Abstract: Plutonium (Pu) is a critical radionuclide of major concern in national security and radiation safety. Accurate determination of Pu in environmental matrices is essential for research on nuclide migration, environmental monitoring around nuclear facilities, and nuclear environmental assessment. Incomplete sample digestion adversely affects the accuracy of both plutonium concentration and isotopic ratio measurements. Therefore, digestion protocols must be adapted to different types of environmental samples. Ammonium bifluoride (NH₄HF₂) was employed to digest granite, achieving complete dissolution at elevated temperatures. To facilitate plutonium speciation determination and the establishment of the separation procedure, ¹⁹F nuclear magnetic resonance (NMR) spectroscopy was used to qualitatively analyze the speciation and concentration of fluorine complexes in the dissolution solution. Aluminum fluoride and silicon fluoride complexes were identified, with no detectable free HF. The total fluoride ion concentration was approximately 0.4 mol/L. Software-based speciation calculations indicate that 99.9% of Pu(Ⅳ) exists as fluoride complexes, resulting in low separation recovery. To address this, N, N'-dimethyl-N, N'-dioctyl-3-oxapentanediamide (DMDODGA) resin was used to efficiently adsorb plutonium from the digestion solution. The adsorption behavior of Pu(Ⅳ) and coexisting elements on this resin was systematically investigated. In 8 mol/L HCl, the distribution coefficient (K_d) of Pu(Ⅳ) is 2.6×10⁴ mL/g. The results indicate that the rigid complexation structure of DMDODGA allows the resin to compete with fluoride ligands, extracting Pu(Ⅳ) from its fluoride complexes with nitrate or chloride as counter anions. Moreover, in dilute acid media, the K_d values for uranium and major rock matrix elements are below 10 mL/g, enabling efficient separation of Pu(Ⅳ) from these interfering elements. This approach improves the Pu(Ⅳ) recovery to 99.3%, eliminating the instability and the low recovery of coprecipitation while simplifying the overall separation procedure. Based on these results, a protocol for trace Pu(Ⅳ) determination in 10 g granite samples is established. The overall Pu(Ⅳ) recovery is 98.2%, with decontamination factors (DFs) exceeding 10⁴ for Am(Ⅲ), U(Ⅵ), and Tc(Ⅶ), and exceeding 10⁶ for matrix elements. This method enables direct adsorption of trace Pu(Ⅳ) from fluoride-containing digests without fluoride masking agents or carrier coprecipitation, overcoming inaccuracies in determination caused by incomplete dissolution. This method provides a robust solution for radiation safety, nuclear source term identification, and nuclear non-proliferation efforts.