Oxidative degradation of bis(2,4,4-trimethylpentyl)dithiophosphinic acid in nitric acid studied by electrospray ionization mass spectrometry

Ratione: The selective separation of the minor actinides (Am, Cm) from the lanthanides is a topic of ongoing nuclear fuel cycle research, and dithiophosphinic acids are candidate ligands in these processes. Ligand instability has been noted under radiolytic and harsh acid conditions but explicit degradation pathways for ligands such as bis(2,4,4-trimethylpentyl)- dithiophosphinic acid (CyxH), the major compound in the commercial product Cyanex 301, have been elusive. Methods: Organic solutions of CyxH were contacted with aqueous solutions of HNO₃, and their degradation was studied by analyzing samples from these experiments by direct infusion electrospray ionization mass spectrometry. Ions were identified using accurate mass measurement and collision-induced dissociation. RESULTS: The positive ion spectra contained cationized CyxH cluster ions, and oxidatively coupled species (designated Cyx₂) cationized by either H or Na. The Cyx ₂-derived ions increased with acid contact time. The negative ion spectra consisted almost entirely of the CyxH conjugate base. The negative ion spectra of the HNO₃-contacted samples also contained conjugate bases corresponding to the dioxo and perthio derivatives of CyxH. CONCLUSIONS: CyxH is oxidized by acid contact to form the coupled species Cyx₂, and the dioxo species arise from subsequent oxidation of Cyx₂. Oxidative coupling increases with contact time, and with higher HNO₃ concentrations. The direct infusion measurements provided a simple approach for assessing degradation pathways and kinetics.