Standards-based EDS (and WDS)
The JSG E-beam lab possesses a collection of hundreds of polished and carbon-coated compositional standards, ranging from minerals to pure elements. Compositional standards are an essential part of any electron microprobe analysis by WDS, with standards chosen for their high concentration of elements of interest and similar ‘matrix’ to the expected unknowns – using garnet standards for analyzing garnets, carbonates for carbonates, and so on. Less common in the earth sciences is the use of standards-based EDS analysis, although it offers a powerful tool when WDS is too time-consuming or expensive. Our lab has developed a standards-based quantitative analytical protocol that allows our JEOL 6490 SEM to function as a sort of probe-lite using it’s outstanding Oxford Instruments X-Max 50mm2 EDS detector. Our system is able to reproduce standard compositions to 1-2 relative deviation (the equivalent of the microprobe) for elemental concentrations down to ~1,000 ppm, even in many x-ray lines that interfere with one another.
Using the EDS for quantitative analysis offers several advantages to the analyst:
– Rapid acquisition, often 10-30 seconds for good precision on all elements.
– Easy workflow by taking an electron image at low (50-100x) mag and then setting points and areas by beam deflection.
– Standardizations hold for weeks or months instead of hours like the WDS, allowing the user to start collecting data immediately.
– Naturally low nA analysis, because the EDS is so much closer to the sample surface than any WDS, most quantification can be performed at <2 nA compared to the typical 20 nA of the electron microprobe. This is very helpful for beam-sensitive materials such as flouro-apatite and alkali glasses.
Our lab uses two standards-based quantification methods for EDS spectra: Oxford Instrument’s Aztec software, and NIST’s DTSA-II. Both use very similar approaches, with the workflow of Aztec being faster but a little more opaque to the analyst. Standards-based quantitative EDS is currently rare in the earth sciences, but has been published in peer-reviewed journals like Nature and Nature Geoscience.
Newbury, Dale E., and Nicholas WM Ritchie. “Quantitative electron-excited X-ray microanalysis of borides, carbides, nitrides, oxides, and fluorides with scanning electron microscopy/silicon drift detector energy-dispersive spectrometry (SEM/SDD-EDS) and NIST DTSA-II.” Microscopy and Microanalysis 21.5 (2015): 1327.
Newbury, Dale E., and Nicholas WM Ritchie. “Performing elemental microanalysis with high accuracy and high precision by scanning electron microscopy/silicon drift detector energy-dispersive X-ray spectrometry (SEM/SDD-EDS).” Journal of materials science 50.2 (2015): 493-518.
Newbury, Dale E., and Nicholas WM Ritchie. “Electron-excited x-ray microanalysis by energy dispersive spectrometry at 50: analytical accuracy, precision, trace sensitivity, and quantitative compositional mapping.” Microscopy and Microanalysis 25.5 (2019): 1075-1105.
Ritchie, N., J. M. Davis, and Dale E. Newbury. “DTSA-II: A new tool for simulating and quantifying EDS spectra-Application to difficult overlaps.” Microscopy and Microanalysis 14.S2 (2008): 1176-1177.
Ritchie, Nicholas WM, Dale E. Newbury, and Jeffrey M. Davis. “EDS measurements of X-ray intensity at WDS precision and accuracy using a silicon drift detector.” Microscopy and Microanalysis 18.4 (2012): 892.