Novel mixed-element particle working standards are needed for instrument calibration and operational quality control within the International Atomic Energy Agency (IAEA) Network of Analytical Laboratories (NWAL). Historically, the microanalysis of mixed plutonium and uranium oxide materials has been challenging due to a deficiency of adequate particle working standards with well-characterized elemental and isotopic compositions. Savannah River National Laboratory (USA) previously developed the THermally Evaporated Spray for Engineered Uniform particulateS (THESEUS) production platform for the synthesis of uranium oxide particulate working standards. THESEUS uses aerosol-based technologies to generate, calcine, and aggregate monodispersed oxidephase particle products. To synthesize Pu-containing U oxide microparticulates, the production platform was re-engineered and the microanalytical characterization methods were refined. THESEUS was redesigned to control the collection of targeted numbers of Pu/U particles on specific locations of microanalytical substrates. Physical modifications included a new thermal source and new electrostatic precipitator collection design. Analytical electron microscopy operations were improved with new automated particle measurement protocols which incorporated uranium oxide particle test specimens for quality assurance and control. A modified counting statistics method was developed to process large-geometry secondary ion mass spectrometry (LG-SIMS) data to assess both inter- and intra-particle elemental and isotopic homogeneity. These THESEUS and microanalytical developments were demonstrated with the syntheses of Pu/U oxide particulates with a 1:100 actinide elemental ratio. Other outcomes included spherical particle morphologies with an average particle diameter of one micrometer, and a constrained particle deposition on microanalytical substrates resulting in particulate loadings of < 2500 particles per planchet.