Time-of-flight Secondary Ion Mass Spectrometry Spectra Research Articles

Embedded structure of silicon monoxide in SiO 2 films

The structure of SiO x ( x = 1.94) films has been investigated using both X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The SiO x films were deposited by vacuum evaporation. XPS spectra show that SiO 1.94 films are composed of silicon suboxides and the SiO 2 matrix. Silicon clusters appeared only negligibly in the films in the XPS spectra. Si 3O + ion species were found in the TOF-SIMS spectra with strong intensity. These results reveal the structure of the films to be silicon monoxide embedded in SiO 2, and this structure most likely exists as a predominant form of Si 3O 4. The existence of Si–Si structures in the SiO 2 matrix will give rise to dense parts in loose glass networks.

Time-of-Flight Secondary Ion Mass Spectrometric Analysis of Polymer Tertiary Structure in Langmuir Monolayer Films of Poly(dimethylsiloxane)

A series of model systems of poly(dimethylsiloxane) (PDMS) of molecular mass 2400 Da and low polydispersity (1.09) were prepared using the Langmuir-Blodgett technique to investigate the effects of tertiary structure on the ion formation probability in time-of-flight secondary ion mass spectrometry (TOF-SIMS). Using data from the measured surface pressure-area isotherm for PDMS ordered monolayer, films were transferred to silver mirror substrates from the various regions in the isotherm where structural changes are observed. Two particular conformations of the polymer are examined here: a linear caterpillar-like structure and a coiled helical structure. Reflection absorption Fourier transform infrared spectroscopic analysis indicates structural changes for the films related to adjustments to the orientation of the polymeric backbone consistent with the formation of two distinct structures. The polymeric changes translate into differences in ion formation probability of fragments in both the low (<250 Da) and high (>1000 Da) mass range. Data are also presented to analyze how tertiary structure may affect the apparent polydispersity index calculated from the TOF-SIMS spectra.

Charge retention by peptide ions soft-landed onto self-assembled monolayer surfaces

Soft-landing of singly and doubly protonated peptide ions onto three self-assembled monolayer surfaces (SAMs) was performed using a novel ion deposition instrument constructed in our laboratory and a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially designed for studying collisions of large ions with surfaces. Modified surfaces were analyzed using in situ 2 keV Cs + secondary ion mass spectrometry or ex situ 15 keV Ga + time-of-flight-secondary ion mass spectrometry (ToF-SIMS). The results demonstrate that a fraction of multiply protonated peptide ions retain more than one proton following soft-landing on the FSAM surface. It is shown that the [M+2H] 2+ ions observed in FT-ICR SIMS spectra are produced by desorption of multiply charged ions from the surface, while re-ionization of singly protonated ions or neutral peptides is a source of [M+2H] 2+ ions in ToF-SIMS spectra. Differences in neutralization efficiency of soft-landed ions following exposure of surfaces to laboratory air has a measurable effect on the results of ex situ ToF-SIMS analysis of soft-landed ions on SAM surfaces.

Mutual information theory for biomedical applications: Estimation of three protein-adsorbed dialysis membranes

Protein-adsorbed dialysis membranes are evaluated with time-of-flight secondary ion mass spectrometry (TOF-SIMS) chemical imaging technique. Protein adsorption causing permeability change is one of big issues in the development of dialysis membranes. Bovine serum albumin adsorption onto three kinds of dialysis membranes has been evaluated with TOF-SIMS. In the present study three kinds of proteins, bovine serum albumin, α-chymotripsinogen A, and cytochrome C adsorbed onto hollow-fiber dialysis membranes, were measured by means of TOF-SIMS and then TOF-SIMS spectra were analyzed using mutual information. Then specific peaks of fragment ions related to α-chymotripsinogen A and bovine serum albumin were found, respectively. In this condition, however, specific peaks to cytochrome C were not able to find compared with other samples. Finally, chemical images of α-chymotripsinogen A and bovine serum albumin, respectively, adsorbed onto the membranes with co-existing proteins were obtained. The results of TOF-SIMS images of the proteins on the membranes show different tendency of adsorption depending on co-existing proteins. Further study is needed to study more detailed protein adsorption onto the membranes with co-existing proteins.

