Advanced Chemical Analysis
structured samples present another application for the annular Bruker XFlash
Results ® FlatQUAD.
Figure 3 : Combined C (blue) and Si (red) element maps overlaid with the corresponding SE images of a low-density polyethylene composite with silicon-containing organo-clay particles. The clay particles have formed large agglom- erates. (a) Map acquired with a conventional 30 mm2 XFlash SDD setup, at 3 kV, 220 pA, input count rate 0.8 kcps, 1024 × 768 pixels, acquired over 320 s. Note the shadowing due to surface roughness. (b) Map acquired with the XFlash ® FlatQUAD detector using the same acquisition conditions as in (a) but resulting in a 12.5 times higher count rate and no shadowing. (c) Magnifi ed map area (green square) of map (b) in annular detection geometry. (d) Magnifi ed map area (green square) of image (c) showing no shadowing effects. Sample courtesy of D. P. da Silva Dalto Center for Mineral Technology (CETEM), Brazil, M. J. O. C. Guimarães and M. E. F. Garcia, Federal University of Rio de Janeiro, Brazil [ 10 ].
Low accelerating voltages and low probe currents can be helpful for the analysis of electron-beam-sensitive materials and of nonconductive samples that charge during examination. Compared to low-vacuum analysis, oſt en used to avoid sample charging, the use of low kV and low probe current in high vacuum circumvents the beam skirting effect. Beam skirting means that the spatial resolution of analysis is degraded by beam broadening because of collisions of probe electrons with gas molecules, atoms, and ions. T erefore low probe current under high vacuum conditions is desired. T in TEM samples or life science samples producing a low X-ray yield are additional situations well suited for analysis with this efficient annular detector. Its geometry was optimized in a way that allows the combination of thin-sample EDS with diff raction experiments in transmission, for example transmission Kikuchi diff raction (TKD) in the SEM [ 8 ]. Small particles and structures in the nanometer range also produce low X-ray yield, but they can be investigated at low accelerating voltages in SEM because of the small interaction volume attained. High-spatial-resolution EDS mapping on bulk specimens is best accomplished at low kV with the annular detector. Furthermore, fast mapping of sensitive samples and of large sample areas, including stitching maps together for complete analysis and statistics, are applications that benefi t greatly from effi cient X-ray collection. Last but not least, element mapping of porous or otherwise topographically
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High-vacuum EDS at low beam current of the historic Mocs Meteorite . This large meteorite fell as a bright ball of fi re in February 1882 near the village of Mocs, Romania. A sample of this stony meteorite, officially named NHMW-H9898 ( Figure 2a ), entered the Natural History Museum Vienna (NHMV) collection in 1908 as a donation. A large surface of 60 mm × 47 mm revealing a network of shock veins up to 10 mm, cut and polished but uncoated, was investi- gated. For the current SEM-EDS investigation, a region of interest was identifi ed aſt er acquiring an 800 pixel by 600 pixel overview map at a 2 µm by 2 µm pixel size over 47 minutes ( Figure 2b ). T e Pb M-line and S K-line were deconvoluted during measurement using a physical background subtraction and a least square fi t to library spectra ( Figure 2c ). To avoid charging of the uncoated nonconductive meteorite surface, a 6 kV beam with a probe current of less than 10 pA was employed under high- vacuum conditions to produce a second
map with an input count rate of 3000 cps ( Figure 2d ). T e 6 kV accelerating voltage kept the interaction volumes small for both electrons and X-rays. A smaller region of interest was chosen for an overnight map with a 130 nm pixel size, but otherwise had the same experimental conditions ( Figure 2e ). T e overview map of the Mocs meteorite ( Figure 2b ) shows lead enrichment in cracks that were most likely caused by an old polishing method employing a lead lapping plate that was used in the past. Suffi cient data quality (large number of counts) allows deconvolution of the Pb M and S K lines ( Figure 2c ), providing a separation of lead contamination from sulfide minerals that are part of the meteorite material beneath the contamination. T e EDS analysis at higher spatial resolution ( Figures 2 d and 2 e) shows the deposition of lead on oxides and sulfi des. Spherical carbon particles and fi laments with sizes <300 nm were observed, a sign of surface contamination with soot, very likely related to furnaces used at the NHMV during the last century or from the time before it was added to the NHMV collection.
Polymer with embedded organo-clay particles . Soſt
and porous materials are challenging not only in terms of beam sensitivity and charging but also in regard to judging the distribution of pores or property-enhancing particles. T e following experiment describes the EDS analysis of a polymer compound containing Si-rich organo-clay particles, derived
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