Barriers to Quantitative Electron Probe X-Ray Microanalysis for Low Voltage Scanning Electron Microscopy

Low voltage x-ray microanalysis, defined as being performed with an incident beam energy ≤5 keV, can achieve spatial resolution, laterally and in depth, of 100 nm or less, depending on the exact selection of beam energy and the composition of the target. The shallow depth of beam penetration, with the consequent short path length for x-ray absorption, and the low overvoltage, the ratio of beam energy to the critical ionization energy, both contribute to minimizing the matrix effects in quantitative x-ray microanalysis when the unknown is compared to pure element standards. The low beam energy restricts the energy of the atomic shells that can be excited, forcing the analyst to choose unfamiliar shells/characteristic peaks. The low photon energy shells are subject to low fluorescence yield, so that the peak-to-continuum background is reduced, severely limiting detectability. The limited resolution of semiconductor energy dispersive spectrometry results in frequent peak interference situations and further exacerbates detection limits. Future improvements to the x-ray spectrometry limitations are possible with x-ray optics-augmented wavelength dispersive spectrometry and microcalorimeter energy dispersive spectrometry.     

X-Ray Microanalysis in the Variable Pressure (Environmental) Scanning Electron Microscope

Electron-excited x-ray microanalysis performed in the variable pressure and environmental scanning electron microscopes is subject to additional artifacts beyond those encountered in the conventional scanning electron microscope. Gas scattering leads to direct contributions to the spectrum from the environmental gas, as well as remote generation of x rays by electrons scattered out of the focussed beam. The analyst can exert some degree of control over these artifacts, but depending on the exact situation, spurious elements can appear at the trace (< 0.01 mass fraction), minor (0.01 mass fraction to 0.1 mass fraction), or even major (> 0.1 mass fraction) levels. Dispersed particle samples give the least compromised results, while fine scale microstructures are the most severely compromised. Procedures to optimize the situation based upon specimen preparation as well as spectral processing are described.     

High Count Rate Electron Probe Microanalysis

Reducing the measurement uncertainty of quantitative analyses made using electron probe microanalyzers (EPMA) requires a careful study of the individual uncertainties from each definable step of the measurement. Those steps include measuring the incident electron beam current and voltage, knowing the angle between the electron beam and the sample (takeoff angle), collecting the emitted x rays from the sample, comparing the emitted x-ray flux to known standards (to determine the k-ratio) and transformation of the k-ratio to concentration using algorithms which includes, as a minimum, the atomic number, absorption, and fluorescence corrections.This paper discusses the collection and counting of the emitted x rays, which are diffracted into the gas flow or sealed proportional x-ray detectors. The representation of the uncertainty in the number of collected x rays collected reduces as the number of counts increase. The uncertainty of the collected signal is fully described by Poisson statistics. Increasing the number of x rays collected involves either counting longer or at a higher counting rate. Counting longer means the analysis time increases and may become excessive to get to the desired uncertainty. Instrument drift also becomes an issue. Counting at higher rates has its limitations, which are a function of the detector physics and the detecting electronics.Since the beginning of EPMA analysis, analog electronics have been used to amplify and discriminate the x-ray induced ionizations within the proportional counter. This paper will discuss the use of digital electronics for this purpose. These electronics are similar to that used for energy dispersive analysis of x rays with either Si(Li) or Ge(Li) detectors except that the shaping time constants are much smaller.     

Accurate Cross Sections for Microanalysis

To calculate the intensity of x-ray emission in electron beam microanalysis requires a knowledge of the energy distribution of the electrons in the solid, the energy variation of the ionization cross section of the relevant subshell, the fraction of ionizations events producing x rays of interest and the absorption coefficient of the x rays on the path to the detector. The theoretical predictions and experimental data available for ionization cross sections are limited mainly to K shells of a few elements. Results of systematic plane wave Born approximation calculations with exchange for K, L, and M shell ionization cross sections over the range of electron energies used in microanalysis are presented. Comparisons are made with experimental measurement for selected K shells and it is shown that the plane wave theory is not appropriate for overvoltages less than 2.5 V.     

Implications of Polishing Techniques in Quantitative X-Ray Microanalysis

Specimen preparation using abrasives results in surface and subsurface mechanical (stresses, strains), geometrical (roughness), chemical (contaminants, reaction products) and physical modifications (structure, texture, lattice defects). The mechanisms involved in polishing with abrasives are presented to illustrate the effects of surface topography, surface and subsurface composition and induced lattice defects on the accuracy of quantitative x-ray microanalysis of mineral materials with the electron probe microanalyzer (EPMA).     

材料电子显微分析与应用

电子显微分析就电子束形式可分为扫描电子分析和透射电子分析两大类,就分析类别可分为显微形貌分析、成分分析和结构分析三部分。并论述了材料电子显微分析常用的技术以及应用实例。     

硒光导膜表面缺陷的电子显微分析

采用以电子探针为主要手段对硒光导膜表面缺陷进行了研究，结果表明硒膜表面斑点区富集有含硫为主的杂质元素，有些还包裹了非硒微粒，讨论了斑点缺陷的形成机制。微量杂质硫在硒膜和原料中均存在不均匀分布，它是造成硒感光鼓斑点缺陷和表面剩余电位高的主要原因。     

