Standards-Based Quantification in DTSA-II
opens a spectrum file browser. Each time you open a spectrum, you will be asked various questions. If any critical information is missing, you will be asked to provide it. Tis may include beam energy, probe current, and/or live time. Unless the program can deduce it without operator intervention, you will also be asked which element or elements to associate with each standard spectrum.
7. Te spectra will be listed in the table associated with the elements for which they are standards. If the composi- tion of a standard is not available in the spectrum file, then the final column may not be correct! Be sure to check that the composition listed in the final column matches the composition of the material from which the standard spectrum was collected. If it does not, highlight the appropriate composition cell and select the correct composition from the drop-down list box. If an appropriate material is not listed, use the “New material” option to define the correct material. (Note: the material editor was described in a previous article in this series [2].)
8. Once you have identified standards for each element in the unknown, review the information in the table. You may use the “Properties” button to edit the probe current or live time. You may replace erroneous standards using the “Remove” button. Make certain all the information is correct. Any mistakes will lead to mistakes in the final result. When you are confident of the information, press “Next” to proceed to the next step.
9. On page D you can specify how to account for unmeasured elements. If you omitted a standard for one element in Step (6), you may specify how to estimate this element. If you specified a standard for all elements, select “No extra element.” If you want to assert that an unmeasured element represents all the remaining unmeasured material, use the “Element by difference.” Most oſten this mechanism is used to
estimate the oxygen or carbon content because these elements are hard to measure directly. Alternatively, you may select “Oxygen by stoichiometry” to calculate the oxygen content based on the quantity of the other elements and an assumed stoichiometry. You may edit the assumed stoichiometry in the table to the right. When you have configured the desired option, use the “Next” button to proceed to the next step.
10. On page E you will select reference spectra. Many times, standard spectra can also be used as references. When this is not the case, there will remain blank entries in the final column of the references table. You must specify spectra to serve as references for each of these vacancies. Te quality of the reference is evaluated in the final column. You may select the reference spectrum from a file using the “File” button. You may unassign a reference using the “Remove” button. Note that the “Strip” button allows you to fit, but otherwise ignore, certain elements. If there is an element (like carbon), which is known to coat the sample but does not represent an element present in the unknown material, you may use “Strip” to specify that element that will be fit but otherwise ignored in the final quantification. Select “Next” to proceed to the next step.
11. Usually the program selects which characteristic lines to use for optimal results. However, on page F you may override this behavior for elements for which there is a choice. On an element-by-element basis you may use the drop-down list box in the “Line family” column to select a set of characteristic lines. Select “Next” to proceed to the next step.
12. On page G you get one last chance to select spectra to quantify and to ensure that the data associated with the spectrum are correct. You can use the “Properties” button to edit the probe current and live time. You can select additional spectra to quantify or remove spectra
Figure 4: The unknown and good and bad residual spectra. A good residual (in red) shows that all the major characteristic lines have been accounted for in the quantification process. A bad residual (in green) suggests that some element has been omitted. In this example, the peaks between 4 keV and 5 keV are due to Ba and Ti. If we neglect to include Ti (a reasonable omission), we get the bad residual shown in green. The slight S-curve at about 1.8 keV is probably due to slight peak shift possibly due to input count rate shifts and does not suggest a missing element.
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www.microscopy-today.com • 2011 September
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