Rare event searches demand extremely low background levels, necessitating ever-advancing screening techniques to enhance sensitivity. Liquid scintillators are highly attractive as detector media due to their inherent radiopurity and scalability in mass. In this work, we present a screening procedure to measure ultra-trace concentrations of natural contaminants -- $^{ 238}$U and $^{ 232}$Th -- with sensitivities at the \qty{ E-15}{ g/g} level. Our method combines neutron activation analysis with radiochemical techniques, followed by \bg\ coincidence spectroscopy to minimize interference backgrounds. This approach achieves sensitivities of \qty{ 0.65E-15}{ g/g} for $^{ 238}$U and \qty{ 2.3E-15}{ g/g} for $^{ 232}$Th, among the best reported worldwide. Potential pathways for further sensitivity improvements are outlined in the conclusions.
Categories: physics.ins-det