Investigation of methods to enhance the secondary ion yields in TOF-SIMS of organic samples

New preparation techniques for molecular and in-situ analysis of ancient organic micro- and nanostructures

Organic microfossils preserved in three dimensions in transparent mineral matrices such as cherts/quartzites, phosphates, or carbonates are best studied in petrographic thin sections. Moreover, microscale mass spectrometry techniques commonly require flat, polished surfaces to minimize analytical bias. However, contamination by epoxy resin in traditional petrographic sections is problematic for the geochemical study of the kerogen in these microfossils and more generally for the in situ analysis of fossil organic matter. Here, we show that epoxy contamination has a molecular signature that is difficult to distinguish from kerogen with time-of-flight secondary ion mass spectrometry (ToF-SIMS). This contamination appears pervasive in organic microstructures embedded in micro- to nano-crystalline carbonate. To solve this problem, a new semi-thin section preparation protocol without resin medium was developed for micro- to nanoscale in situ investigation of insoluble organic matter. We show that these sections are suited for microscopic observation of Proterozoic microfossils in cherts. ToF-SIMS reveals that these sections are free of pollution after final removal of a <10 nm layer of contamination using low-dose ion sputtering. ToF-SIMS maps of fragments from aliphatic and aromatic molecules and organic sulfur are correlated with the spatial distribution of organic microlaminae in a Jurassic stromatolite. Hydrocarbon-derived ions also appeared correlated with kerogenous microstructures in Archean cherts. These developments in analytical procedures should help future investigations of organic matter and in particular, microfossils, by allowing the spatial correlation of microscopy, spectroscopy, precise isotopic microanalyses, and novel molecular microanalyses such as ToF-SIMS.

Comparison of xenon and gallium sources on the detection and mapping of lithium in Li-containing materials by using ToF-SIMS combined FIB-SEM

Li can find itself a wide range of applications since it is the lightest metal. However, Li detection by microscopy-based techniques is problematic because of the highly susceptible nature during electron beam irradiation. ToF-SIMS is a versatile technique to detect Li but the detection of light materials is also problematic due to the large ion contaminated zone and low sputtering yield. By combining ToF-SIMS with a recently launched Xe ion source FIB-SEM, which has small ion contamination and high sputtering yield features, can produce more realistic data at near surface and below the surface region especially for the detection of lightweight materials such as Li. In this study, Li detection and mapping capabilities of ToF-SIMS attached to the FIB-SEM with Ga and Xe ion sources were discussed for Al incorporated Li₇ La₃ Zr₂ O₁₂ solid electrolyte sample that contains Li and Al rich regions at triple junctions. In spite of smoother milling from Ga source, Xe performs more precisely in Li mapping. Low ion contaminated zone, high sputtering yield and low straggling obtained from Monte Carlo simulations are the main advantages of Xe ion sources. The Li detection efficiency for Xe is higher than Ga source discriminating the LiAlO₂ phase placed at the triple junctions of grains and La₂ Zr₂ O₇ regions placed at the outer side of LLZO neighbouring the LiAlO₂ phase. LAY DESCRIPTION: Li can find itself a wide range of applications since it is the lightest metal. However, Li detection by microscopy-based techniques is problematic because of the highly susceptible nature during electron beam irradiation. ToF-SIMS is a versatile technique to detect Li but the detection of light materials is also problematic due to the large ion contaminated zone and low sputtering yield. By combining ToF-SIMS with a recently launched Xe ion source FIB-SEM, which has small ion contamination and high sputtering yield features, can produce more realistic data at near surface and below the surface region especially for the detection of lightweight materials such as Li. In this study, Li detection and mapping capabilities of ToF-SIMS attached to the FIB-SEM with Ga and Xe ion sources were discussed for Al incorporated Li₇ La₃ Zr₂ O₁₂ solid electrolyte sample that contains Li and Al rich regions at triple junctions. In spite of smoother milling from Ga source, Xe performs more precisely in Li mapping. Results were also supported from Monte Carlo simulations of ion-atom interactions. The Li detection resolution of xenon is much higher than gallium source discriminating the LiAlO₂ phase placed at the triple junctions of grains and La₂ Zr₂ O₇ regions placed at the outer side of LLZO neighbouring the LiAlO₂ phase.

Metal-assisted polyatomic SIMS and laser desorption/ionization for enhanced small molecule imaging of bacterial biofilms

Mass spectrometry imaging (MSI) has become an important analytical tool for many sectors of science and medicine. As the application of MSI expands into new areas of inquiry, existing methodologies must be adapted and improved to meet emerging challenges. Particularly salient is the need for small molecule imaging methods that are compatible with complex multicomponent systems, a challenge that is amplified by the effects of analyte migration and matrix interference. With a focus on microbial biofilms from the opportunistic pathogen Pseudomonas aeruginosa, the relative advantages of two established microprobe-based MSI techniques-polyatomic secondary ion mass spectrometry (SIMS) and laser desorption/ionization-are compared, with emphasis on exploring the effect of surface metallization on small molecule imaging. A combination of qualitative image comparison and multivariate statistical analysis demonstrates that sputtering microbial biofilms with a 2.5 nm layer of gold selectively enhances C60-SIMS ionization for several molecular classes including rhamnolipids and 2-alkyl-quinolones. Metallization also leads to the reduction of in-source fragmentation and subsequent ionization of media-specific background polymers, which improves spectral purity and image quality. These findings show that the influence of metallization upon ionization is strongly dependent on both the surface architecture and the analyte class, and further demonstrate that metal-assisted C60-SIMS is a viable method for small molecule imaging of intact molecular ions in complex biological systems.