Using time-of-flight secondary ion mass spectrometry and multivariate statistical analysis to detect and image octabenzyl-polyhedral oligomeric silsesquioxane in polycarbonate

Silsesquioxane, with an empirical formula of RSiO3/2, has the potential to combine the mechanical properties of plastics with the oxidative stability of ceramics in one material [D.W. Scott, J. Am. Chem. Soc. 68 (1946) 356; K.J. Shea, D.A. Loy, Acc. Chem. Res. 34 (2001) 707; K.-M. Kim, D.-K. Keum, Y. Chujo, Macromolecules 36 (2003) 867; M.J. Abad, L. Barral, D.P. Fasce, R.J.J. William, Macromolecules 36 (2003) 3128]. The high sensitivity, surface specificity, and ability to detect and image high mass additives make time-of-flight secondary ion mass spectrometry (ToF-SIMS) a powerful surface analytical instrument for the characterization of polymer composite surfaces in an analytical laboratory [J.C. Vickerman, D. Briggs (Eds.), ToF-SIMS Surface Analysis by Mass Spectrometry, Surface Spectra/IMPublications, UK, 2001; X. Vanden Eynde, P. Bertand, Surf. Interface Anal. 27 (1999) 157; P.M. Thompson, Anal. Chem. 63 (1991) 2447; S.J. Simko, S.R. Bryan, D.P. Griffis, R.W. Murray, R.W. Linton, Anal. Chem. 57 (1985) 1198; S. Affrossman, S.A. O’Neill, M. Stamm, Macromolecules 31 (1998) 6280]. In this paper, we compare ToF-SIMS spectra of control samples with spectra generated from polymer nano-composites based on octabenzyl-polyhedral oligomeric silsesquioxane (BnPOSS) as well as spectra (and images) generated from multivariate statistical analysis (MVSA) of the entire spectral image. We will demonstrate that ToF-SIMS is able to detect and image low concentrations of BnPOSS in polycarbonate. We emphasize the use of MVSA tools for converting the massive amount of data contained in a ToF-SIMS spectral image into a smaller number of useful chemical components (spectra and images) that fully describe the ToF-SIMS measurement.

Static ToF-SIMS analysis of plasma chemically deposited ethylene/allyl alcohol co-polymer films

A plasma co-polymerization of ethylene as a “chain extending” monomer and allyl alcohol as a carrier monomer for hydroxyl groups was studied. The composition of the feed gas was systematically varied and the plasma co-polymers were analyzed in terms of their relative concentrations of OH functional groups by static Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) immediately after their preparation without any air contact, i.e., under so called “in situ” conditions. The relative OH group concentration involved in the –CH 2–OH groups was derived from the normalized yields of the CH 3O + secondary fragment ion. The density of functional groups in the plasma co-polymers was found to vary non-linearly with respect to the mole percentage of the monomers in the feed gas. Co-polymerization phenomena, i.e. chemical interactions and recombination reactions taking place between monomer molecules in the plasma and/or during the deposition process, become evident in ToF-SIMS spectra.

Simplifying the interpretation of ToF-SIMS spectra and images using careful application of multivariate analysis

As analytical problems addressed using time-of-flight secondary ion mass spectrometry (ToF-SIMS) increase in chemical complexity, multivariate analysis (MVA) methods have become standard tools for simplifying the interpretation of ToF-SIMS spectra and images. MVA methods can significantly simplify ToF-SIMS datasets by providing a comprehensive description of the data using a small number of variables, typically in an automated fashion requiring minimal user intervention. However, successful and widespread application of MVA methods to SIMS data analysis is limited by a lack of understanding of the outputs of MVA methods and optimization of these methods for ToF-SIMS data analysis. Appropriate selection of data pre-processing and MVA tools are critical for accurate interpretation of ToF-SIMS spectra and images. As an example, an image dataset of a selectively ion-etched polymer film was analyzed to identify and characterize the chemically distinct regions in the image. Principal component analysis (PCA) and multivariate curve resolution (MCR) after pre-processing using normalization or Poisson-scaling were compared to identify the etched and non-etched regions of the image. The utility of each pre-processing and MVA method was examined, with MCR coupled with Poisson-scaling being the appropriate choice for identifying the different chemical phases present in the image. However, appropriate selection of data pre-processing and MVA methods generally depends on the specific dataset being analyzed and the goals of the analysis.

