John J. Donovan

Improved electron probe microanalysis of trace elements in quartz

Abstract Quartz occurs in a wide range of geologic environments throughout the Earth’s crust. The concentration and distribution of trace elements in quartz provide information such as temperature and other physical conditions of formation. Trace element analyses with modern electron-probe microanalysis (EPMA) instruments can achieve 99% confidence detection of ~100 ppm with fairly minimal effort for many elements in samples of low to moderate average atomic number such as many common oxides and silicates. However, trace element measurements below 100 ppm in many materials are limited, not only by the precision of the background measurement, but also by the accuracy with which background levels are determined. A new “blank” correction algorithm has been developed and tested on both Cameca and JEOL instruments, which applies a quantitative correction to the emitted X-ray intensities during the iteration of the sample matrix correction based on a zero level (or known trace) abundance calibration standard. This iterated blank correction, when combined with improved background fit models, and an “aggregate” intensity calculation utilizing multiple spectrometer intensities in software for greater geometric efficiency, yields a detection limit of 2 to 3 ppm for Ti and 6 to 7 ppm for Al in quartz at 99% t-test confidence with similar levels for absolute accuracy

Secondary fluorescence in electron probe microanalysis of material couples

We describe a semi-analytical method for the fast calculation of secondary fluorescence in electron probe microanalysis of material couples. The calculation includes contributions from primary K-, L- and M-shell characteristic x-rays and bremsstrahlung photons. The required physical interaction parameters (subshell partial cross sections, attenuation coefficients, etc) are extracted from the database of the Monte Carlo simulation code system PENELOPE. The calculation makes use of the intensities of primary photons released in interactions of beam electrons and secondary electrons. Since these intensities are not readily available and do not allow analytical calculation, they are generated from short Monte Carlo simulation runs. The reliability of the proposed calculation method has been assessed by comparing calculated, distance-dependent k-ratios with experimental data available in the literature and with results from simulations with PENELOPE. Numerical results are found to be in close agreement with both simulated and experimental data.

Trace analysis in EPMA

Trace element micro-analysis has evolved steadily since the early days of EPMA, yet remains an extraordinarily challenging subject. The enhanced capabilities of modern instrumentation, including the use of spectrometers with high X-ray collection efficiencies, high brightness electron sources, and improved stability all contribute to our ability to improve detection limits and analytical spatial resolution. Along with much improved software for data acquisition and analysis, recent progress in EPMA has made the trace realm more accessible than ever. High count precision can be obtained in order to easily bring analytical sensitivity into the single ppm range, but accuracy remains the greatest struggle. With the exception of the calibration, all sources of error encountered in major element analysis are magnified in trace analysis, and other sources become apparent where high spatial resolution is needed. Beam damage and charge effects are difficult problems in high sensitivity, high spatial resolution analysis, particularly in the analysis of insulators. Software can minimize some of the resulting effects on count rates during acquisition in order to improve accuracy, and analysts can empirically evaluate the conditions of analysis (count time, voltage, current, etc.) to try to minimize these effects. Trace analysis is fundamentally an exercise in background characterisation, and the acquisition and evaluation of background is a subject of developing methodology. Background curvature and interferences can result in considerable inaccuracy, but can be dealt with via detailed quantitative wavelength scanning or multi-point spectral acquisitions which allow proper regression of the background shape. In the absence of excellent quality trace element secondary standards of similar matrix to unknowns, blank testing and consistency standards can be used to test at least some aspects of the methods employed. Ultimately, the analyst must rely on accuracy evolving from application of the most rigorous protocols.

A new EPMA method for fast trace element analysis in simple matrices

Abstract It is well known that trace element sensitivity in electron probe microanalysis (EPMA) is limited by intrinsic random variation in the X-ray continuum background and weak signals at low concentrations. The continuum portion of the background is produced by deceleration of the electron beam by the Coulombic field of the specimen atoms. In addition to the continuum, the background also includes interferences from secondary emission lines, “holes” in the continuum from secondary Bragg diffraction, non-linear curvature of the wavelength-dispersive spectrometer (WDS) continuum and other background artifacts. Typically, the background must be characterized with sufficient precision (along with the peak intensity of the emission line of interest, to obtain the net intensity for subsequent quantification), to attain reasonable accuracy for quantification of the elements of interest. Traditionally we characterize these background intensities by measuring on either side of the emission line and interpolate the intensity underneath the peak to obtain the net intensity. Instead, by applying the mean atomic number (MAN) background calibration curve method proposed in this paper for the background intensity correction, such background measurement artifacts are avoided through identification of outliers within a set of standards. We divide the analytical uncertainty of the MAN background calibration between precision errors and accuracy errors. The precision errors of the MAN background calibration are smaller than direct background measurement, if the mean atomic number of the sample matrix is precisely known. For a simple matrix and a suitable blank standard, a high-precision blank correction can offset the accuracy component of the MAN uncertainty. Use of the blank-corrected-MAN background calibration can further improve our measurement precision for trace elements compared to traditional off-peak measurements because the background determination is not limited by continuum X-ray counting statistics. For trace element mapping of a simple matrix, the background variance due to major element heterogeneity is exceedingly small and high-precision two-dimensional background correction is possible.