Changes in the molecular ion yield and fragmentation of peptides under various primary ions in ToF-SIMS and matrix-enhanced ToF-SIMS

Time of flight secondary ion mass spectrometry (ToF-SIMS) is a powerful technique for the nanoanalysis of biological samples, but improvements in sensitivity are needed in order to detect large biomolecules, such as peptides, on the individual cell level at physiological concentrations. Two promising options to improve the sensitivity of SIMS to large peptides are the use of cluster primary ions to increase desorption of intact molecules or the use of matrix-assisted laser desorption/ionization (MALDI) matrices to increase the ionization probability. In this paper, the authors have combined these two approaches in order to improve understanding of the interaction between ionization and fragmentation processes. The peptides bradykinin and melittin were prepared as neat monolayers on silicon, in a Dextran-40 matrix and in two common MALDI matrices, 2,5-dihydroxybenzoic acid (DHB) and α-cyano-4-hydroxy cinnamic acid (HCCA). ToF-SIMS spectra of these samples were collected using a range of small Bi cluster primary ions and large Ar cluster primary ions. The trends observed in the molecular ion yield and the [M+H](+)/C4H8N(+) ratio with primary ion cluster size were sample system dependent. The molecular ion yield of the bradykinin was maximized by using 30 keV Bi3 (+) primary ions in a DHB matrix but in the HCCA matrix, the maximum molecular ion yield was obtained by using 30 keV Bi7 (+) primary ions. In contrast, the molecular ion yield for melittin in both matrices was greatest using 20 keV Ar2000 (+) primary ions. Improvements in the molecular ion yield were only loosely correlated with a decrease in small fragment ions. The data indicate a complex interplay between desorption processes and ion formation processes which mean that the optimal analytical conditions depend on both the target analyte and the matrix.

Enhancing secondary ion yields in time of flight-secondary ion mass spectrometry using water cluster primary beams

Low secondary ion yields from organic and biological molecules are the principal limitation on the future exploitation of time of flight-secondary ion mass spectrometry (TOF-SIMS) as a surface and materials analysis technique. On the basis of the hypothesis that increasing the density of water related fragments in the ion impact zone would enhance proton mediated reactions, a prototype water cluster ion beam has been developed using supersonic jet expansion methodologies that enable ion yields using a 10 keV (H₂O)₁₀₀₀⁺ beam to be compared with those obtained using a 10 keV Ar₁₀₀₀⁺ beam. The ion yields from four standard compounds, arginine, haloperidol, DPPC, and angiotensin II, have been measured under static+ and high ion dose conditions. Ion yield enhancements relative to the argon beam on the order of 10 or more have been observed for all the compounds such that the molecular ion yield per a 1 μm pixel can be as high as 20, relative to 0.05 under an argon beam. The water beam has also been shown to partially lift the matrix effect in a 1:10 mixture of haloperidol and dipalmitoylphosphatidylcholine (DPPC) that suppresses the haloperidol signal. These results provide encouragement that further developments of the water cluster beam to higher energies and larger cluster sizes will provide the ion yield enhancements necessary for the future development of TOF-SIMS.

Effects of metal nanoparticles on the secondary ion yields of a model alkane molecule upon atomic and polyatomic projectiles in secondary ion mass spectrometry

A model alkane molecule, triacontane, is used to assess the effects of condensed gold and silver nanoparticles on the molecular ion yields upon atomic (Ga(+) and In(+)) and polyatomic (C60(+) and Bi3(+)) ion bombardment in metal-assisted secondary ion mass spectrometry (MetA-SIMS). Molecular films spin-coated on silicon were metallized using a sputter-coater system, in order to deposit controlled quantities of gold and silver on the surface (from 0 to 15 nm equivalent thickness). The effects of gold and silver islets condensed on triacontane are also compared to the situation of thin triacontane overlayers on metallic substrates (gold and silver). The results focus primarily on the measured yields of quasi-molecular ions, such as (M - H)(+) and (2M - 2H)(+), and metal-cationized molecules, such as (M + Au)(+) and (M + Ag)(+), as a function of the quantity of metal on the surface. They confirm the absence of a simple rule to explain the secondary ion yield improvement in MetA-SIMS. The behavior is strongly dependent on the specific projectile/metal couple used for the experiment. Under atomic bombardment (Ga(+), In(+)), the characteristic ion yields an increase with the gold dose up to approximately 6 nm equivalent thickness. The yield enhancement factor between gold-metallized and pristine samples can be as large as approximately 70 (for (M - H)(+) under Ga(+) bombardment; 10 nm of Au). In contrast, with cluster projectiles such as Bi3(+) and C60(+), the presence of gold and silver leads to a dramatic molecular ion yield decrease. Cluster projectiles prove to be beneficial for triacontane overlayers spin-coated on silicon or metal substrates (Au, Ag) but not in the situation of MetA-SIMS. The fundamental difference of behavior between atomic and cluster primary ions is tentatively explained by arguments involving the different energy deposition mechanisms of these projectiles. Our results also show that Au and Ag nanoparticles do not induce the same behavior in MetA-SIMS of triacontane. The microstructures of the metallized layers are also different. While metallic substrates provide higher yields than metal islet overlayers in the case of silver, whatever the projectile used, the situation is reversed with gold.