Quantitative WDS compositional mapping using the electron microprobe

Abstract

While much progress has been made in electron-probe microanalysis (EPMA) to improve the accuracy of point analysis, the same level of attention has not always been applied to the quantification of wavelength-dispersive spectrometry (WDS) X-ray intensity maps at the individual pixel level. We demonstrate that the same level of rigor applied in traditional point analysis can also be applied to the quantification of pixels in X-ray intensity maps, along with additional acquisition and quantitative processing procedures to further improve accuracy, precision, and mapping throughput. Accordingly, X-ray map quantification should include pixel-level corrections for WDS detector deadtime, corrections for changes in beam current (beam drift), changes in standard intensities (standard drift), high-accuracy removal of background intensities, quantitative matrix corrections, quantitative correction of spectral interferences, and, if required, time-dependent corrections (for beam and/or contamination sensitive materials). The purpose of quantification at the pixel level is to eliminate misinterpretation of intensity artifacts, inherent in raw X-ray intensity signals, that distort the apparent abundance of an element. Major and minor element X-ray signals can contain significant artifacts due to absorption and fluorescence effects. Trace element X-ray signals can contain significant artifacts where phases with different average atomic numbers produce different X-ray continuum (bremsstrahlung) intensities, or where a spectral interference, even an apparently minor one, can produce a false-positive intensity signal. The methods we propose for rigorous pixel quantification require calibration of X-ray intensities on the instrument using standard reference materials, as we already do for point analysis that is then used to quantify multiple X-ray maps, and thus the relative time overhead associated with such pixel-by-pixel quantification is small. Moreover, the absolute time overhead associated with this method is usually less than that required for quantification using manual calibration curve methods while resulting in significantly better accuracy. Applications to geological, synthetic, or engineering materials are numerous as quantitative maps not only show compositional 2D variation of fine-grained or finely zoned structures but also provide very accurate quantitative analysis, with precision approaching that of a single point analysis, when multiple-pixel averaging in compositionally homogeneous domains is utilized.

Mapping mineralogical heterogeneities at the nm-scale by scanning electron microscopy in modern Sardinian stromatolites: Deciphering the origin of their laminations

“…A specific library of standards with synthetic and natural oxides and silicates (calcite, apatite, albite, augite, celestine, diopside, forsterite, garnet, orthose, TiO5, almandine, and anhydrite) were acquired using the same parameters. Then, each EDXS spectrum for each pixel was quantified using the ESPRIT, Bruker software based on the following procedure: 1) X-ray background generated by Bremsstrahlung was modeled accounting for X-ray absorption by the sample/carbon coating layer / detector window (Zanetta et al, 2019) and removed to obtain unbiased elemental maps in comparison with raw count maps (Donovan et al, 2021) 2) Quantification by a standard-based extended Pouchou-Pichoir (XPP) matrix correction was applied to provide a weight percentage for each element per pixel.…”

Quantitative WDS Compositional Mapping Using the Electron Microprobe

“…Trace element X-ray signals can contain significant artifacts where phases with different average atomic number produce different Xray continuum (bremsstrahlung) intensities, or where a spectral interference, even an apparently minor one, can produce a false-positive intensity signal [2,3]. The methods we propose for rigorous pixel quantification requires calibration of X-ray intensities on the instrument using standard reference materials, as we already do for point analysis, which are then used to quantify multiple X-ray maps, and thus the relative time overhead associated with such pixel-by-pixel quantification is small [4].…”

Practical Aspects of X-Ray Mapping in Electron Probe Microanalysis of Minerals

“…Often only qualitative x-ray intensity maps are presented where real compositional features are superimposed with instrumental artefacts such as x-ray continuum variations and spectral interferences. To provide reliable, quantitative information, WDS maps should be processed in the same way as point measurements in terms of background, interference and matrix corrections [1].Natural perovskite (CaTiO 3 ) crystals from the Mark kimberlite (Lac De Gras, Canada) were mapped on a JEOL JXA-8530F Plus instrument at 15 kV accelerating voltage, beam current of 50 nA (Ca, Ti, Mg, Si, Fe, all K) or 300 nA (NaK, NbL, CeL, NdL, ThM), beam diameter of 0.2 µm, step size of 0.5 µm and dwell time of 200 ms. Oxygen was calculated by stoichiometry and the mean atomic number (MAN) background correction was employed [2].…”

“…Often only qualitative x-ray intensity maps are presented where real compositional features are superimposed with instrumental artefacts such as x-ray continuum variations and spectral interferences. To provide reliable, quantitative information, WDS maps should be processed in the same way as point measurements in terms of background, interference and matrix corrections [1].…”