Quantitative analysis with EDS can be done very rigorously and lead to very accurate answers. There are also some situations (albeit a minority) where EDS provides better quantitative answers due to issues such as severe peak overlaps. Steve Seddio has a presentation at M&M comparing EDS and WDS quant for rare earth elements. The KEY REQUIREMENT is that rigorous standards, a very stable beam current and a systematic approach to data collection are required to achieve accurate Quant with an EDS. Not so different than the requirements for accurate Quant in a WDS.
Hi Keith,
I'm going to agree with most of what you said and nitpick the rest. First I would say that since WDS (e.g., Probe for EPMA at least) has had a fully quantitative iterated interference correction implemented, spectral overlaps are easy to diagnose and very easy to correct. EDS is getting there, but still there are more such situations in EDS with its 10 to 20 times lower spectral resolution. I agree that standards is the key and as I stated at the Castaing session in Nashville a few years ago: "There is one advantage that WDS will *always* have over EDS, and that is the fact that you can't do WDS quant without standards!"
HOWEVER, the EDS community at large has lost sight of both the natural limitations of EDS quantitative analysis and the rigor required for good quantitative analysis. There are two major problems with how the community at large approaches quant with EDS.
1. Several posts here have really hit the first misconception right on its head. Most EDS users don't use standards and simply normalize the "standardless" data and pretend that the results are perfect. In fact a certain EDS company actually advertises standardless EDS quant "as good as full standards quantitative analysis." Work on enough different SEMs - better yet plug in a good oscilloscope and watch the actual outbound signals from an EDS detector before it hits the DPP - and this statement becomes comical. To further exacerbate the error, It's amazing how much EDS analysis is done in variable pressure mode, without standards, with the assumed pretense that there is no loss in low energy x-ray intensity (read as boron or oxygen) relative to the transition metals. It's easy to prove: Good quant on any EDS system requires the use of standards.
I could not agree more- it's basically a training issue, but also an issue of making the application so easy to do the right thing, that even a lazy man (such as myself) will do it!
2. The systematic accuracy in EDS quant is ALWAYS reported based on counting statistics. Want accuracy to 0.001 %wt? Just collect 10 million x-rays. Only need 0.1% accuracy? Collect only a couple thousand x-rays. This neglects entire concepts such as peak to background, Gauge R&R, and basic machine error.
OK, just a semantic nitpick here: when speaking of counting statistics, you'll want to say "precision" or "sensitivity" as opposed to accuracy, which is determined by secondary standards or a blank specimen.
The EDS technology leaders have all done an outstanding job in making EDS more powerful and more available - i.e. easier to use. This has dramatically expanded the community of EDS users, which is a very, very good thing. The sad drawback is that this rapid expansion in the size of the EDS user base comes at the expense of the basic understanding of underlying principles of EDS.
Yes, make the system easy enough to use so that even an idiot can use it and... well you know.
My bottom line is this:
1. Use EDS for major elements and, *when possible*, even for light elements because peak shape issues are automatically dealt with.
2. Use WDS for trace elements because of its intrinsic better sensitivity.
As most of you know, this is the reason Probe for EPMA automatically acquires an EDS spectra with every standard and unknown analysis point- this allows for much flexibility when off line processing and which elements you decide to quantify using which technique. This means we construct a normal unk/std net intensity k-ratio from the unknown and standard EDS spectra which can be calculated for any element in the EDS spectra (assuming you have a standard with the element). See here for more details:
http://probesoftware.com/smf/index.php?topic=226.msg1052#msg1052