= Solution
Use complementary composition, spatial and population measurements:
* \b[Insufficient Pt:] digest representative aliquots and measure calibrated ICP-OES or ICP-MS to obtain bulk Pt/Pd ratio and trace Fe. Blanks, recovery checks and standards control contamination and matrix effects. This is sensitive and quantitative but destructive and supplies no particle-by-particle spatial information.
* \b[Poor Pt-Pd mixing:] use STEM-EDS maps or line profiles of many individual particles. Characteristic X-ray energies identify the elements; background subtraction and calibrated sensitivity factors permit local composition estimates. HAADF-STEM supplies complementary atomic-number-sensitive contrast, with Pt generally brighter than Pd for comparable thickness. Core-shell segregation or distinct domains can then be tested. Projected overlap can imitate mixing, and low counts, thickness and beam damage limit nanoscale quantification; tomography or multiple views can resolve important ambiguities.
* \b[Fe impurities:] <EDS>/<EELS> can locate Fe-rich inclusions or surface-associated material, while the bulk digest establishes whether a trace signal is representative. Failure to detect Fe in a few <TEM> fields does not rule out dilute contamination below local detection limits. <XPS> can additionally assess near-surface composition and oxidation state, but is not a bulk assay.
* \b[Size uniformity:] obtain calibrated <TEM> images from randomly chosen independent fields, segment individual particles and report a number-weighted size histogram with sample count and shape descriptors. Avoid overlap and selection of only well-dispersed or attractive particles. A projected diameter is not a complete three-dimensional volume; scattering methods can corroborate ensemble size but weight particles differently.
The strength of <TEM> is correlating structure, size and local chemistry; its limited sampling is why a representative bulk chemical assay remains valuable.
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