Dissolved oxygen:

The paramagnetic substances such as dissolved oxygen and several transition metals along with unpaired electrons dramatically decrease fluorescence and cause interference within fluorimetric determinations. A paramagnetic nature of molecular oxygen promotes intersystem crossing from singlet to triplet states in another molecules. The longer lifetimes of the triplet states increases the opportunity for radiationless deactivation to occur.

Presence of dissolved oxygen influences phosphorescence too and causes a big decrease in the phosphorescence intensity. It is because of the fact that oxygen that is in triplet state at the ground state gets the energy from an electron in the triplet state and gets excited. This is fact the oxygen emission and not the phosphorescence. Thus, it is advisable to make phosphorescence measurement in the absence of dissolved oxygen.