Spectra data is listed for each dye on our website and in our catalog. These values are provided by the manufacturer of the fluorescent dye and are generally calculated from the free dye not attached to an oligonucleotide. As a general rule, these values work fine for the common oligonucleotide user as there is little change, if any, when the dye is attached to an oligonucleotide. The particular base composition of an oligonucleotide can play a role as can pH in many cases, particularly with fluorescein and related dyes.