It has already been noted that fluorescent dyes have a finite lifetime (t) which can be measured. It is possible to make use of temporal as well as spectral information in a number of ways. The sources of background in fluorescence typically have lifetimes of 0.1-10 ns ( 1 nanosecond = 10^-9 seconds). Several sources of background such as Raman and Rayleigh scatter can be considered as instantaneous events. So by delaying the measurement of the fluorescent emission until the background has decayed to zero it should be possible to eliminate background entirely (figure 21 and figure 22).

In order for this approach to succeed the lifetime of the dyes used in this type of experiment would ideally be >10 ns. Unfortunately almost all of the organic dyes in use have lifetimes <10 ns and none of the dyes with very high extinction coefficients have lifetime >10 ns. It is however, possible to carry out time resolved work using the chelates of some metals. The lanthanide chelates for example have lifetimes in the 100 ms to 2 ms region. The chelates of ruthenium have lifetimes of around 400 ns. These and several other metal chelates have been extensively used in time-resolved fluorescence studies. Further information about the chelates is given below in the Dyes section.