spectrometry (CI/FABMS) is discussed with respect to enhancing the molecular ions of the fatty acids and triacylglycerols from these materials. The use of FABMS requires little sample preparation, and FABMS enables rapid fatty acid screening, directly from crude biological materials.

Ultra-High Mass Spectrometry

Massive cluster ions of CsI (mass-to-charge ratio > 25,000) have been produced, mass analyzed, and detected with a conventional double-focusing mass spectrometer. Variations in the ion intensity distributions depend on the relative lifetime of the ions. These results are of fundamental interest, but they also impact the practical limits of mass spectrometry.

Regularities and irregularities in SIMS/FAB spectra of alkali halides analyzed via the bond‐breaking model

Relative ion intensities for cluster ions of the type [M(MX)n]+ and [X(MX)n]− from the SIMS spectra of alkali halides for n<100 are presented. The envelopes of the positive and negative ion spectra are quite similar and can be fit using a random bond‐breaking model on the Bethe lattice approximating the bulk crystal structure. Irregularities occur in the vicinity of certain n values with ion intensity enhanced relative to that of the envelope. The greater intensity at these ‘‘magic numbers’’ is consistent with the bond‐breaking model if the overall probability of breaking any bond of the bulk crystal is small. However, the reduced ion intensity immediately following the enhancement is only consistent with the subsequent unimolecular cluster ion decomposition. Mass‐resolved mass spectra are reported exceeding m/z 25u2009000 from neutral atom bombardment of alkali iodides.

Characterization of polymeric thin films by low-damage secondary ion mass spectrometry

Abstract Static or low-damage secondary ion mass spectrometry (SIMS) is used to characterize thin films of a series of poly(alkyl)-methacrylates consisting of the methyl, ethyl, isobutyl, n-butyl and lauryl substituents. Relative ion intensities at four m/z values are used to distinguish between the various homologs. Ion intensities at m/z values characteristic of the substituent group appear to be significant relative to the backbone (hydrocarbon type) ion intensities. Low-damage SIMS offers a means of direct and “apparent” nondestructive analysis of polymeric thin films.

Gas-phase phenylium and acyclic [C6H5]+ isomers

Abstract The structures of [C6H5]+ species, formed from a variety of precursors, have been investigated by ion/molecule reactivity studies with pulsed ion cyclotron resonance mass spectrometry and by collision spectroscopy with mass-analyzed ion kinetic energy spectrometry. Formed by sequential ion/molecule reactions in gaseous acetylene, the [C6H5]+ species are present as a mixture of isomers, those that react with acetylene and those that do not. Ion-assisted dehalogenation reactions were used to establish that the unreactive [C6H5]+ isomer was the phenylium ion. These data were used to study and confirm collisional decomposition reactions postulated to be diagnostic of the acyclic and phenylium isomers. These diagnostic reactions were then used to study the composition of the [C6H5]+ isomers produced by electron ionization of a number of precursors. These results are contrasted with previous studies and the collision spectroscopy experiments are described in detail. The possible role of [C6H5]+ species in flame processes is discussed.

Fast-atom molecular secondary-ion mass spectrometry

Abstract A commercial ion gun has been modified to generate a diffuse fast-atom beam for molecular secondary-ion mass spectrometry (SIMS). Ion-to-neutral conversion mechanisms have been investigated to determine the nature of the ion neutralization process. When the ion beam passes through a narrow selected-metal aperture, so that ion/surface interactions are increased, the fast-atom component of the beam is enhanced. The potential and kinetic emission of electrons from the metal surface can account for an ion and electron recombination mechanism. Residual charge-exchange reactions between the primary ions and neutral species in the ion gun assembly are a contributing ion neutralization process. The diffuse fast-atom component of the ion/atom beam is obtained by electrostatic deflection of the ions out of the beam following ion neutralization. The fast-atom beam provides several analytical advantages over an ion beam. These advantages and applications of the fast-atom molecular SIMS approach to the analysis of polymers and insulators are discussed.