although tr-fret can be accomplished with a variety of fluorophore combinations, lanthanide metals are particularly useful. certain life science applications take advantage of the unique fluorescence properties of lanthanide ion complexes (ln(iii) chelates or cryptates). these are well-suited for this application due to their large stokes shifts and extremely long emission lifetimes (from microseconds to milliseconds) compared to more traditional fluorophores (e.g. fluorescein, allophycoyanin, phycoerythrin, and rhodamine). the biological fluids or serum commonly used in these research applications contain many compounds and proteins which are naturally fluorescent. therefore, the use of conventional, steady-state fluorescence measurement presents serious limitations in assay sensitivity. long-lived fluorophores, such as lanthanides, combined with time-resolved detection (a delay between excitation and emission detection) minimizes prompt fluorescence interference. this method (commonly referred to as time-resolved fluorometry or trf) involves two fluorophores: a donor and an acceptor. excitation of the donor fluorophore (in this case, the lanthanide ion complex) by an energy source (e.g. flash lamp or laser) produces an energy transfer to the acceptor fluorophore if they are within a given proximity to each other (known as the förster's radius). the acceptor fluorophore in turn emits light at its characteristic wavelength.