Improved methods employed at Los Alamos National Laboratory for 231Pa/235U radiochronometry are outlined. We present elution curves obtained during 233Pa tracer preparation from 237Np. Additionally, we report model ages for uranium certified reference materials (CRMs) exhibiting a range of 235U enrichments including the first 231Pa/235U model ages for CRMs U200 and U900. Our results enable these CRMs to be used, with increased confidence, as quality control materials during nuclear forensics investigations.