Polyatomic Ion Sources Research Articles

TOF-SIMS imaging of lipids on rat brain sections.

Since several decades, secondary ion mass spectrometry (SIMS) coupled to time of flight (TOF) is used for atomic or small inorganic/organic fragments imaging on different materials. With the advent of polyatomic ion sources leading to a significant increase of sensitivity in combination with a reasonable spatial resolution (1-10 μm), TOF-SIMS is becoming a more and more popular analytical platform for MS imaging. Even if this technique is limited to small molecules (typically below 1,000 Da), it offers enough sensitivity to detect and locate various classes of lipids directly on the surface of tissue sections. This chapter is thus dedicated to the TOF-SIMS analysis of lipids in positive and negative ion modes on rat brain tissue sections using a bismuth cluster ion source.

Material dependence of argon cluster ion sputter yield in polymers: Method and measurements of relative sputter yields for 19 polymers

There is a pressing need for reference data to allow sputter depth-profiling of polymers using cluster and polyatomic ion sources for the quantification of depth in XPS and SIMS. The authors have developed a new method of sputter rate measurement based on a combination of contact masking and white-light interferometry. This allowed us to measure sputter rates for 19 different polymers of technological significance, a much wider set of data than any available previously. The results show a much larger range of sputter yield than might previously have been expected. For example, the sputter yield of PMMA being more than ten times that of poly ether ether ketone when using argon ion clusters of around 4 eV/atom, with other polymers being widely distributed between these extremes. Without reference data for sputter rate this wide range could lead to major errors in depth estimation in sputter depth-profiling of polymer coatings, biomaterials, nanostructures, polymer electronic and polymer photovoltaic devices.

Lipid mapping of colonic mucosa by cluster TOF-SIMS imaging and multivariate analysis in cftr knockout mice

The cftr knockout mouse model of cystic fibrosis (CF) shows intestinal obstruction; malabsorption and inflammation; and a fatty acid imbalance in intestinal mucosa. We performed a lipid mapping of colon sections from CF and control (WT) mice by cluster time of flight secondary-ion mass spectrometry (TOF-SIMS) imaging to localize lipid alterations. Data were processed either manually or by multivariate statistical methods. TOF-SIMS analysis showed a particular localization for cholesteryl sulfate at the epithelial border, C16:1 fatty acid in Lieberkühn glands, and C18:0 fatty acid in lamina propria and submucosa. Significant increases in vitamin E (vE) and C16:0 fatty acid in the epithelial border of CF colon were detected. Principal component analysis (PCA) and partitioning clustering allowed us to characterize different structural regions of colonic mucosa according to variations in C14:0, C16:0, C16:1, C18:0, C18:1, C18:2, C20:3, C20:4, and C22:6 fatty acids; phosphatidylethanolamine, phosphatidylcholine, and phosphatidylinositol glycerolipids; cholesterol; vitamin E; and cholesteryl sulfate. PCA on spectra from Lieberkühn glands led to separation of CF and WT individuals. This study shows for the first time the spatial distribution of lipids in colonic mucosa and suggests TOF-SIMS plus multivariate analyses as a powerful tool to investigate disease-related tissue spatial lipid signatures.