Contamination In The Rare-Earth-Element Orthophosphate Reference Samples

Several of the fourteen rare-earth element (plus Sc and Y) orthophosphate standards grown at Oak Ridge National Laboratory in the 1980s and widely distributed by the Smithsonian Institution’s Department of Mineral Sciences, are significantly contaminated by Pb. The origin of this impurity is the Pb2P2O7 flux that is derived from the thermal decomposition of PbHPO4. The lead pyrophosphate flux is used to dissolve the oxide starting materials at elevated temperatures (≈1360 °C) prior to the crystal synthesis. Because these rare-earth element standards are extremely stable under the electron beam and considered homogenous, they have been of enormous value to electron probe micro-analysis (EPMA). The monoclinic, monazite structure, orthophosphates show a higher degree of Pb incorporation than the tetragonal xenotime structure, orthophosphates. This paper will attempt to describe and rationalize the extent of the Pb contamination in these otherwise excellent materials.

Numerical correction for secondary fluorescence across phase boundaries in EPMA

A fast calculation method to compute secondary fluorescence near phase boundaries in electron probe microanalysis is described. Secondary fluorescence intensities are calculated by numerically integrating the equations that describe the emission of fluorescence by photoelectric absorption of primary X-rays (characteristic and bremsstrahlung) from a couple of two semi-infinite, adjacent materials, when the electron beam impacts on one of them. The reliability of the developed calculation is assessed by comparing calculated fluorescence k-ratios, as functions of the distance of the electron beam to the interface, with experimental data available in the literature and with the results of Monte Carlo simulation using code PENELOPE.

Trace Element Analyses by EMP: Pb-in-Monazite and New Multipoint Background Method

University of Oregon, CAMCOR, Eugene (OR), USA Precision in electron microprobe analysis is primarily a matter of counting statistics, and involves voltage, beam current, counting time, and geometric efficiency. Besides potential issues with dead-time correction and beam damage, counting statistics do not significantly affect the accuracy of major and minor element analyses. Accuracy chiefly depends on standards and matrix correction. Additional challenges with both accuracy and high precision appear when measuring trace elements [1]: 1) Analytical precision requires the use of high intensity monochromators, optimized PHA settings, high beam current, higher voltage, and/or lengthy count time. These latter requirements can lead to beam damage, surface contamination, and internal charge effects. 2) Characterization and measurement of the background become crucial for accurate results when the peak-to-background ratio approaches 1. 3) Accurate results require correction or avoidance of peak and background interferences, even for some 2

CALCZAF, TRYZAF and CITZAF: The Use of Multi-Correction-Algorithm Programs for Estimating Uncertainties and Improving Quantitative X-ray Analysis of Difficult Specimens

Much work has been done in the last 50 years in developing comprehensive correction algorithms for quantitative electron microbeam analysis. A number of correction methods – both theoretically and empirically based – have been used, incorporating or parameterizing various compilations of physical constants like mass absorption coefficients and mean ionization potentials. There is no single, universally accepted correction method used for microbeam analysis and the correction programs employed by the commercial instrument manufacturers give different results for the same input data.

A New EPMA Method For Fast Trace Element Analysis In Simple Matrices

Traditionally Electron Probe Micro Analysis (EPMA) has relied upon precise characterization of the continuum intensities adjacent to the emission line of interest using Wavelength Dispersive Spectrometry (WDS) for determination of the background under the peak. Recent improvements including new hardware designs with large area Bragg crystals, new software methods implementing exponential and polynomial interpolations to more accurately characterize the curvature of the background, and aggregated spectrometer signals to improve sensitivity, have enabled the EPMA to attain detection limits as low as 2 to 3 PPM in some materials.[1]

Quantitative Mapping of and Secondary Fluorescence Effects in Olivine Hosted Melt Inclusions

Quantitative x-ray maps were acquired to investigate the homogeneity of melt inclusions and explore possible secondary fluorescence effects from the host material. By using elemental maps, one is more easily able to distinguish large scale patterns from localized features. It can also illustrate instrument effects on measurements, such as detector position (relative to sample orientation) or Bragg defocusing. These attributes are useful when trying to determine authenticity of chemical zonations in samples.