Variable Take-off Angle Research Articles

Basic interpretation of intracoronary ultrasound and optical coherence tomography images: examples

Basic interpretation of intracoronary ultrasound and optical coherence tomography images: examples

The Hemodynamic Impact of Misalignment of Fenestrated Endografts: A Computational Study

The hemodynamic consequences of misaligned stent-grafts (SG) in fenestrated endografts (EG) have not been adequately studied. Our aim was to study the hemodynamic effects of positional variations of SG, investigating the potential influence on the total displacement forces acting on the EG and the shear stress values at the stented segments. This was a computational study. An idealized EG model with two renal fenestrations was computationally reconstructed and centrally extended up to the suprarenal level to treat a suprarenal aneurysm. The misalignment of SG was represented by a variable take-off angle between the SG and the EG centerline axis, corresponding to angles of 90°, 176°, 142°, 38°, and 4°, respectively. Accordingly, the maximum EG displacement forces and the shear stress within the stented segments were calculated, using commercially available software. The variable positions of the SG caused no effect on the maximum displacement force acting on the EG, being quite steady and equal to 5.55 N. On the contrary, the values of maximum shear stress acting on the stented segments were influenced by their orientation. The narrow transition zone between the distal end of the mating stent and the target artery showed higher stresses than any other segment. The right-angle take off SG position (90°) was associated with the lowest stresses (12.5 Pa), whereas the highest values were detected at 38° and 142° (16.5 and 16.1 Pa, respectively). The vessel segments distal to the SG exhibited constantly lower stress values (1.9-2.2 Pa) than any other segment. We detected differences in the values of shear stress exerted on the stented arteries, depending on different positions that SG can adapt after the deployment of fenestrated EG. The pathophysiologic implication of our findings and their potential association with clinical events deserve further investigation and clinical validation.

X-ray photoelectron spectroscopy as a tool for studies of the surface layer of microspheres. The case of polystyrene and poly(styrene-acrolein) microspheres with attached human serum albumin

The application of X-ray photoelectron spectroscopy (XPS) for studies of surface layers of objects with spherical shape was investigated using as examples polystyrene and poly(styrene–acrolein) microspheres with attached human serum albumin (HSA). The amounts of immobilized protein were determined by the standard biochemical Lowry method and by XPS, using the intensity of the N1s signals of HSA as a basis for evaluation. The XPS data were treated by taking into account the spherical shape of the particles analyzed (variable take-off angle of ejected electrons). The best agreement between the results of the biochemical and XPS determinations was found assuming that for the average particle the takeoff angle varies from 0° to 72.7°. This reflects the fact that in the multilayer arrangement of particles, placed onto the support of the XPS apparatus, the particles from the upper layer partially screen the edges of the particles in the layer below.

Interactions between Organized, Surface-Confined Monolayers and Vapor-Phase Probe Molecules. 12. Two New Methods for Surface-Immobilization and Functionalization of Chemically Sensitive Dendrimer Surfaces

Abstract : In this report we demonstrate two new methods for covalently linking dendrimers to surfaces. In the first method (Method 1) a poly(iminopropane-1,3,-diyl) dendrimer with 64 terminal-amine groups is first attached to a mixed mercaptoundecanoic-acid (MUA)/mercaptopentane (MP) SAM, and then the unreacted terminal-amine groups of the dendrimer are converted to amide-linked functional groups by condensation with acid chlorides. The second method (Method 2) involves bulk-phase coupling of suitable functional groups with the primary-amine-terminated dendrimer followed by reaction of the few unfunctionalized primary amines with the MUA component of the SAM to yield amide linkages. Five different dendrimer terminal groups are considered: primary amine, benzamide, 4- (trifluoromethyl)benzamide butanamide, and triphenylacetamide. FTIR external reflection spectroscopy, ellipsometry, variable take-off angle XPS, and surface acoustic wave (SAW) device-based gravimetry reveal that these two approaches result in very different types of dendrimer monolayers. When the dendrimers are prepared by Method 2, their surface concentration is lower than when the functionalization is done after attachment. However, the density of surface functionalities on each dendrimer is higher when dendrimer modification is performed prior to surface attachment. When the benzamido-terminated dendrimer surfaces are dosed with a variety of volatile organic compounds (VOCs), we find that the surface prepared by Method 2 is more sensitive and that there is enhanced selectivity for the VOCs having pi electrons. This result is interpreted in terms of pi-stacking interactions with the aromatic groups on the dendrimer surfaces.

