Albert J. Fahey

Multifunctional ToF-SIMS: combinatorial mapping of gradient energy substrates

Abstract We present a simple method for chemical modification of chlorosilane self-assembled monolayers (SAMs) on Si surfaces by exposure to a gradient of UV-ozone radiation to create stable substrates with a range of contact angles ( θ H 2 O ≈5–95°) and surface energies on a single substrate. These gradient energy substrates are developed to potentially generate libraries for combinatorial studies of thin film phenomenology, where a systematic variation of interfacial surface energy represents one of the significant parameters along one axis. The graded oxidation process presents a systematic variation of surface chemical composition. We have utilized contact angle measurements and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to investigate this variation for a series of ions, among which are SiCH 3 + , SiOH + and COOH − . We show that the macroscopic measurements of surface free energy/contact angle correlate with the detailed analysis of surface chemistry (as assessed by ToF-SIMS) on these test substrates.

Secondary Ion Mass Spectrometry Using Cluster Primary Ion Beams

Abstract We have a developed a capability for conducting cluster secondary ion mass spectrometry (SIMS) experiments on commercially available SIMS instrumentation. This paper reviews our recent work on cluster ion source development, elemental depth profiling with cluster primary ion beams and the use of cluster ion beams for organic surface characterization. An area of particular interest is the observation that beam-induced damage for some organic materials is substantially reduced under cluster bombardment. This unique feature of cluster SIMS is utilized for molecular depth profiling of selected polymer films and for studying the spatial distribution of high explosive particles by SIMS imaging. We also describe recent studies that may provide additional insight into possible mechanisms for the molecular secondary ion yield enhancement observed for organic thin films under cluster bombardment.

Three-Dimensional Compositional Analysis of Drug Eluting Stent Coatings Using Cluster Secondary Ion Mass Spectrometry

Cluster secondary ion mass spectrometry (cluster SIMS) employing an SF5+ polyatomic primary ion sputter source in conjunction with a Bi3+ analysis source was used to obtain three-dimensional molecular information in polymeric-based drug-eluting stent coatings. The formulations of the coatings varied from 0% to 50% (w/w) sirolimus drug in poly(lactic-co-glycolic acid) and were prepared on both MP35N metal alloy coupons and bare metal stents. All cluster SIMS depth profiles obtained indicated a drug-enriched surface region, followed by a drug-depletion region, and finally a constant bulk composition region, similar to previous data obtained in polymeric blend systems. The drug overlayer thickness was determined to increase with increasing sirolimus content. Sample temperature was determined to play an important role in the resulting depth profiles, where it was shown that the best profiles were obtained at low temperatures (-100 degrees C). At these temperatures, molecular signals typically remained constant through the entire depth of the film (approximately 6.5 microm) in some cases, as opposed to the typical 1 microm-2 microm depth limit, which is achievable at room temperature. The 3-D imaging capabilities of cluster SIMS were successfully demonstrated and indicated a significant amount of subsurface domain formation in the 25% and 50% sirolimus samples, but not in the 5% sample, which was homogeneous. These results clearly illustrate the utility of cluster SIMS for probing the 3-D structure in polymeric-based drug delivery devices.

Measurements of dead time and characterization of ion counting systems for mass spectrometry

A method for characterization of electron multiplier-based pulse counting systems is presented. Specific attention is paid to measurement of the counting system dead time. The systems discussed here are typically used to measure the abundance of ions passing through a mass spectrometer. A method for the determination of dead time by measurement of at least two isotopic ratios of an element with at least three isotopes is given. This method differs from the “text book” method typically used to measure the dead time. The theoretical basis for the measurements is given and an example of measurements of Ti isotopes by secondary ion mass spectrometry (SIMS) is shown. Dead time measurements on SIMS instruments can be performed in a few hours of machine time. A brief description of the pulse detection and counting circuit and its operation is also given. Guidelines for the proper measurement of pulse shapes and the determination of reasonable settings for electron multiplier gain, preamplifier gain, and discrimi...