Investigating Activated Sludge Flocs using Microanalytical Techniques: Demonstration of Environmental Scanning Electron Microscopy and Time‐of‐Flight Secondary Ion Mass Spectrometry for Wastewater Applications

Environmental scanning electron microscopy (ESEM) with an energy-dispersive X-ray spectrometer (EDS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were demonstrated to be useful analytical tools for investigating surface and bulk components of individual floc particles from both full- and bench-scale activated sludge systems. Detailed surface imaging of various hydrated biological floc particles by ESEM revealed substantial differences in surface features between treatment systems, while EDS identified spatial differences in the iron and the aluminum distributions. The ToF-SIMS spectra had signature fragments of protein and polysaccharide material from the floc surface, suggesting that this technique is capable of surface profiling extracellular polymeric substances. Principal-component analysis of the positive ion ToF-SIMS spectra from the mixed-liquor-suspended solid (MLSS) samples and reference aquatic organic materials found slight differences between the full- and bench-scale MLSS surface properties but substantial differences among MLSS and treated effluent from the same facility.

Surface segregational behaviour studied as an effect of thickness by SIMS and AFM in annealed PS-PMMA blend and block copolymer thin films

The surface behaviour of a two-phase polymer mixture depends on the chemical structure of the polymer components, the interaction between the two polymers and the processing conditions. The microscopic morphology and the surface composition need to be known in order to fully utilize the thin film properties. The technique of static time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used to obtain the molecular surface composition of thin films of blends and block copolymers. The depth profiling tool of Nano-SIMS, a dynamic SIMS technique, helps to provide the chemical mapping of the surface in 2D and 3D. The surface morphology is investigated using AFM. Thin films of PS and PMMA diblock copolymers with molecular weight of 12K-12K and 10K-10K and blends of PS/PMMA (10K/10K) for thicknesses ranging from 5 nm to 50 nm are examined. For the blends, the ToF-SIMS spectra for all the thicknesses show the same behaviour of a high increase of PMMA on the surface after annealing. Nano-SIMS images reveal the formation of nanostructures on the annealed surfaces and AFM studies show these nanostructures to be droplets having distinct phase shift from the surrounding matrix. The droplet dimensions increase with the increase of the thickness of the film but the absolute intensity from the ToF-SIMS spectra for all the annealed films remains almost the same. For the copolymers, the ToF-SIMS spectra show that there is a decrease of PMMA on the surface for the annealed films when compared to the as-cast ones. AFM morphology reveals that, for different thicknesses, annealing induces different topographical features like droplets, holes, spinodal patterns, etc. but with no distinct phase shift between the patterns and the surrounding matrix. The two different copolymers of comparable molecular weight are found to exhibit very different topography even when the thickness of the films remained the same. The surface composition from the ToF-SIMS data, however, was not found to vary even when the topography was completely different.

Time of Flight-Secondary Ion Mass Spectrometry on isolated extracellular fractions and intact biofilms of three species of benthic diatoms

Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) was used to study compositional characteristics of Extracellular Polymeric Substances (EPS) and compared these to characteristics of the EPS-matrix of intact diatom biofilms. Three benthic diatoms species were investigated, Cylindrotheca closterium, Navicula mutica and Nitzschia cf. brevissima. Comparison of the ToF-SIMS spectra of sequentially extracted EPS-fractions by cluster analysis and multidimensional scaling analysis (MDS) indicated that soluble and bound EPS were not distinguishable based on their ion spectra. On the contrary the water insoluble bicarbonate soluble (WIBS)-EPS-fraction formed a distinct cluster showing that this material was compositionally different from the other EPS-fractions. Ion spectra of the EPS-fractions were dissimilar to results obtained from intact biofilms. This suggested that during the extraction procedure, the structure of the EPS irreversibly changed, which alters the fragmentation patterns of the extracellular surface layer. Furthermore, from the examination of the positive ion spectra it was shown that the overall composition of EPS in the intact biofilms was different between diatom species. In spite of these differences, several common peak patterns were shared between different species. This suggests the presence of common structural components in the EPS of these diatoms that may play a role in building the surface EPS-layer.

