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Covering: 2009-2019 Over the last decade, methods in imaging mass spectrometry (IMS) have progressively improved and diversified toward a variety of applications in natural products research. Because IMS allows for the spatial mapping of the production and distribution of biologically active molecules in situ, it facilitates phenotype and organelle driven discovery efforts. As practitioners of IMS for natural products discovery, we find one of the most important aspects of these experiments is the sample preparation and compatibility with different ionization sources that are available to a given researcher. As such, we have focused this mini review to cover types of ionization sources that have been used in natural products discovery applications and provided concrete examples of use for natural products discovery while discussing the advantages and limitations of each method. We aim for this article to serve as a resource to guide the broader natural product community interested in IMS toward the application/method that would best serve their natural product discovery needs given the sample and analyte(s) of interest. This mini review has been limited to applications using natural products and thus is not exhaustive of all possible ionization methods which have only been applied to image other types of samples such as mammalian tissues. Additionally, we briefly review how IMS has been coupled with other imaging platforms, such as microscopy, to enhance information outputs as well as offer our future perspectives on the incorporation of IMS in natural products discovery.

Impacts of energetic C60 ions with energies of the order of MeV are excellent ionization methods for highly sensitive secondary ion (SI) mass spectrometry because they can provide SIs necessary for the identification of target materials in high emission yields with high reproducibility. As part of an investigation into their SI emission phenomena with the aim of controlling the mass-analyzed depth range, we measured and characterized the impact angle dependence of SI mass spectra for MeV C60 ion impacts, and found that shallower angle impacts of MeV C60 ions can provide surface-sensitive information through intense emission of surface-derived SIs. [ABSTRACT FROM AUTHOR]

Imaging mass spectrometry (IMS), based on linking mass-analysis data for ions and their original positions in a sample, allows the visualization of the spatial distribution of atomic and/or molecular species in the sample, and is therefore an essential analytical method for the characterization of chemical and biological materials. For sensitive elemental and chemical imaging using IMS, a sufficient amount of ions to be analyzed should be supplied from the analyzing area on the sample surface. Secondary ion mass spectrometry (SIMS) using energetic cluster ions (ECIs) with energies of sub-MeV and above as primary ions (ECI-SIMS) can provide the secondary ions necessary for the characterization of organic materials with high emission yields. Hence, ECI-SIMS is a good method for highly sensitive IMS. We report recent progress in ECI beam manipulation techniques for applications of ECI-SIMS to highly sensitive IMS together with the characteristics of ECI impacts on organic materials. [ABSTRACT FROM AUTHOR]

We have investigated the negative and positive secondary ions emitted from ethanol droplets by 4.0-MeV C 3+ impact to reveal the characteristic features of the reaction processes induced by fast heavy ions at the liquid ethanol surface. Analysis of the secondary ions was performed by time-of-flight mass spectrometry for microdroplet targets in a high vacuum environment. Fragment ions, deprotonated cluster ions, and trace amounts of the reaction product ions are observed in the negative secondary ions. The main fragment anions are C 2 H m O − ( m  = 1, 3, and 5) and C 2 H − generated by loss of hydrogen and oxygen atoms. The reaction product anions include deprotonated glycols, larger alcohols, and their dehydrated and dehydrogenated forms generated by secondary reactions between fragments and radicals. Furthermore, C 3 H m − ( m  = 0–2) and C 4 H m − ( m  = 0 and 1) are observed, which could be produced through a plasma state generated in the heavy ion track. Deprotonated ethanol cluster ions, [(EtOH) n  − H] − , are observed up to about n  = 25. [(EtOH) n  − H] − have smaller kinetic energies than the protonated cluster ions (EtOH) n H + . This probably represents the effect of the positive Coulomb potential transiently formed in the ion track. We also discuss the size distributions and structures of the water- and CH 2 OH-radical-attached ethanol cluster ions. [ABSTRACT FROM AUTHOR]

Abstract: A mass spectrometric technique using nuclear microprobes is presented in this paper for biological applications. In recent years, imaging mass spectrometry has become an increasingly important technique for visualizing the spatial distribution of molecular species in biological tissues and cells. However, due to low yields of large molecular ions, the conventional secondary ion mass spectrometry (SIMS), that uses keV primary ion beams, is typically applied for imaging of either elements or low mass compounds. In this study, we performed imaging mass spectrometry using MeV ion beams collimated to about 10μm, and successfully obtained molecular ion images from plant and animal cell sections. The molecular ion imaging of the pollen section showed high intensities of ions in the pollen cytoplasm, compared to the pollen wall, and indicated the heterogeneous distribution in the cytoplasm. The 3T3-L1 cell image revealed the high intensity of ions, in particular from the cell nucleus. The result showed that not only the individual cell, but also the cell nucleus could be identified with the present imaging technique. [Copyright &y& Elsevier]

