X is NH for APF and O for HPF (hydroxyphenyl fluorescein); hROS is a highly reactive oxygen species. APF is specific to OH radical and ONOO▪- and HPF is specific to OH radical. HPF does not detect OH radical, which indicates that the APF intensity is mostly due to ONOO▪- formation. When ONOO▪- combines with APF (the left reagent, X=NH), fluorescein compounds are produced at an excitation wavelength of 490 nm and emission wavelength of 515 nm.

Figure 20: Peroxynitrite anion radical (ONOO▪-) detection using aminophenyl fluorescein (APF).