XPS and SSIMS Analysis Revealing Surface Segregation and Short-Range Order in Solid Films of Block Copolymers of PEO and PLGA

The interfacial chemistry of solvent cast films of block copolymers of poly(ethylene oxide) (PEO) and poly(lactic and glycolic acid) (PLGA) have been investigated over a wide range of compositions. X-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectrometry (SSIMS) were utilized to provide both elemental and structural data. In all cases it is found that the PLGA component preferentially resides at the copolymer surface, and this is demonstrated by the use of the variable electron takeoff angle XPS. SSIMS confirms the low concentrations of PEO at the copolymer surface. Radical cation intensities within the SSIMS spectra have been correlated to PLGA block compositions. It is demonstrated that these ion intensities may be used to estimate the PLGA surface chain sequence composition and its short-range order.

Adsorption of Zirconium−Phosphonate Multilayers onto Phosphate-Derivatized Glassy Carbon Substrates

Two methods were developed to modify glassy carbon surfaces with phosphoric acid groups. In the first method, phosphoryl chloride was reacted directly with oxides on the glassy carbon surface. Oxidative and reductive pretreatment of the polished surface improved the phosphate site density, as determined by X-ray photoelectron spectroscopy. In the second method, the carbon surface was modified with an ω-aminoalcohol by electrochemical oxidation, prior to phosphorylation, yielding similiar phosphate site densities. Zirconium 1,10-decanediylbis(phosphonic acid) (Zr-DBP) multilayers were deposited upon these modified glassy carbon substrates. Variable takeoff angle XPS studies and the steady increase of zirconium and of phosphorus with each metal−phosphonate deposition indicate regular adsorption of each multilayer. Higher than ideal zirconium-to-phosphorus ratios are attributed to deposition kinetics, signal attenuation, and the random character of the glassy carbon surface. Zr−DBP layers exhibited ellipsometric thickness of 17 Å or higher, plausibly due to adsorption of excess ionic species into disordered regions or pockets of the film. Contact angle measurements concur with this picture of an imperfect film. The ability of the films to insulate the carbon interface from solution species was investigated by cyclic voltammetry of potassium ferricyanide; heterogeneous electron transfer was significantly hindered after one to two Zr−DBP adsorption steps.

Low temperature oxidation and reduction of ceria particles on Rh(111)

X-ray photoelectron spectroscopy (XPS) was used to study the oxidation and reduction of Ce vapor deposited onto a Rh(111) single crystal in a system which couples a moderate pressure reactor to an ultrahigh vacuum (UHV) analysis chamber. This article compares the behavior of thick (≥50 Å) ceria layers to that of thin (4–8 Å) ceria islands. Using variable takeoff angle XPS and difference spectra we find that the interior of both the thick Ce layers and the thin Ce islands oxidize completely to CeO2 with 1 Torr of oxygen at 475 K; however, the surfaces of both of the cerias are a mixture of Ce+3 and Ce+4. As for reduction, under our conditions (≤525 K) both the thick layers and the thin islands undergo a surface reduction that generates Ce+3. This Ce+3 surface is quite stable and no reduction of the bulk of the material occurs. Whereas the thick ceria layers require high pressures (8 Torr) of CO in order to reduce the surface, the thin islands reduce under UHV conditions—CO adsorbed at 300 K forms CO2 at 475 K during desorption. Further reduction of the thin ceria islands does not occur if they are treated with 8 Torr of CO at 525 K. We conclude that intimate contact with Rh can enhance ceria reduction via reaction with CO which initially adsorbs on Rh. Further, these two cerias are in fact very similar in that the amount of oxygen that they store is proportional to their surface area.