ToF-SIMS Characterization of Thermal Modifications of Bixin from Bixa orellana Fruit

Bixa orellana fruit extracts were studied by time-of-flight secondary ion mass spectrometry (ToF-SIMS). The intensity of the peak at m/z 396, assigned to the bixin molecular ion plus two hydrogen atoms (C25H(32)O(4+), decreased as the extract was heated and nearly disappeared with heating above 150 degrees C. Simultaneously, the formation of dimers at m/z 790, 804, and 818 was observed. The ToF-SIMS spectrum is characterized by a large amount of peaks generated by the principal ions and their multiple fragmentation patterns. To extract maximum information from the data set, multivariate statistical analysis was applied. Principal component analysis revealed important structural changes of the bixin molecule upon heating at different temperatures. This information can be used by the food industry as by controlling the temperature of the heating process the red/yellow balance of this colorant can be tuned.

Multivariate Analysis of TOF-SIMS Spectra of Monolayers on Scribed Silicon

Static time-of-flight secondary ion mass spectrometry (TOF-SIMS) was performed on monolayers on scribed silicon (Si(scr)) derived from 1-alkenes, 1-alkynes, 1-holoalkanes, aldehydes, and acid chlorides. To rapidly determine the variation in the data without introducing user bias, a multivariate analysis was performed. First, principal components analysis (PCA) was done on data obtained from silicon scribed with homologous series of aldehydes and acid chlorides. For this study, the positive ion spectra, the negative ion spectra, and the concatentated (linked) positive and negative ion spectra were preprocessed by normalization, mean centering, and autoscaling. The mean centered data consistently showed the best correlations between the scores on PC1 and the number of carbon atoms in the adsorbate. These correlations were not as strong for the normalized and autoscaled data. After reviewing these methods, it was concluded that mean centering is the best preprocessing method for TOF-SIMS spectra of monolayers on Si(scr). A PCA analysis of all of the positive ion spectra revealed a good correlation between the number of carbon atoms in all of the adsorbates and the scores on PC1. PCA of all of the negative ion spectra and the concatenated positive and negative ion spectra showed a correlation based on the number of carbon atoms in the adsorbate and the class of the adsorbate. These results imply that the positive ion spectra are most sensitive to monolayer thickness, while the negative ion spectra are sensitive to the nature of the substrate-monolayer interface and the monolayer thickness. Loadings show an inverse relationship between (inorganic) fragments that are expected from the substrate and (organic) fragments expected from the monolayer. Multivariate peak intensity ratios were derived. It is also suggested that PCA can be used to detect outlier surfaces. Partial least squares showed a strong correlation between the number of carbon atoms in the adsorbate and the number it predicted.

Quantitative Chemical Derivatization Technique in Time-of-Flight Secondary Ion Mass Spectrometry for Surface Amine Groups on Plasma-Polymerized Ethylenediamine Film

A chemical derivatization technique in time-of-flight secondary ion mass spectrometry (TOF-SIMS) has been developed to quantify the surface density of amine groups of plasma-polymerized ethylenediamine thin film deposited on a glass surface by inductively coupled plasma chemical vapor deposition. Chemical tags of 4-nitrobenzaldehyde or pentafluorobenzaldehyde were hybridized with the surface amine groups and were detected in TOF-SIMS spectra as characteristic molecular secondary ions. The surface amine density was controlled in a reproducible manner as a function of deposition plasma power and was also quantified using UV-visible spectroscopy. A good linear correlation was observed between the results of TOF-SIMS and UV-visible measurements as a function of plasma power. This shows that the chemical derivatization technique in TOF-SIMS analysis would be useful in quantifying the surface density of specific functional groups that exist on the organic surface.