Isotopic ratio measurements by time-of-flight secondary ion mass spectrometry

Abstract Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is often considered synonymous with SIMS in the static limit where the ion fluence on the sample surface is so low that damage is negligible. For this same reason, its use in measuring isotopic ratios has generally been ruled out. However, the high-spatial-resolution Ga+ ion beams typically used in ToF-SIMS make it a potentially attractive technique for the isotopic characterization of small features such as particles. We have developed a technique to measure isotopic ratios by ToF-SIMS with a spatial resolution of

Characterization of Composition C4 Explosives using Time-of-Flight Secondary Ion Mass Spectrometry and X-ray Photoelectron Spectroscopy

The application of surface analytical techniques such as time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) is explored as a means of differentiating between composition C4 plastic explosives (C-4). Three different C-4 samples including U.S. military grade C-4, commercial C-4 (also from the United States), and C-4 from England (PE-4) were obtained and analyzed using both ToF-SIMS and XPS. ToF-SIMS was able to successfully discriminate between different C-4 samples with the aid of principal component analysis, a multivariate statistical analysis approach often used to reduce the dimensionality of complex data. ToF-SIMS imaging was also used to obtain information about the spatial distribution of the various additives contained within the samples. The results indicated that the samples could potentially be characterized by their 2-D chemical and morphological structure, which varied from sample to sample. XPS analysis also showed significant variation between samples, with changes in the atomic concentrations, as well as changes in the shapes of the high-resolution C 1s and O 1s spectra. These results clearly demonstrate the feasibility of utilizing both ToF-SIMS and XPS as tools for the direct characterization and differentiation of C-4 samples for forensic applications.

Bridging the micro-to-macro gap: a new application for micro X-ray fluorescence.

X-ray elemental mapping and X-ray spectrum imaging are powerful microanalytical tools. However, their scope is often limited spatially by the raster area of a scanning electron microscope or microprobe. Limited sampling size becomes a significant issue when large area (>10 cm²), heterogeneous materials such as concrete samples or others must be examined. In such specimens, macro-scale structures, inclusions, and concentration gradients are often of interest, yet microbeam methods are insufficient or at least inefficient for analyzing them. Such requirements largely exclude the samples of interest presented in this article from electron probe microanalysis. Micro X-ray fluorescence-X-ray spectrum imaging (μXRF-XSI) provides a solution to the problem of macro-scale X-ray imaging through an X-ray excitation source, which can be used to analyze a variety of large specimens without many of the limitations found in electron-excitation sources. Using a mid-sized beam coupled with an X-ray excitation source has a number of advantages, such as the ability to work at atmospheric pressure and lower limits of detection owing to the absence of electron-induced bremsstrahlung. μXRF-XSI also acts as a complement, where applicable, to electron microbeam X-ray output, highlighting areas of interest for follow-up microanalysis at a finer length scale.

Details of the measurement of rare earth and other trace element abundances by secondary ion mass spectrometry

Abstract Details of a method for quantitative measurement of rare earth element (REE) abundances by secondary ion mass spectrometry are presented. The specifics of the multipass deconvolution algorithm are given. Direct measurements of the ratios of REE to REE oxide ion signals ( η i ) as a function of energy are shown and investigated. The η i are important quantities used to determine the intensities of the heavy REE ion signals. Relative sensitivity factors for the REE are given for silicate and sulfide minerals and a comparison of measured relative sensitivity factors among three laboratories is shown. The occurrence of artifacts in the analysis is also discussed.

Time-Of-Flight Secondary Ion Mass Spectrometry (Tof-SIMS) For High-Throughput Characterization Of Biosurfaces

A graded oxidation process, involving UV-ozone (UVO) treatment, was used to create a poly(e-caprolactone) (PCL) surface with a systematic variation in surface chemistry. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) has proved useful in characterizing the chemical composition of these surfaces and in monitoring the oxidation process. The TOF-SIMS data correlates with contact angle data and the results of the binding studies performed with mouse calvarial cells. UVO treatment resulted in a PCL surface with improved wettability and cellular adhesion.

Phase separation at the surface of poly(ethylene oxide)-containing biodegradable poly(L-lactic acid) blends.

The surface chemistry and in-depth distribution of the composition of a poly(ethylene oxide) (PEO)-containing biodegradable poly(L-lactic acid) (PLLA) blend matrix system have been investigated using X-ray photoelectron spectroscopy (XPS). This study reports detailed quantitative compositional information using a novel numerical method for determining depth profiles. The PEO system studied is an amphiphilic Pluronic P104 surfactant, PEO-b-poly(propylene oxide) (PPO)-b-PEO. The extent of phase separation is analyzed by determining the surface enrichment of the PEO component via measurement of chemical composition at the polymer-air interface. For this blend system, the combination of the PPO component in the Pluronic surfactants drives the formation of a surface excess of Pluronic in the blends with PLLA. The surface excess profile shows a rapid increase in Pluronic surface composition versus bulk Pluronic mass fractions of 1-5%, but the profile levels off above bulk Pluronic mass fractions of 5%.