Standards-Based Quantification in DTSA-II
using the “Add file” and “Remove” buttons respec- tively. (Te “Sample Shape” option will be covered in a later article.) Select “Next” to perform the calculations and proceed to review the results.
13. Page H allows you to review the results. Te results are tabulated on a spectrum-by-spectrum basis. Te first column identifies the spectrum. Te second tabulates the analytical total: the sum by mass fraction of all elements quantified. Te analytical total should be approximately 1.0. With care, a total between 0.95 and 1.05 can be regularly achieved. A total outside this range suggests that there might be a problem worthy of investigation. When you are done reviewing the results, select “Finish.”
14. Te quantitative results will be tabulated at the end of the “Report.” Te quantified spectrum, the residual spectrum, and a simulated spectrum will be displayed in the spectrum display.
15. Review the residual spectrum (see Figure 4). Te residual represents the unknown spectrum minus all the characteristic peaks accounted for in the quanti- fication process. If an element was overlooked in the quantification, the residual will not look “clean,” and the analytical total is likely to be low. Tere will be intensity in a characteristic peak, which remains in the residual. When this happens, it suggests that you are likely to have omitted an element and that you should collect a standard for this element and repeat the quantification process.
16. Te “Report” (see Figure 5) summarizes the standards, references, and other parameters selected to perform the quantification. It also summarizes the results and some other useful pieces of information used during the quantification process.
Helpful Suggestions a. It saves time and effort if the standard spectra record all the necessary information within the file. Te most important pieces of information are the beam energy, probe current, live time, and composition. It saves time if each time you collect a standard spectrum you open the spectrum and edit the properties to ensure they are correct. Use the “Assign material” item in the “Tools” menu to associate a material with the spectrum. Save the standard spectrum back to disk as an EMSA file. I usually name the edited spectrum aſter the material and append the letters “std” to indicate that the file contains all the standard-related information.
b. You can select more than one spectrum to quantify during each pass through the “quantification alien.” All the spectra should contain the same elements and have been collected on the same detector with the same beam energy.
c. If you control your acquisition conditions and check that your detector’s performance has not changed, you can keep an archive of standard and reference spectra. I usually archive them in directories first organized by detector/process time and then by beam energy. Te
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results are likely to be slightly less accurate than standards collected on the same day, but the results are still likely to be more accurate than standard-less quantification.
d. If you don’t have standard samples for the elements you anticipate measuring in your laboratory, you might consider purchasing one or more standard blocks from a suitable vendor. Vendors can provide you with a single 25.4 mm diameter block containing as many as sixty suitable standard materials. Te block is typically polished to a sub-micrometer finish and coated with a conductive layer of carbon to eliminate surface charging. A good quality block is not cheap, but the cost represents a small fraction of the cost of your instrument or X-ray detector and will improve the quality of your results more than any other item you can purchase.
Conclusion Te quantification process is complicated. It takes time
and an attention to detail. Although I have tried to be concise and complete in this description, a few examples will help to clarify the process. Part II of this article will provide more details and a couple of examples.
References [1] NWM Ritchie, Microscopy Today 19(1) (2011) 26–31. [2] NWM Ritchie, Microscopy Today 19(3) (2011) 30–34. [3] JI Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, Springer, New York, 2003.
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