Observation of fluorescence-labeled protein A on a biosensor surface by means of TOF-SIMS

The ability to carry out evaluations of proteins on biomaterial surfaces such as the distribution of each protein and the orientation of immobilized proteins are required for the development of high performance materials. Since proteins all have the same basic chemical structure it is difficult to distinguish one from the others by means of spectroscopy, even though such techniques do provide useful chemical information. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) provides an avenue into the submicron-scale distribution of specific chemicals and enables investigation of chemical conditions on materials surfaces. Though TOF-SIMS also has problems with the characterization of the mass spectra of protein samples, the mass spectra can be interpreted using data analysis techniques such as are provided by information theory. In this study, the distribution of fluorescence-labeled protein A immobilized on glass plates was chemically visualized by means of TOF-SIMS and a spectra analysis technique making use of mutual information. With this analysis technique, TOF-SIMS spectra can be interpreted such that it allows one to select specific peaks for chemical imaging of proteins. In addition, fragment ions from protein A labeled with different fluorescence substances, FITC and quantum dots, are characterizable by means of the mutual information, and their chemical conditions are investigated.

Identifying the Characteristic Secondary Ions of Lignin Polymer Using ToF−SIMS

The chemical structure of lignin, a complex, irregular polymer of phenylpropane units that occurs in plant cell walls, was investigated using time-of-flight secondary ion mass spectrometry (ToF-SIMS). The positive ToF-SIMS spectra of lignin isolated from pine and beech wood showed prominent secondary ions possessing guaiacyl (at m/z 137 and 151) or syringyl (at m/z 167 and 181) rings, which are the basic building units of lignin polymer. This shows that ToF-SIMS is a useful tool for lignin structural analysis. The peaks at m/z 137 and 167 were assigned as the C6-C1 ion, and the peaks at m/z 151 and 181 may be double-component, the C6-C1 ion and the C6-C2 ion. We confirmed the characteristic guaiacyl ions using a synthetic lignin model compound, dehydrogenation polymer (DHP), which was formed by polymerizing of unlabeled and deuterium-labeled coniferyl alcohols. The formation mechanism of the main secondary ions was deduced by labeling specific positions of coniferyl alcohols with a stable isotope to study the relationship between chemical structure and secondary ion formation in ToF-SIMS.

Quantitative analysis of styrene-pentafluorostyrene random copolymers by ToF-SIMS and XPS

Poly(styrene) (PS), poly(2,3,4,5,6-pentafluorostyrene) (5FPS) and their random copolymers were prepared by bulk radical polymerization. The spin-cast polymer films of these polymers were analyzed using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The surface and bulk compositions of these copolymers were found to be same, implying that surface segregation did not occur. The detailed analysis of ToF-SIMS spectra indicated that the ion fragmentation mechanism is similar for both PS and 5FPS. ToF-SIMS quantitative analysis using absolute peak intensity showed that the SIMS intensities of positive styrene fragments, particularly C7H7+, in the copolymers are higher than the intensities expected from a linear combination of PS and 5FPS, while the SIMS intensities of positive pentafluorostyrene fragments are smaller than expected. These results indicated the presence of matrix effects in ion formation process. However, the quantitative approach using relative peak intensity showed that ion intensity ratios are linearly proportional to the copolymer mole ratio when the characteristic ions of PS and 5FPS are selected. This suggests that quantitative analysis is still possible in this copolymer system. Copyright © 2005 John Wiley & Sons, Ltd.

Effective monitoring of protein reaction on glass plate surfaces by TOF-SIMS

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is capable of chemically visualizing proteins on insulated samples. Distribution of an immobilized probe protein, fluorescent-labeled protein A-immobilized on a glass plate, and that of a sample protein, immunogloblin G (IgG) in solution, reacting with protein A on the biosensor surface, were evaluated with TOF-SIMS (TFS-2100, Physical Electronics). TOF-SIMS spectra and images of the protein on the glass plates were obtained, and this “mutual information”, as defined by information theory, was employed to analyze the TOF-SIMS spectra of proteins. Fragment ions from protein A and IgG were distinguished by the mutual, reinforcing information and specific fragment ions to each protein were selected to obtain the TOF-SIMS image of the protein. It is evident from the TOF-SIMS images of each protein that protein A was immobilized on the substrate homogeneously and that the reaction between the immobilized protein A and IgG is not localized in this condition. Chemical images of the proteins by TOF-SIMS will contribute to a better understanding of the reaction on the biosensor surface, and thus will help the development of more sophisticated biosensors. In addition, the requisite chemical conditions as well as the interaction between the biosensor surface and the immobilized proteins were investigated by TOF-SIMS by means of sets of reinforcing, mutually supportive information.