Surface properties of RGD-peptide grafted polyurethane block copolymers: variable take-off angle and cold-stage ESCA studies.

Variable take-off angle and cold-stage ESCA measurements were utilized to analyze the surface composition of five polyurethane block copolymers. The polymers studied included a PTMO-polyurethane control, a carboxylated version of the control polyurethane, and three different peptide grafted (GRGESY, GRGDSY, and GRGDVY) polyurethanes. On dry samples the nitrogen signal detected using ESCA decreased with increasing take-off angle (i.e. as the specimen was probed closer to the surface) for all five polymers. This was believed to be due to the depletion of nitrogen-containing urethane hard segments at the surface. For all five polymers, the surface nitrogen concentration, associated with the hard segment, increased upon hydration. A greater increase of nitrogen concentration was observed for the peptide grafted polymers which suggests that grafting of the hydrophilic peptides to the polyurethane augments the hard segment enrichment at the surface upon hydration. Upon dehydration, the nitrogen concentration decreased for all five polymers suggesting migration of the more hydrophobic PTMO soft segment to the surface. In vitro endothelial cell adhesion showed an increase of cell attachment on prehydrated RGD-containing peptide grafted polyurethanes, but not on the other polymers. This result suggests an enhancement of peptide density at the aqueous interface, in good agreement with the ESCA studies.

A comparison of two XPS methods for quantification of concentration profiles

Binary AlLi alloys are known for well pronounced segregation at ambient temperatures. Literature provides a good knowledge of segregation behaviour of these alloys. An exponential concentration profile with depth can be assumed. We compare the concentration profiles obtained from SIMS measurements with results from XPS. SIMS measurements [M. Vonbank and P. Varga, Vak.-Tech., 37 (1988) 220.] performed on a specimen with 9.1 at.% Li after storage at room temperature for more than 48 h gave a surface composition of 50 at.% Li and a gradient of −20 at.% Li nm −1. We investigated the same specimen after heat treatment for 2 h 20 min at 150°C. For comparison, XPS with variable take-off angle gives 75 at.% Li and −17.8 at.% Li nm −1 assuming an exponential depth profile, 70 at.% Li and −9.9 at.% Li nm −1 with a linear response and our new imaging XPS method with a linear response gives 92 at.% Li and −15.4 at.% Li nm −1. The standard deviations of surface concentration and gradient are approximately 9 at.% Li and 8 at.% Li nm −1. The differences are due to: the statistical significance of measured data (8% relative error in case of variable take-off angle experiments and 25% for imaging experiments) especially due to weak Li1s signals; the necessity of a specimen transfer in ambient atmosphere; and uncertainties in sputter depths of about 10%. In principle the imaging method is similar to methods in which the surface is etched with a constant flux Ar ion beam and changes in the composition of the surface with time are monitored using XPS.

Depth Profiling by Means of X-Ray Fluorescence Analysis

Sherman described the excitation of characteristic radiations by primary x-rays and by secondary excitation. The derivation has been made assuming a homogeneous sample. Criss and Birks inverted the problem from the calculation of fluorescent countrates to the quantitative XFA by means of fundamental parameters. Theoretical and instrumental developments enabled a reduction of the sample area and led to small area XFA and imaging XFA sytems. Depth profiling by means of XFA is a further development. We continue the original concept of variable take-off angle technique for the determination of film thicknesses without reference samples and apply the variation of the incidence angle to depth profiling.

