X-ray fluorescence microscopy of light elements in cells: self-absorption correction by integration of compositional and morphological measurements

A relatively new entry, in the field of microscopy, is the Scanning X-Ray Fluorescence Microscope (SXRFM). Using this type of instrument (e.g. Kevex Omicron X-ray Microprobe), one can obtain multiple elemental x-ray images, from the analysis of materials which show heterogeneity. The SXRFM obtains images by collimating an x-ray beam (e.g. 100 μm diameter), and then scanning the sample with a high-speed x-y stage. To speed up the image acquisition, data is acquired "on-the-fly" by slew-scanning the stage along the x-axis, like a TV or SEM scan. To reduce the overhead from "fly-back," the images can be acquired by bi-directional scanning of the x-axis. This results in very little overhead with the re-positioning of the sample stage. The image acquisition rate is dominated by the x-ray acquisition rate. Therefore, the total x-ray image acquisition rate, using the SXRFM, is very comparable to an SEM. Although the x-ray spatial resolution of the SXRFM is worse than an SEM (say 100 vs. 2 μm), there are several other advantages.

Download Full-text

Gaussian ϕ(ρz) curves: Comparison with other models

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100134557 ◽

1990 ◽

Vol 48 (2) ◽

pp. 192-193

Author(s):

Alberto Riveros ◽

Gustavo Castellano

Keyword(s):

Good Description ◽

Bulk Sample ◽

Incident Electron Energy ◽

Power Mean ◽

General Shape ◽

Ionization Cross ◽

X Ray ◽

Mass Absorption ◽

Absorption Correction ◽

Image Position

X ray characteristic intensity Ii , emerging from element i in a bulk sample irradiated with an electron beam may be obtained throughwhere the function ϕi(ρz) is the distribution of ionizations for element i with the mass depth ρz, ψ is the take-off angle and μi the mass absorption coefficient to the radiation of element i.A number of models has been proposed for ϕ(ρz), involving several features concerning the interaction of electrons with matter, e.g. ionization cross section, stopping power, mean ionization potential, electron backscattering, mass absorption coefficients (MAC’s). Several expressions have been developed for these parameters, on which the accuracy of the correction procedures depends.A great number of experimental data and Monte Carlo simulations show that the general shape of ϕ(ρz) curves remains substantially the same when changing the incident electron energy or the sample material. These variables appear in the parameters involved in the expressions for ϕ(ρz). A good description of this function will produce an adequate combined atomic number and absorption correction.

Download Full-text

point-Analysis Using the Extrapolation Method in the Analytical Electron microscope

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100135757 ◽

1990 ◽

Vol 48 (2) ◽

pp. 430-431

Author(s):

Zenji Horita ◽

Ryuzo Nishimachi ◽

Takeshi Sano ◽

Minoru Nemoto

Keyword(s):

Electron Microscope ◽

Extrapolation Method ◽

X Ray ◽

Absorption Correction ◽

Point Analysis ◽

Analytical Electron ◽

Analytical Electron Microscope ◽

Data Points ◽

Identical Composition ◽

X Ray Absorption

Absorption correction is often required in quantitative x-ray microanalysis of thin specimens using the analytical electron microscope. For such correction, it is convenient to use the extrapolation method[l] because the thickness, density and mass absorption coefficient are not necessary in the method. The characteristic x-ray intensities measured for the analysis are only requirement for the absorption correction. However, to achieve extrapolation, it is imperative to obtain data points more than two at different thicknesses in the identical composition. Thus, the method encounters difficulty in analyzing a region equivalent to beam size or the specimen with uniform thickness. The purpose of this study is to modify the method so that extrapolation becomes feasible in such limited conditions. Applicability of the new form is examined by using a standard sample and then it is applied to quantification of phases in a Ni-Al-W ternary alloy.The earlier equation for the extrapolation method was formulated based on the facts that the magnitude of x-ray absorption increases with increasing thickness and that the intensity of a characteristic x-ray exhibiting negligible absorption in the specimen is used as a measure of thickness.

Download Full-text