Abstract: We performed secondary ion mass spectrometry for arginine thin films in the incident energy range from 10keV to 2MeV, in which electronic energy loss in the surface region increases with energy and exceeds the nuclear energy loss. The yield of protonated arginine increased with incident energy, and at 2MeV it was approximately two orders of magnitude larger than at 10keV, where the nuclear collision process is dominant. It was found that the molecular ion desorption is enhanced by the electronic excitation induced in the near-surface region. In addition, the yield ratios of the fragment ions to the protonated arginine decreased with incident energy, which indicates that the nuclear energy deposition significantly affects molecular fragmentation. These results demonstrate a soft desorption and ionization of biomolecules by the MeV ion-induced electronic excitation. [Copyright &y& Elsevier]

Abstract: CO–NH3 ice at 25K is bombarded by 65MeV fission fragments and the emitted secondary ions are analyzed by time-of-flight mass spectrometry. The yields of the specific ion species (those formed only from CO or from NH3 molecules) and of the hybrid ion species (formed from both CO and NH3 molecules) are determined as a function of the ice temperature. The time-temperature dependence of desorption yields has been used for secondary ion identification because its behavior characterizes the ion''s origin around the sublimation temperature of CO ice (∼30K). The mass spectrum of positive ions measured before CO sublimation is decomposed into three spectra corresponding to CO specific ions, NH3 specific ions and hybrid molecular ions, respectively. The observed spectrum after CO sublimation is very similar to that of a pure NH3 specific spectrum. The total yield of all positive hybrid molecular ions over 600u mass range is found to be about 2 ions/impact: 20% of this is attributed to N and NH3 containing ions and 80% are ions having the C n O m Hl + structure. The ions C n≤3Hl,2,3 +, COH+, NO+, NH3OH+ and NH3CO+ were identified. At T =25K, the total yield of negative ions is about 20 times lower than that of positive ions, the most abundant species being the cyanide ion CN−. Observation of cluster ions based on (HCN) n indicates the formation of hydrogen cyanide. [Copyright &y& Elsevier]

Abstract: We propose to use cluster ions that are much larger than the molecular ions as primary ions for organic secondary ion mass spectrometry. Incident Ar cluster ion beams with energies from 10 to 20keV and a mean size of about 1000 atoms/cluster were used. Secondary ions were measured for a thin arginine film target (200nm) bombarded with large Ar cluster ions using a time-of-flight technique. Molecular ions of arginine and characteristic fragment ions were detected with high sensitivity. When large Ar cluster ions such as Ar1500 were incident on the arginine target, molecular ions of arginine were detected with little fragment ions. This indicates that large cluster ions can ionize arginine molecules without damaging them. [Copyright &y& Elsevier]

The present paper reports the effect of oxygen activity p(O2) on Nb segregation in a polycrystalline specimen of Nb doped TiO2 (2·91 at.-%). The determined segregation induced enrichment factor following annealing at 1773 K in p(O2) = 10 Pa and p(O2) = 21 kPa is 3·1 and 7·3 respectively. The data on the effect of p(O2) on the segregation induced Nb enrichment factor are considered in terms of defect disorder of Nb doped TiO2 and the related lattice charge neutrality requirements. The reported segregation data may be used for controlled modification of surface concentration gradients in TiO2 through the imposition of well defined gas phase composition during processing procedures. [ABSTRACT FROM AUTHOR]

The present paper reports the effect of oxygen activity p(O2) on Nb segregation in a polycrystalline specimen of Nb doped TiO2 (2·91 at.-%). The determined segregation induced enrichment factor following annealing at 1773 K in p(O2) = 10 Pa and p(O2) = 21 kPa is 3·1 and 7·3 respectively. The data on the effect of p(O2) on the segregation induced Nb enrichment factor are considered in terms of defect disorder of Nb doped TiO2 and the related lattice charge neutrality requirements. The reported segregation data may be used for controlled modification of surface concentration gradients in TiO2 through the imposition of well defined gas phase composition during processing procedures. [ABSTRACT FROM AUTHOR]

Abstract: We have measured both secondary neutral and ionized particles from an InSb target under 3.0MeV Si ion bombardment. Measurements of both ions and neutrals have not been carried out so far in the MeV-energy range. The mass spectra and axial emission energy distributions of secondary particles were investigated. Secondary ions were measured with a linear- and a reflective-type time-of-flight technique, whereas secondary neutral particles were photo-ionized by a UV pulsed laser (ArF: 193nm) and measured with a reflective-type time-of-flight technique. Different results were obtained for neutral particles in comparison with ionized particles. The mean energy of neutral Sb atoms was much lower than that of neutral In atoms, whereas the mean energies of secondary In and Sb ions were nearly equal. [Copyright &y& Elsevier]