What are the different isomers of FITC?

There are two isomers of FITC – fluorescein 5-isothiocyanate and fluorescein 6-isothiocyanate. 

Fluorescein 5-isothiocyanate, also known as 5-FITC or isomer I, contains an isothiocyanate group at position 5 (isomer I) of the bottom benzene ring. It has an excitation maximum of 491 nm and an emission maximum of 516 nm. 5-FITC is used as a label in fluorescent antibody staining procedures as well as protein- and amino acid-binding techniques. 

Fluorescein 6-isothiocyanate, also known as 6-FITC or isomer II, contains an isothiocyanate group at position 6 (isomer 2) of the bottom benzene ring. It has an excitation maximum of 490 nm and an emission maximum of 525 nm. 6-FITC can be incorporated directly at the time of synthesis. 

Both FITC isomers are bright green fluorophores that are sensitive to changes in pH. The variation of the excitation and emission wavelengths among the two FITC isomers is minor.