Yes, this is a very interesting approach John. The zero test tells you a lot, and, as you suggest, the analysis of inhomogeneous materials can be quite useful indeed, and matrix matching is a critical part of this. Of course, synthetic monazite would be useful as suggested by crystalgrower, maybe even as a primary standard if it could be synthesized homogeneously. Any such reference material for blank testing, however, would need a range of Th concentrations, and a few wt.% Y, and at least a typical La, Ce, Nd concentration. The range of Th in natural monazite can be very large (less than 1 wt.% to almost 20 wt.% but is most typically in the 1-8 wt.% range. The measurements of Pb and U are complicated by interferences by Th and Y mostly, and in the case of UMb, by the Th M4 and M5 absorption edges. When Philippe Goncalves synthesized some monazite here years ago, he could get the REE phosphates, and a mixed LREE phosphate, but adding Th became interesting as the products were very heterogeneous. But this is still useful as you suggest!