Characterization of alkyl grafted polyurethane block copolymers by variable takeoff angle X‐ray photoelectron spectroscopy

Variable takeoff angle x-ray photoelectron spectroscopy was used to determine the surface composition of five polyurethane block copolymers. The high-resolution C1s spectra from all five polyurethane samples had peaks at binding energies of 285.0, 286.5, and 289.5 eV, which are consistent with the presence of hydrocarbon, ether, and urethane carbon species. Both the measured elemental compositions at the low takeoff angles (deepest sampling depths) and the calculated elemental compositions for depths greater than 15 A generally showed good agreement with the expected bulk compositions. The outer 15 A of the surface of all five samples was depleted in the nitrogen-containing hard segment. For the base polyurethane sample in this series, an enrichment of the poly(tetramethylene oxide) soft segment and the presence of an organic silicone impurity were detected. Doubling the chain length of both the hard and soft segments resulted in a further decrease of the hard segment concentration in the outer surface of the sample. The grafting of C2 and C18 alkyl groups onto 10 or 20% of the urethane linkages resulted in an increase in the percentage of aliphatic carbon species present on the surface. In the case of the C18-alkylated material, the presence of alkyl chains at the surface was inferred using contact angle measurements.

Determination of reduced thicknesses by means of the variable take-off angle technique

Reduced thicknesses can be determined by means of substrate, layer and ratio methods. All these methods require the determination of two unknown quantities — the reduced thickness and a proportional factor which is specific for the method applied. Experimental results can be evaluated by regression, employing equations derived for plane, smooth, homogeneous layers. The aim of this paper is to show how the measured data are influenced by surface roughness, by the energy-dependence of the mean free path of the photoelectrons, and by measurement errors. Moreover, a method is presented for applying the equations derived for ideal layers to results obtained from real structures, taking these influences into account. This evaluation procedure is demonstrated by two examples.

Evaluation of XPS-data of oxide layers

XPS with variable take-off angle has been applied to the determination of the thickness of thin oxide layers (SiO 2 on Si). The information about D/λ ox gained by such measurements is strongly influenced by surface roughness. This influence can be demonstrated when the parameters R and D/λ ox are calculated from sets of the experimental results and for each set the corresponding pair of R and D/λ ox is plotted in a diagram D/λ ox = f( R). Having the true value of R it is possible to determine D/λ ox. With the XPS-results of at least three different oxide layers and their ellipsometric thicknesses D e one is able to calculate λ Si, λ ox/λ Si and the difference Δ D between D e and the XPS-thickness D.

Zur bestimmung der reduzierten dicke D/λ dünner Schichten mittels XPS

Based on a theoretical introduction to the method of variable take-off angle for the determination of D/λ by means of XPS an algorithm has be In addition there is now strong evidence that it is impossible to give reliable D/λ values from measurements performed on substrate or film or fr

Praktische anwendung der röntgenfluoreszenzanalyse mit variabler strahlengeometrie

An outline of the theory for the method of quantitative XRFA with variable take-off angle is given. After a description of the instrument, which has been, developed for the application of this method, as an example, the quantitative analysis of a Cr-Ni-steel is presented.

Die sekundäranregung bei der quantitativen röntgenfluoreszenzanalyse mit variablem beobachtungswinkel

Based on the theory the influence of secondary X-ray excitation on different XRFA-methods is treated. This influence is not to be considered for multi-component films and solution-techniques. On the contrary, in the case of bulk materials and fixed-, or variable take-off angle technique, the secondary radiation can reach up to 30% of the total. From theory an approximation is developed, which makes the XRFA with variable take-off angle free from a comparison with chemically analysed reference samples. The quality of the approximation is proofed theoretically. The evaluation of the measurements can be done easily together with a computer. Experimental results show an agreement between XRFA and other methods of about 1 wt. %. This accuracy contains errors caused by the XRFA method, by the present knowledge of mass-absorption coefficients and specific constants of the elements, which are used for the calculations and finally by the reference method.

Die effektive wellenlänge in der quantitativen röntgenfluoreszenzanalyse—II.: Diskussion

The known methods of quantitative XRPA are discussed, based on the experimental investigation of the dependence of the effective wavelength on the composition, the excitation conditions and the geometry. It can be shown that the methods given by C riss and B irks—computer evaluation—, T ertian—solution technique—and E bel—variable take-off angle—either take this dependence of the effective wavelength into account or are independent of this quantity.