Static secondary ion mass spectrometry (SSIMS) of biological compounds in tissue and tissue-like matrices

Developments in molecular SIMS depth profiling and 3D imaging of biological systems using polyatomic primary ions

In principle mass spectral imaging has enormous potential for discovery applications in biology. The chemical specificity of mass spectrometry combined with spatial analysis capabilities of liquid metal cluster beams and the high yields of polyatomic ion beams should present unprecedented ability to spatially locate molecular chemistry in the 100 nm range. However, although metal cluster ion beams have greatly increased yields in the m/z range up to 1000, they still have to be operated under the static limit and even in most favorable cases maximum yields for molecular species from 1 µm pixels are frequently below 20 counts. However, some very impressive molecular imaging analysis has been accomplished under these conditions. Nevertheless although molecular ions of lipids have been detected and correlation with biology is obtained, signal levels are such that lateral resolution must be sacrificed to provide a sufficient signal to image. To obtain useful spatial resolution detection below 1 µm is almost impossible. Too few ions are generated! The review shows that the application of polyatomic primary ions with their low damage cross-sections offers hope of a new approach to molecular SIMS imaging by accessing voxels rather than pixels to thereby increase the dynamic signal range in 2D imaging and to extend the analysis to depth profiling and 3D imaging. Recent data on cells and tissue analysis suggest that there is, in consequence, the prospect that a wider chemistry might be accessible within a sub-micron area and as a function of depth. However, these advances are compromised by the pulsed nature of current ToF-SIMS instruments. The duty cycle is very low and results in excessive analysis times, and maximum mass resolution is incompatible with maximum spatial resolution. New instrumental directions are described that enable a dc primary beam to be used that promises to be able to take full advantage of all the capabilities of the polyatomic ion beam. Some new data are presented that suggest that the aspirations for these new instruments will be realized. However, although prospects are good, the review highlights the continuing challenges presented by the low ionization efficiency and the complications that arise from matrix effects.

New Cs sputter ion source with polyatomic ion beams for secondary ion mass spectrometry applications

A simple design for a cesium sputter ion source compatible with vacuum and ion-optical systems as well as with electronics of the commercially available Cameca IMS-4f instrument is reported. This ion source has been tested with the cluster primary ions of Si(n)(-) and Cu(n)(-). Our experiments with surface characterization and depth profiling conducted to date demonstrate improvements of the analytical capabilities of the secondary ion mass spectrometry instrument due to the nonadditive enhancement of secondary ion emission and shorter ion ranges of polyatomic projectiles compared to atomic ones with the same impact energy.

Static secondary ion mass spectrometry for biological and biomedical research

Static secondary ion mass spectrometry (SSIMS) is a capable of providing detailed atomic and molecular characterization of the surface chemistry of biological and biomedical materials. The technique is particularly suited to the detection and imaging of small molecules such as membrane lipids, metabolites, and drugs. A limit of detection in the ppm range and spatial resolution <1 microm can be obtained. Recent progress in instrumental developments, notably cluster ion beams, and the application of multivariate data analysis protocols, promise further advances. This chapter presents a brief overview of the theory and instrumentation of static secondary ion mass spectrometry followed by examples of a range of biological and biomedical applications. Because of the ultrahigh vacuum requirements and extreme surface sensitivity of the technique, appropriate sample preparation and handling is essential. These protocols, and the analysis methodology required to ensure high-quality, reliable data are described.

Characterization of Poly(l-lysine)-graft-Poly(ethylene glycol) Assembled Monolayers on Niobium Pentoxide Substrates Using Time-of-Flight Secondary Ion Mass Spectrometry and Multivariate Analysis

Control of protein adsorption onto solid surfaces is a critical area of biomaterials and biosensors research. Application of high performance surface analysis techniques to these problems can improve the rational design and understanding of coatings that control protein adsorption. We have used static time-of-flight secondary ion mass spectrometry (TOF-SIMS) to investigate several poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) adlayers adsorbed electrostatically onto negatively charged niobium pentoxide (Nb(2)O(5)) substrates. By varying the PEG graft ratio (i.e., the number of lysine monomers per grafted PEG chain) and the molecular weights of the PLL and PEG polymers, the amount of protein adsorption can be tailored between 1 and 300 ng/cm(2). Detailed multivariate analysis using principal component analysis (PCA) of the positive and negative ion TOF-SIMS spectra showed changes in the outermost surface of the polymer films that were related to the density and molecular weight of the PEG chains on the surface. However, no significant differences were noted due to PLL molecular weight, despite observed differences in the serum adsorption characteristics for adlayers of PLL-g-PEG polymers with different PLL molecular weights. From the PCA results, multivariate peak intensity ratios were developed that correlated with the thickness of the adlayer and the enrichment of the PEG chains and the methoxy terminus of the PEG chains at the outermost surface of the adlayer. Furthermore, partial least squares regression was used to correlate the TOF-SIMS spectra with the amount of protein adsorption, resulting in a predictive model for determining the amount of protein adsorption on the basis of the TOF-SIMS spectra. The accuracy of the prediction of the amount of serum adsorption depended on the molecular weight of the PLL and PEG polymers and the PEG graft ratio. The combination of multivariate analysis and static TOF-SIMS provides detailed information on the surface chemistry and insight into the mechanism for protein resistance of the coatings.

