I would like to apply isotope dilution mass spectrometry (IDMS)* to quantify lead at trace levels. A good spike solution for the process would be the NIST SRM 991 which is an enriched material of almost only lead-206, but this material is now discontinued and the replacement is a radioactive material NIST SRM 983, which I think, will be a little difficult to import.
The NRC of Canada has produced the lead-204 and lead-207 double spike isotopic standard BLED-1 which contains 49% and 33% the two isotopes, respectively, and I was wondering if it could be suitable for IDMS considering that the natural relative abundance of those isotopes is low. I have only performed IDMS using the (single spike) NIST SRM 991, and although I think the same IDMS equations will apply to each pair of isotopes, would like to know please if there is something I am not taking into account.
*) An overview about this technique is provided by Vogel, J.; Prizkow, W. Isotope dilution mass spectrometry — A primary method of measurement and its role for RM certification. MAPAN 25, 2010, 135–164, doi 10.1007/s12647-010-0017-7. The author's copy on ResearchGate.