Peter J. Todd

Peer Reviewed: Organic SIMS of Biologic Tissue

Instrumental developments over the past 20 years now make it possible to identify and map some biologically important molecules by SIMS.

Identification and Mapping of Phosphocholine in Animal Tissue by Static Secondary Ion Mass Spectrometry and Tandem Mass Spectrometry

Secondary ion mass spectra and images were obtained from animal tissue samples using less than 10(13) primary ions/cm2. The mass spectra showed abundant peaks at m/z 184 and m/z 86. Tandem mass spectrometry (MS/MS) was used to identify the source of these ions as phosphocholine. Secondary ion images obtained using MS/MS were used to show that m/z 86 is an abundant gas-phase fragment ion derived from m/z 184. These results are discussed in terms of the physiology of the samples investigated.

MS/MS spectra of organic ions generated by secondary ion mass spectrometry

Abstract Organic ions are withdrawn directly from the condensed phase in molecular secondary ion mass spectrometry (SIMS). In common with other desorption ionization methods, SIMS provides both molecular weight and structural information about complex molecules; other characteristics include a persistence of ion emission, a propensity for cluster ion formation, and a sensitivity to matrix effects. Tandem mass spectrometry (MS/MS) can be advantageously coupled with SIMS to establish ionic relationships in the gas phase, to deduce ion structure, and in analytical studies. In this work, capabilities for isomer differentiation are demonstrated in the natural products field, the value of solid matrices in desorption ionization is illustrated, and in situ isotopic selection is used to interpret spectra. Various types of cluster ions generated in SIMS are analyzed by MS/MS, which allows a distinction between weakly and strongly bound complexes. The cluster ions formed between the cation of thiamine and glycerol shows a strong sample/solvent interaction and consequently glycerol is too reactive a solvent to be used in obtaining the mass spectrum of thiamine. Gas phase fragmentations of the intact cation of carnitine are strikingly different from those observed in the SIMS spectrum, in contrast to the behavior of thiamine, cocaine and candicine for which collision-induced dissociations provide a good guide to desorption ionization mass spectra.

Organic ion imaging beyond the limit of static secondary ion mass spectrometry

Secondary ion mass spectra and images were obtained from spikes of choline chloride, acetylcholine chloride, and methylphenylpyridinium iodide deposited onto specimens of porcine brain tissue. Samples were subsequently subjected to a dose of 10-keV Cs+ sufficient to suppress secondary ion emission characteristic of the targeted analytes. Following ablation of the samples by massive glycerol clusters generated by electrohydrodynamic emission, secondary ion mass spectra and images could be obtained that reflected the identity and location of the spiked analytes. The absolute intensity of secondary ion emission that followed ablation was found to be between 30 and 100% of the intensity obtained prior to exposure to the high dose of Cs’. Not all chemical noise is removed by ablation, however, so that the signal-to-noise ratios after ablation correspond to between 10 and 85% of their values observed under conditions of low primary ion dose.

Solution chemistry and secondary ion emission from amine-glycerol solutions

Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA Secondary ion mass spectra were obtained from a series of C4-C10n-alkylamines introduced via the gas phase onto glycerol. It was found that the amine-characteristic secondary ion intensity varied linearly with amine partial pressure. Henrys law constants and surface activity constants for each of the amines in glycerol solution were measured. A linear correlation was found between amine-characteristic secondary ion intensity and Henrys law concentrations. The concentrations calculated from Henrys law were too low to yield the intensities observed, indicating that secondary ion precursors were not free-base amine molecules but ions in solution. Explicit kinetic equations describing glycerol and amine protonation and deprotonation as a result of primary ion damage to the solutions are derived to rationalize the observed spectra.

Secondary ion images of the rodent brain.

Sections of biologic tissue obtained from laboratory rodents are prepared and analyzed by secondary ion mass spectrometry. The intensity of phosphocholine secondary ions is used to identify anatomical features of the brain from secondary ion images and to evaluate the effectiveness of procedures developed. Secondary ion emission of phosphocholine (m/z 184), is found to be abundant and its intensity is heterogeneous. Effects of sample thickness are addressed. Correspondence between conventional optical images of stained tissue and secondary ion images shows that successive ion images may be used to produce a three-dimensional map of the brain, i.e., an atlas.

Formation and growth mechanism of B10N nanotubes via a carbon nanotube–substitution reaction

A substitution-reaction route has been demonstrated as an efficient synthesis route for producing single-walled and multiwalled B10N nanotubes (BNNTs). The nanotubes have diameters smaller than those of the starting carbon nanotubes and also have similar lengths to the starting carbon nanotubes. The isotopic ratio of B10 in BNNTs depends on the isotopic ratio of the starting B2O3. A detailed growth model is also given for the carbon nanotube–substitution reaction.

A Wide-angle Secondary Ion Probe for Organic Ion Imaging

A secondary ion source has been developed for an organic ion microprobe capable of imaging samples up to 2 em in diameter. The source uses a focused 5 keY Cs+ ion beam which is rastered across the sample surface, and secondary ions from each point on the sample are collected and formed into a low energy beam to be analyzed by a quadrupole mass filter. Dynamic emittance matching is employed to deflect ions from off-axis points on the sample back onto the mass analyzer axis. Rastering and dynamic emittance matching are rapidly controlled by assembly language programs using an IBM/AT (80286) type computer. A low energy ion monitor was used to tune and evaluate the secondary ion source by providing a magnified cross-sectional image of the ion beam at the source exit aperture. A well-focused and centered secondary ion beam was obtained from each point on the sample, indicating that large-scale dynamic emittance matching with high collection efficiency is possible. Mass resolved images of grids and glycerol samples are shown to demonstrate the performance of the integrated secondary ion source mass analyzer and control system.

A molecular secondary ionization source for use with a high performance tandem mass spectrometer

Abstract A secondary ion source has been designed, constructed and mounted to a triple-sector (EBE) mass spectrometer. The intense secondary ion beam produced by the source permits routine tandem mass spectrometry of involatile analytes. The gas phase fragmentation of secondary protonated argenine has been investigated using this instrument.

Secondary ion emission and images from a biologic matrix

Abstract Secondary ion mass spectra and images were obtained from organic compounds deposited on gold and 30–50 μm thick biologic tissue substrates. Analyte solutions were prepared from acetylcholine chloride, choline chloride, and methylphenylpyridinium (MPP + ) iodide. Tandem mass spectrometry (MS/MS) was used to distinguish secondary ions characteristic of the analyte from secondary ions characteristic of the tissue itself. Effects of primary ion damage appear similar regardless of substrate. Samples of choline chloride deposited on a gold substrate are exceptional; secondary ion emission from such samples appeared unaffected by primary ion dose. Emission of acetylcholine secondary ions was found to decay with a rate independent of primary ion dose, but dependent on the substrate. These results show that the distribution of organic compounds can be mapped from biologic tissue under conditions of static SIMS, but matrix effects and chemical noise must be considered.