Editorial

SIMS, as a non-destructive surface analysis technique for polymeric materials, started about 30 years ago; which happened co-instantaneously with the foundation of the journal Surface and Interface Analysis (SIA). Over the 30-year developments, SIMS has become one of the most important surface analysis techniques for polymeric materials owing to the combination of its unique capabilities: high molecular specificity, high surface sensitivity and sub-micrometer imaging resolution. In recent years, many advances have been made in the SIMS analysis of organic material surfaces such as molecular dynamics simulations, instrumental developments (e.g. cluster ion beams), data interpretation (e.g. multivariate analysis) and new methodology (metal-assisted SIMS). These advances have not only improved the SIMS capabilities for polymer analysis but also opened up new application perspectives. For example, it is now possible for some polymers to perform molecular depth profiles and construct 3D molecular images. Consequently, it seems timely and appropriate to gather research articles on the recent advancements of SIMS analysis on polymers into this special issue of SIA. The papers in this special issue are organized as follows. The volume starts with a concise and informative review by Delcorte et al. on their recent studies on the use of atomic and polyatomic ions in SIMS analysis of organic materials. The review covers both the theoretical study using molecular dynamics simulation and the experimental SIMS work and presents a nice overview on the progress on several areas such as molecular depth profiling, metal-assisted SIMS, molecular SIMS imaging. The strength and weakness of polyatomic versus atomic ion sources in these areas has also been critically assessed. The subsequent paper by Piwowar and Vickerman systematically studies the effects of molecular weights on the ToF SIMS spectra of poly(methyl methacrylate) (PMMA) when Au+ and C60+ are used as primary ions respectively. Their results have clearly shown that the C60+ ion source produces much higher secondary ion yields than Au+ ion source and the ion yield enhancements are consistent across the entire investigated molecular weight range. Three papers in this special issue are focused on the depth profiling of polymers. The paper by Mahoney presents a thorough investigation of the effect of polymer stereochemistry on depth profiling. Three stereo-specific PMMA samples (isotactic, atactic and syndiotactic) were depth profiled using SF5+ polyatomic ion source at different temperatures and the damage characteristics of these samples were compared. The results clearly showed that a polymer's physical properties also play an important role in molecular depth profiling. The paper by Houssiau and Mine provides a detailed investigation on the feasibility of molecular depth profiling of PMMA, polycarbonate (PC) and polystyrene (PS) using low energy (250 eV) Cs+ and Xe+ ions. All the three polymers can be successfully profiled using Cs+ but not all by Xe+. These results allowed the authors to point out the special beneficial effects of low energy Cs+ in polymer depth profiling, namely the negative ion signal enhancement and the inhibition of cross-linking reactions due to Cs reactivity. Finally, the paper by Lee et al. presents a nice application of ToF SIMS depth profiling using Cs+ and C60++ to a polystyrene-poly(n-propyl methacrylate) diblock copolymer system. Their results clearly showed that the microstructure changes of the copolymer due to annealing temperatures and times are reflected in the SIMS profiles. The last three papers in this special issue provide real applications of SIMS in combination with XPS to different polymeric materials. Baytekin et al. showed how the combination of ToF SIMS and XPS can be used to provide full surface chemistry and wettability of micro-fluidic devices and to monitor the effectiveness of their processing. On the other hand, Shimizu et al. have used ToF SIMS and XPS to study the interfacial interaction between polymeric methylene diphenyl diisocyanate (PMDI) and aluminum. Their results clearly showed that the reaction of PMDI with water can be quantitatively followed by ToF SIMS and XPS and a covalent bonding between PMDI and aluminum was unambiguously proven from ToF SIMS results. Finally, the paper by Lau et al. demonstrated that the surface composition changes of liquid crystalline polymers as a result of molecular ordering can be easily monitored using ToF SIMS combined with principal component analysis.

Sources of polyatomic ions of organic liquids

We have developed two types of liquid ion sources, one of which was a polyatomic ion source using liquid organic materials with a high-vapor pressure. Liquid materials such as octane and ethanol could be heated up to a maximum temperature of 100 degrees C, and the vapors were introduced into an ion source. They were ionized by an electron bombardment method and extracted from the ionizer. The ion current obtained at an extraction voltage of 2 kV was 230 microA for octane and several fragment ions such as alkyl ions were produced. On the other hand, another type of polyatomic ion source using alkyl naphthalene mixed with ionic liquid such as imidazolium dicyanamide has been developed. Instead of the electron bombardment method, a high-electric field method was used for the ion-emission from a sharp tip, because the vapor pressure of the liquid materials was relatively low. The threshold voltage was approximately 4.5 kV and the ion current of approximately 250 nA was obtained at an extraction voltage of 9.5 kV.

Three-Dimensional Time-of-Flight Secondary Ion Mass Spectrometry Imaging of a Pharmaceutical in a Coronary Stent Coating as a Function of Elution Time