Limits of detection for time of flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS): detection of low amounts of adsorbed protein

Characterization of biomaterial surfaces requires analytical techniques that are capable of detecting a wide concentration range of adsorbed protein. This range includes detection of low amounts of adsorbed protein (<10 ng/cm2) that may be present on non-fouling biomaterials. X-ray Photoelectron Spectroscopy (XPS) and Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) are surface sensitive techniques capable of detecting adsorbed proteins. We have investigated the lower limits of detection of both XPS and ToF-SIMS on four model substrates each presenting unique challenges for analysis by XPS and ToF-SIMS: mica, poly(tetrafluoroethylene), allyl amine plasma polymer and heptyl amine plasma polymer. The detection limit for XPS ranged from 10 ng/cm2 of fibrinogen (on mica) to 200 ng/cm2 (on allyl amine plasma polymers). The detection limit for ToF-SIMS ranged from 0.1 ng/cm2 of fibrinogen to 100 ng/cm2, depending on the substrate and data analysis. Optimal conditions provided detection limits between 0.1 ng/cm2 and 15 ng/cm2 on all of the substrates used in this study. While both techniques were shown to be effective in detecting protein, the sensitivity of both XPS and ToF-SIMS was shown to be dependent on substrate surface chemistry and the organization of the adsorbed protein film. This study specifically highlights the applicability of ToF-SIMS in the characterization of low level protein adsorption.

Characterizing multicomponent adsorbed protein films using electron spectroscopy for chemical analysis, time-of-flight secondary ion mass spectrometry, and radiolabeling: capabilities and limitations

Characterization of complex adsorbed protein films is a critical aspect of biomaterials science, particularly in understanding the in vivo response to biomaterials. The surface analysis techniques electron spectroscopy for chemical analysis (ESCA) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) are particularly suited to the analysis of complex adsorbed protein films due to their wide applicability to a variety of materials. We have investigated the applicability of ESCA for studying the structure of adsorbed serum and plasma protein layers. ESCA was able to monitor the thickness of the adsorbed protein film. Due to its chemical specificity, ToF-SIMS was used to estimate the composition of the plasma and serum protein layers by comparison of their spectra with the spectra of single protein films. The limit of detection of ToF-SIMS for the plasma protein fibrinogen was determined by comparison with independent radiolabeled fibrinogen adsorption measurements. While ToF-SIMS was able to determine some qualitative trends in the composition of the plasma protein films as a function of adsorption time, the detection limit of the minor components in multicomponent adsorbed protein films ultimately limits the ability of ToF-SIMS to quantify the composition of these films. However, both ESCA and ToF-SIMS can provide useful information on adsorbed plasma protein films without further sample treatment. This study outlines the strengths and weaknesses of ESCA and ToF-SIMS for studying multicomponent adsorbed plasma protein films.

A C60 primary ion beam system for time of flight secondary ion mass spectrometry: its development and secondary ion yield characteristics

A buckminsterfullerene (C60)-based primary ion beam system has been developed for routine application in TOF-SIMS analysis of organic materials. The ion beam system is described, and its performance is characterized. Nanoamp beam currents of C60+ are obtainable in continuous current mode. C60(2+) can be obtained in pulsed mode. At 10 keV, the beam can be focused to less than 3 microm with 0.1 nA currents. TOF-SIMS studies of a series of molecular solids and a number of polymer systems in monolayer and thick film forms are reported. Very significant enhancement of secondary ion yields, particularly at higher mass, were observed using 10-keV C60+ for all samples other than PTFE, as compared to those observed from 10 keV Ga+ primary ions. Three materials (PS2000, Irganox 1010, PET) were studied in detail to investigate primary ion-induced disappearance (damage) cross sections to determine the increase in secondary ion formation efficiency. The C60 disappearance cross sections observed from monolayer film PS2000 and self-supporting PET film are close to those observed from Ga+. The resulting C60 efficiencies are 30-100 times those observed from gallium. The cross sections observed from C60 bombardment of multilayer molecular solids are approximately 100 times less, such that essentially zero damage sputtering is possible. The resulting efficiencies are > 10(3) greater than from gallium. It is also shown that C60 primary ions do not generate any more low-mass fragments than any other ion beam system does. C60 is shown to be a very favorable ion beam system for TOF-SIMS, delivering high yield, close to 10% total yield, favoring high-mass ions, and on thick samples, offering the possibility of analysis well beyond the static limit.