Determining the Uncertainty of X-Ray Absorption Measurements

X-ray absorption (or more properly, x-ray attenuation) techniques have been applied to study the moisture movement in and moisture content of materials like cement paste, mortar, and wood. An increase in the number of x-ray counts with time at a location in a specimen may indicate a decrease in moisture content. The uncertainty of measurements from an x-ray absorption system, which must be known to properly interpret the data, is often assumed to be the square root of the number of counts, as in a Poisson process. No detailed studies have heretofore been conducted to determine the uncertainty of x-ray absorption measurements or the effect of averaging data on the uncertainty. In this study, the Poisson estimate was found to adequately approximate normalized root mean square errors (a measure of uncertainty) of counts for point measurements and profile measurements of water specimens. The Poisson estimate, however, was not reliable in approximating the magnitude of the uncertainty when averaging data from paste and mortar specimens. Changes in uncertainty from differing averaging procedures were well-approximated by a Poisson process. The normalized root mean square errors decreased when the x-ray source intensity, integration time, collimator size, and number of scanning repetitions increased. Uncertainties in mean paste and mortar count profiles were kept below 2 % by averaging vertical profiles at horizontal spacings of 1 mm or larger with counts per point above 4000. Maximum normalized root mean square errors did not exceed 10 % in any of the tests conducted.     

浅谈用氢化物——原子吸收分光光度法测定化妆品中的砷

砷是化妆品中最常见的有害元素之一,用氢化物-原子吸收分光光度法测定微量砷,具有较高的灵敏度,而且分析方法简便、快速、准确度高,测定过程中砷损失少。     

原子吸收光谱法测定水中铅的不确定度分析

目的:为提高检测结果精确度,评定水中铅的不确定度。方法:分析水中铅不确定度的来源,通过计算得出该法测定水中铅的扩展不确定度。结果:测量结果表明扩展不确定度U95=2.3μg/L,适用于每个水样的检测结果。结论:该方法简便,适合于每一个样本的检测结果,可参考用于水中某些检测参数的不确定评定。     

火焰原子吸收法测定土样中铬的不确定度评定

实验室认可和资质认定都要求检测实验室具备评定测量不确定度的能力,现依据JJF1059-1999<测量不确定度评定与表示>,对火焰原子吸收法测定土样中铬的含量进行不确定度评估,分析测定过程中不确定度来源,量化不确定度分量,求出合成不确定度和扩展不确定度,给出测定结果的表示式.     

聚全氟乙丙膜电子束引发预辐射接枝AA和SSS制备阳离子交换膜的性能

利用电子束引发预辐射接枝技术,在聚全氟乙丙烯(FEP)膜上接枝丙烯酸(AA)和对苯乙烯磺酸钠(SSS)两单体,制备出一种含羧酸基团和磺酸基团的阳离子交换膜,接枝率越高,接枝膜的吸碱性能越好。差示量热扫描及X射线衍射测试表明,接枝膜结晶度随接枝率的升高而降低。这是接枝膜中FEP组份的“结晶变形”及接枝链对晶区的“稀释作用”综合影响的结果。     

原子吸收法测定酱油中铅不确定度的评定

通过对测定酱油中铅含量的不确定度评定分析,找出不确定度产生的主要因素,评定确认最小二乘法拟合校准标准曲线及测量重复性是影响结果的最主要因素,使用标准物质(标准储备液)及稀释过程引入的不确定度也应引起重视,此方法对类似的火焰原子吸收法测定样品中待测元素含量有借鉴和参考作用。     

扫描电镜中颗粒能谱定量分析的质量效应

利用扫描电镜和能谱仪对精密合金中第二相颗粒的表面形貌和化学成分进行了分析,研究了颗粒能谱定量分析的质量效应。结果表明:颗粒能谱定量分析时有明显的质量效应;颗粒与基体的成分差异越大,质量效应越明显,加速电压对质量效应也有影响;颗粒能谱定量分析中存在一个临界尺寸,当颗粒尺寸大于临界尺寸时,质量效应消失;由于样品抛光表面上颗粒的二维表观尺寸与实际的三维尺寸不尽相同,使得质量效应的表观临界尺寸有别于理论临界尺寸,但颗粒能谱定量分析质量效应的变化趋势和统计规律是十分明显的。     

石墨炉原子吸收法测定化探样品中痕量金——石墨管使用次数对分析准确度的影响

本文报道了普通石墨管在化探样品中痕量金测试过程中,分析准确随石墨管使用次数的增加而变化的现象,研究了其引起的主要原因,简述了这类问题解决的办法。     

LIPS-200离子推力器电子反流极限不确定度分析

针对延长离子推力器栅极极限寿命的恒定加速电压和步进调节加速电压两种优化方法的工程应用需求,结合现有LIPS-200离子推力器产品技术参数,基于电子反流极限的Williams解析表达并应用QMU工程方法,定量计算了LIPS-200离子推力器电子反流极限的不确定度.计算结果表明:不确定度主要影响因素包括束流电子温度、加速栅...     

软X射线质量吸收系数的测定

制备了厚度为２５－２２５ｎｍ的Ｃ、Ａｌ、Ｔｉ、Ｃｕ、Ｎｂ、Ｍｏ、Ａｇ、Ｔａ和Ｗ９种元素薄膜样品,用化学分析方法测定了薄膜的质量厚度ｐｚ,并用电子探针测定了软Ｘ射线ＣＫα、ＢＫα的出射强度Ｉ０和穿过薄膜对ＣＫα、ＢＫα辐射的质量吸收数μ,并与以往的数值进行比较。