Three-dimensional (3D) chemical images reveal the surface and subsurface distribution of pharmaceutical molecules in a coronary stent coating and are used to visualize the drug distribution as a function of elution time. The coronary stent coating consists of 25% (w/w) sirolimus in a poly(lactic-co-glycolic acid) (PLGA) matrix and is spray-coated onto metal coupons. Information regarding the 3D distribution of sirolimus in PLGA as a function of elution time was obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS) imaging using a Au(+) ion beam for analysis in conjunction with a C(60)(+) ion beam for sputter depth profiling. The examined formulation is shown to have large areas of the surface as well as subsurface channels that are composed primarily of the drug, followed by a drug-depleted region, and finally, a relatively homogeneous dispersion of the drug in the polymer matrix. Elution is shown to occur from the drug-enriched surface region on a relatively short time scale and more gradually from the subsurface regions of homogeneously dispersed drug. Bulk composition was also probed by X-ray photoelectron spectroscopy (XPS) depth profiling and confocal Raman imaging, the results of which substantiate the TOF-SIMS 3D images. Finally, the effectiveness of a C(60)(+) ion beam for use in 3D characterization of organic systems is demonstrated against another polyatomic ion source (e.g., SF(5)(+)).

Static time-of-flight secondary ion mass spectrometry analysis of microelectronics related substrates using a polyatomic ion source

Recent time-of-flight secondary ion mass spectrometry studies using primary ion cluster sources such as Au n +, SF 5 +, Bi n + or C 60 + have shown the great advantages in terms of secondary ion yield enhancement and ion formation efficiency of polyatomic ion sources as compared to monoatomic ion sources like the commonly used Ga +. In this work, the effective gains provided by such a source in the static ToF-SIMS analysis of microelectronics devices were investigated. Firstly, the influence of the number of atoms in the primary cluster ion on secondary ion formation was studied for physically adsorbed di-isononyl phthalate (DNP) (plasticizer) and perfluoropolyether (PFPE). A drastic increase in secondary ion formation efficiency and a much lower detection limit were observed when using a polyatomic primary ion. Moreover, the yield of the higher mass species was much enhanced indicating a lower degree of fragmentation that can be explained by the fact that the primary ion energy is spread out more widely, or that there is a lower energy per incoming ion. Secondly, the influence of the number of Bi atoms in the Bi n primary ion on the information depth was studied using reference thermally grown silicon oxide samples. The information depth provided by a Bi n cluster was shown to be lowered when the number of atoms in the aggregate was increased.

Investigation of molecular weight effects of polystyrene in ToF-SIMS using C 60+ and Au + primary ion beams

In secondary ion mass spectrometry, polyatomic primary ion sources are known to enhance yields from many surfaces including polymers. In order to understand the fundamental causes for these increases, the enhancement as a function of material type and molecular weight needs to be delineated. In this article, we report results from a systematic investigation of polymeric films of polystyrene (PS) with varying molecular weights to examine the influence of the primary ion beam on the secondary ion yields in time of flight secondary ion mass spectrometry (ToF-SIMS). The masses of the polymers investigated ranged from 1000 to 20,000 Da, or from about n = 10 to 200 where n indicates the number of polymeric units in a polymer chain. The polymers had a narrow molecular weight range (PDI < 1.07). The multilayer polymeric films (10–30 nm) characterized by AFM were prepared by spin-casting onto silicon substrates and were analyzed using Au + and C 60 + primary ion beams. The analysis with the two beams provided a useful comparison between atomic and polyatomic primary ion sources. Information gathered from this study provides insight into the role of molecular weight on the observed yield enhancement from polyatomic ion sources.

Fragmentation of molecular compounds on silicon wafers and low dielectric constant materials studied by time‐of‐flight secondary ion mass spectrometry using a polyatomic ion source

AbstractRecent time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) studies using primary cluster ion sources such as Aun+, SFn+, Bin+ or C60+ have shown polyatomic ions to be more appropriate for the detection of high mass molecular secondary ions than monoatomic ion sources like Ga+, thanks to secondary ion yield and ion formation efficiency enhancements.This work is part of a study aiming at improving molecular compound quantification at the surface of microelectronics industry substrates by taking advantage of a polyatomic ion source in ToF‐SIMS analysis. It focuses on the case of a phthalate‐related contamination. Fundamental parameters like secondary ion yield, damage cross section and secondary ion efficiency were studied as a function of the size and the energy of the primary ions. The substrate composition influence on the ToF‐SIMS results was also studied by comparing several substrates of interest in the microelectronics industry such as silicon wafers and low dielectric constant materials SiOxCyHz.Phthalate‐related secondary ions detected at the surface of the contaminated surfaces were compared and a fragmentation pathway of the di‐isononyl phthalate (DNP) molecule was determined. A drastic improvement of the secondary ion yield was provided when using a polyatomic ion source. Furthermore, a clear increase in damage cross section was observed as a function of the secondary ion size for a given Bin primary ion as well as an increase for a given fragment as a function of the number of Bi atoms in the Bin aggregate. This led to a lower static limit when taking into account higher phthalate fragments and primary ions of larger size. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Recent Advances in Biological Tissue Imaging with Time-of-Flight Secondary Ion Mass Spectrometry: Polyatomic Ion Sources, Sample Preparation, and Applications

