Problems With the Analysis
Performing quantitative analysis is always beset with many difficulties and it is often difficult to pinpoint the cause of "bad" analyses. More often than not, the effect is the sum of more than one problem. However, there are several things that can be checked. First of all start by examining the standards. It is in situations like this that the use of secondary standards can really be helpful in determining what is going on. If the primary and secondary standards do not agree with each other, there is a problem.
First of all, are the standards really "good" standards? That is, have their compositions been determined by a reliable analytical method? In general, major element standard concentrations need to be determined using classical wet chemistry methods. Second, are the concentrations entered into the STANDARD.MDB database correct and without typographical errors? Third, was the right standard actually acquired? It is easy to get lost at 300-400 magnifications when using a standard mount which contains many standards.
Some other things to consider :
1. Is the operating voltage correct specified? Is the correct x-ray line tuned for each element? Check the on-peak position offsets from the Peak/Scan Option dialog and see if they are reasonable. The program will usually type a warning if the actual and calculated peak positions are very different. Be sure that the spectrometer is not tuned on a nearby line of another element if using multi-element standards.
2. Be sure that no bad data points are in the standards samples used for the quantitative calibration. The best way to check for this is to analyze each standard and examine the results to look for points with obviously bad or low totals (epoxy, inclusions, bad surface polish, bad carbon coat, etc.). If a bad point is found, one can disable it. Remember, one can always enable the data later on. A disabled point is simply not used in calculating the analytical calculations but is still present. A good rule of thumb is to only disable points that have low totals since generally most of the problems mentioned above will result in fewer x-ray counts. Avoid disabling points just to get better agreement between the primary and secondary standards. Points that have high totals should not be arbitrarily disabled. It may be necessary to look for other problems such as points with low totals in the primary standards.
3. Look for interferences on the analyzed elements. One easy way to do this is to use the Interferences button in the Element Parameters window (see the Elements/Cations button in the Acquire! or Analyze! window). The best method is to perform a wavelength scan and display possible interfering peak markers. Then not only can the interferences be seen but one an also select background positions free from interferences. If the element causing the interference is present in significant concentrations in your unknowns, and is not being analyzed for, it may be necessary to add the interfering element to the run by creating a new sample with the interfering element as an analyzed element. Be sure that the proper standards are available to use for the interference correction.
4. If none of the above suggestions seem to help, try acquiring the standards again. Probe for EPMA uses an automatic standard drift corrections which can make a significant difference in situations where one or more of the standard intensities are drifting. Note that since the program will perform an automatic drift correction not only on the standards, but also the interference standards and the MAN background standards, it might be also be necessary to run additional sets of those standards or MAN standards.
5. In the case of trace or minor elements, also check to see that none of the off-peak positions are interfered with by another peak. This can cause a reduction in the on-peak counts, sometimes enough to result in a negative k-ratio. Always run at least one wavelength scan on a sample, using the same count time as your quantitative analyses, and if a peak is seen interfering with the off-peak marker, use the Low and/or High buttons in the Graph Data window to select a new off-peak position that is not interfered with.
6. Finally are the standards really "good" standards? That is, homogeneous and whose compositions are accurately and precisely known. Some may have been analyzed by classical "wet" chemical methods (e.g. Smithsonian mineral and glass standards) of small quantities of separates. Second, are the concentrations entered into the STANDARD.MDB database correctly? (no typos)
7. Low totals may result from charging due to a poor conductive (e.g. carbon) coating on insulated materials. Many year-old carbon coats have been known to fail. Also, a carbon-sputtered coating is not necessarily equivalent to a carbon-evaporated coating; both standards and unknowns may need to be recited by the same process
8. Peaks shifts both may need to be considered. The later are generally well recognized for the K lines of "light" elements (including Al), but less so for some L lines. If you recall that this results from the outer shell electron bonding or configuration being involved in X-ray production, those elements with M shell electrons (e.g. Co) also can have significant peak shifts.
9. Peak positioning: you may want to verify that the peak centering method you have selected is giving you optimal peak centers. It would not hurt to run the post-scan verification option where you step across the peak that the automation has chosen.