Checking (Secondary) Standards
Once the primary standards are assigned it is recommended to analyze the standards to check for internal consistency. For this, the acquisition of secondary standards is preferred. Secondary standards are standards in which no elements (or only some) are not assigned as the primary standard for the analyzed elements.
There are two ways to check your standards, either using the Evaluate utility to graphically view together all standards that have an element acquired, or by viewing the result (in Analyze!) of using the primary standard to analyze a secondary standard.
The application EVALUATE.EXE runs as a standalone utility, with the MDB file being read in and the user then selecting an element-spectrometer pair. All standards with counts on that pair (e.g. Si Ka-Sp4) will be plotted with the declared element wt.% on the X axis. The user selects the specific type of count data to be plotted on the Y axis, e.g. ZAF-corrected average counts. A set of good standards would then all fall on a straight line through all the points as well as (0,0). This is discussed in more detail elsewhere.
The other method is to review the secondary standards one by one, in the Analyze! Window. First select all the standards. Then check the Pause Between Sample check box. Then click the Analyze button. The program will pause between each sample analysis to allow the user to view the results. Look over the results carefully. Standards whose x-ray intensities have been assigned as the standard for that element will normally look close to perfect since they are by definition the primary standard. The other unassigned elements are treated as unknowns and by observing the "%VAR" (percent variance) line, one can get an idea of the quality of the standardization by examining these secondary standards.
For example, assume that you are analyzing for Si, Al, Na and K and your assigned standards are SiO2 for Si, NaAlSi3O8 (albite) for Al and Na, and KAlSi3O8 (orthoclase) for K. In this case SiO2 is your primary standard for Si, and both albite and orthoclase can serve as secondary standards to check the quality of the Si calibration since they are measured for but not assigned as calibration standards for Si. In addition, the orthoclase standard can be used as a secondary standard in the example above, to check the Al calibration. Therefore, orthoclase is the primary standard for K, but is a secondary standard for both Si and Al, and albite is the primary standard for Al and Na, but is a secondary standard for Si only, while SiO2 is the primary standard for Si only. Note that a pure standard such as synthetic SiO2 can be used to check the quality of the background correction on the Al, Na and K channels in this example, since pure SiO2 should not contain those elements in significant quantities.
While looking over the standard results, look carefully for "bad" points. By this, it is meant points whose total compositions are anomalously low. These could be points where the beam was on epoxy or perhaps the surface was poorly polished. In any case you will see this as a low total. These points may be disabled.
Note that points whose totals are high, on the other hand, should not be disabled arbitrarily. High totals indicate that something else is wrong. Check for low total points on another standard or for the presence of spectral interferences or inclusions.