Silver deposition on freeze-dried cells allows subcellular localization of cholesterol with imaging TOF-SIMS.

Imaging time-of-flight secondary ion mass spectrometry (TOF-SIMS) was used for characterization and subcellular localization of organic ions in leucocytes adhering to glass surfaces. The cells were fixed by freeze drying in 0.15 m ammonium formate buffer at pH 7.2-7.4. The freeze-dried cells were sputter-coated with silver, and the silver surface was analysed with imaging TOF-SIMS. TOF-SIMS spectra were recorded by scanning the primary ion beam over the analysis area and acquiring positive mass spectra of the ions leaving the surface. The relative brightness of each pixel within the analysis area reflects the signal intensity of a selected ion in that pixel. Data were collected separately at high mass resolution m/delta m > 7000 and at high lateral resolution (= 0.5 micro m). The images were analysed by principal component analysis (PCA). The glass-adhering cells showed a well defined attachment area with a diameter of up to 20 micro m, and an equally well defined cell body, containing the nucleus, with a diameter of 8-10 micro m. On the raw data images, the obtained cholesterol distributions were consistent with a higher cholesterol content of the cell membrane in the attachment area than in the cell body. Using PCA analysis, silver-cationized molecular cholesterol was found localized mainly in the attachment area of the cells. Cholesterol was also seen at higher concentration in circular spots of </= 1 micro m in diameter, probably representing caveolae.

Maximizing information obtained from secondary ion mass spectra of organic thin films using multivariate analysis

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) can give a detailed description of the surface chemistry and structure of organic materials. The high mass resolution and high mass range mass spectra obtainable from modern ToF-SIMS instruments offer the ability to rapidly obtain large amounts of data. Distillation of that data into usable information presents a significant problem in the analysis of ToF-SIMS data from organic materials. Multivariate data analysis techniques have become increasingly common for assisting with the interpretation of complex ToF-SIMS data sets. This study presents an overview of principal component analysis (PCA) and partial least squares regression (PLSR) for analyzing the ToF-SIMS spectra of alkanethiol self-assembled monolayers (SAMs) adsorbed onto gold substrates and polymer molecular depth profiles obtained using an SF 5 + primary ion beam. The effect of data pretreatment on the information obtained from multivariate analysis of these data sets has been explored. Multivariate analysis is an important tool for maximizing the information obtained from the ToF-SIMS spectra of organic thin films.

Mass spectrometric imaging of lipids in brain tissue.

The spatial distributions of various specific lipids in freeze-dried mouse brain sections were monitored using time-of-flight secondary ion mass spectrometry (TOF-SIMS). Mouse brain sections were prepared by cryosectioning, rinsing in 0.15 M NH3HCOO, and freeze-drying, after which the samples were analyzed directly by TOF-SIMS, using Au3+ ions as primary ions. Positive and negative TOF-SIMS spectra of the tissue surface contained peaks from quasimolecular ions of a variety of specific lipids, including cholesterol, sulfatides, phosphatidylinositols, and phosphatidylcholines. Images showing the spatial signal intensity distributions of specific ions were recorded across analysis areas ranging from 100 x 100 microm(2) to 9 x 9 mm(2). The results demonstrate a highly complementary localization of cholesterol and phosphatidylcholine over dimensions from millimeter to micrometer range. Characteristic spatial distributions of several other lipids, including sulfatides and phosphatidylinositols, were observed. Principal component analysis was used to localize regions of the sample surface that show common spectral features. Spectra from different such regions showed large variations in lipid ion signals, indicating large variations in the lipid composition in different regions.