Recent technological and methodological improvements have greatly enhanced the sensitivity of the Time-of-flight Secondary Ion Mass Spectrometry (TOF-SIMS), thus making this technique now very attractive in the field of molecular imaging of biological samples such as tissue sections or cells. This paper reviews the most recent advances in this field. After a short reminder of the basic physics involved, the instruments are described, as well as the primary ion sources, including the different cluster ion sources. The sample preparation methods are also described and compared, such as the matrix coating and the metal coating. The capabilities of the technique are finally illustrated with the most recent applications published in the last years.

Attempts for molecular depth profiling directly on a rat brain tissue section using fullerene and bismuth cluster ion beams

The capabilities of time of flight secondary ion mass spectrometry (TOF-SIMS) have been recently greatly improved with the arrival in this field of polyatomic ion sources. This technique is now able to map at the micron scale intact organic molecules in a range of a thousand Daltons or more, at the surface of tissue samples. Nevertheless, this remains a surface analysis technique, and three-dimensional information on the molecular composition of the sample could not be obtained due to the damage undergone by the organic molecules during their irradiation. The situation changed slightly with the low damage and low penetration depth of the C 60 fullerene ion beams. Recent promising studies have shown the possibility of organic molecular depth profiling using this kind of beams onto model samples. This possibility has been tried out directly onto a rat brain tissue section, which is the most commonly used biological tissue model in TOF-SIMS imaging method developments. The tissue surface has been sputtered with a 10 keV energy fullerene ion beam, and surface analyses were done with a 25 keV Bi 3 + ion beam at regular time intervals. The total depth which was analysed was more than two microns, with total primary ion doses of more than 10 16 ions cm −2. Although not in contradiction with results previously published but with much lower doses, it is found that the molecular damage remains too large, thus making molecular imaging very difficult. In addition, most of the lipids, which are usually the main observable molecules in TOF-SIMS, are concentrated close to the sample surface in the first hundreds of nanometers.

ToF-SIMS studies as a tool to discriminate between spores and vegetative cells of bacteria

Within the field of microbiology rapid bacterial identification is a subject of growing interest. Research into food borne pathogens and disease causing bacteria as well as dormant bacterial spores has highlighted the need for new, sensitive and rapid diagnostic tools to avoid human health risks. In this work we investigate the application of ToF-SIMS for the discrimination between spores and vegetative bacterial cells of Bacillus megaterium. A custom-built imaging ToF-SIMS equipped with polyatomic ion sources (Au n +, and C 60 +) and an in situ freeze-fracture stage was used for the analysis of spore and vegetative cell samples. The study assesses the suitability of ToF-SIMS analysis in combination with computatorial methods (principal component analysis and variance analysis) for the discrimination between bacterial spores and vegetative cells. We demonstrate that the combination of the two methods gives a tentative identification of surface phospholipids of spores and vegetative cells and that differences in the phospholipid pattern between spore samples and vegetative cells can be highlighted. The method is especially useful in the determination of phospholipids with higher abundance in spores (diglyceryldiacylglycerols and cardiolipins). The possible use of ToF-SIMS analysis as a tool for the rapid detection of bacterial spores in food samples or tap water is discussed.

A spontaneous desorption-based polyatomic ion source

We describe an ion source based on the spontaneous desorption process. The production, acceleration, and separation processes allow for the production of keV energy polyatomic ions while maintaining a high signal-to-background ratio. Both atomic and polyatomic ions with up to 37 keV energy were generated. Because the spontaneous desorption process is naturally pulsed, the ion source was attached to a dual time-of-flight mass spectrometer for secondary ion mass spectrometry experiments. In this article, we describe the design of both the polyatomic ion source and the mass spectrometer.