Hi all, I thought you might want to know about the first results from the "EXAFS Divination Set" project I've been plugging. I don't think the results will be any surprised to the veterans on this list, but might give novices an idea of what's possible. To briefly recap, my students made up mixtures of various iron compounds in various proportions, and then I analyzed them "blind." The idea was to see how well XAFS analysis as practiced by an "expert'" can do in that kind of case. Unfortunately, unbeknownst to me, the standards were poorly prepped (i.e. they were very uneven). So these results show what can be done with poor samples. I'm planning a follow-up for this fall using an entirely new set of samples and standards prepped as carefully as we can. Presumably better samples will generate more accurate results. Nonetheless, it's nice to know what can be done under conditions that are far from ideal. The first two samples I analyzed were mixtures of known compounds in unknown amounts (iron and hematite in one case; iron, goethite, and humboltine in the other). My analysis gave the correct percent composition to within 15 percentage points in all cases. The next four samples were mixtures of two or three compounds from a list (iron, hematite, goethite, humboltine, FeO, lepidocrocite, and magnetite). I didn't know which compounds were chosen, whether there were two or three, and how much of each. In two of the four cases I correctly identified the compounds present. In one case I identified two of the three compounds, and noted that there might be a third present in a small concentration but that I couldn't confirm that; the three compounds proved to be present. In the last case I correctly identified the majority compound but misidentified the minor contributor. In three cases I found the correct percent compositions to within 10 percentage points; the last was off badly. The next three samples involved a compound completely unknown to me mixed with one or two compounds known to me. I correctly identified the class of the completely unknown compound. In two cases the percent compositions I found were correct within 10 percentage points; the other within 16. The last sample was a mixture of the complete unknown with one or two compounds chosen from the list, but without me knowing which ones. As already indicated, I correctly identified the class of the complete unknown. I also identified one of the two other compounds as definitely present and the other as most likely if there were a third compound involved. Percent compositions for this sample were not accurate. You could therefore argue that this shows that bad data yields bad results. But I'd tend to look at it another way--even questionable data can yield pretty good results. In all cases but one the substances were correctly identified (twice provisionally as a possible minor contributor), despite the fact that many of the candidate substances are structurally similar in that they involve octahedrally coordinated iron. Percent compositions were usually good rough guides, although on occasion they went astray. I'll leave these data up at http://xafs.org/EXAFS_Divination_Set and let you know when I get the high-quality follow-up project posted... --Scott Calvin Sarah Lawrence College